152880920 lte transport tutorial

1 (86) Radio Transport PM LTE Transport Confidential

LTE Transport Tutorial

Nokia Siemens Networks Radio Transport Product Management

Confidential

____________________________________________________________________________

Version Date Author

4.0 24-Jun-2010 Torsten Musiol ____________________________________________________________________________


Terminology .....................................................................................................................................5 Glossary of Acronyms ......................................................................................................................6

1. Introduction ..................................................................................................................................9 2. LTE Basics.................................................................................................................................12

2.1 Network Architecture Evolution ............................................................................................12 2.2 Protocol Stacks....................................................................................................................15

2.2.1 User Plane....................................................................................................................15 2.2.2 Control Plane................................................................................................................15

2.3 Intra-LTE Handover .............................................................................................................16 2.4 Transport Performance Requirements .................................................................................18

2.4.1 Throughput (Capacity) ..................................................................................................18 2.4.2 Delay (Latency), Delay Variation (Jitter)........................................................................23 2.4.3 TCP Issues ...................................................................................................................25

3. Mobile Backhaul Architecture for LTE ........................................................................................26 3.1 X2 Connectivity Requirements.............................................................................................26 3.2 Implementation Options .......................................................................................................28

3.2.1 L2 vs. L3.......................................................................................................................28 3.2.2 Ethernet Based Backhaul Network................................................................................29 3.2.2.1 E-Line .......................................................................................................................30 3.2.2.2 E-LAN .......................................................................................................................31 3.2.2.3 E-Tree.......................................................................................................................32 3.2.2.4 Comparison...............................................................................................................33 3.2.3 Implementation Examples.............................................................................................34

3.3 Transport Service Attributes.................................................................................................36 3.3.1 Capacity .......................................................................................................................36 3.3.2 Performance .................................................................................................................36

3.4 Traffic Differentiation with VLAN and IP Addressing ............................................................37 3.4.1 Generic eNB IP Addressing Model................................................................................37 3.4.2 Network Reference Configurations ...............................................................................41

3.5 Transport Redundancy ........................................................................................................44 3.5.1 General.........................................................................................................................44 3.5.2 SCTP Multi-homing.......................................................................................................45

3.6 Base Station Co-location .....................................................................................................46 4. LTE Transport Interfaces and Protocols .....................................................................................47

4.1 Ethernet ...............................................................................................................................47 4.2 IP.........................................................................................................................................48

4.2.1 General.........................................................................................................................48 4.2.2 MTU Size Issues...........................................................................................................49 4.2.2.1 Enforcement of lower MTU size ................................................................................50 4.2.2.2 IP Fragmentation and Reassembly ...........................................................................51 4.2.2.3 Ethernet Jumbo Frames............................................................................................52 4.2.3 IPv6 ..............................................................................................................................53

5. QoS............................................................................................................................................55 5.1 End-to-End QoS ..................................................................................................................55 5.2 Transport QoS Features ......................................................................................................56

5.2.1 Traffic Prioritization on IP Layer ....................................................................................56 5.2.2 Traffic Prioritization on Ethernet Layer ..........................................................................57 5.2.3 Packet Scheduling ........................................................................................................58 5.2.4 Traffic Shaping .............................................................................................................59

5.3 Mapping RRM QoS onto Transport QoS..............................................................................60 5.4 Congestion Control ..............................................................................................................61


6. Synchronization..........................................................................................................................62 6.1 Synchronization from GPS...................................................................................................62 6.2 Synchronization from Transport Network .............................................................................63

6.2.1 IEEE1588-2008 Precision Time Protocol (Timing-over-Packet) ....................................63 6.2.2 Synchronous Ethernet ..................................................................................................65

6.3 Solutions for Co-location with Legacy Equipment ................................................................66 6.3.1 Synchronization from PDH interface .............................................................................66 6.3.2 Synchronization from 2.048MHz signal.........................................................................67

7. Transport Security......................................................................................................................68 7.1 General................................................................................................................................68 7.2 X2 Issues.............................................................................................................................70 7.3 IPsec Tunnel Mode vs. Transport Mode...............................................................................72 7.4 Security Associations and IP Addressing .............................................................................72 7.5 Public Key Infrastructure (PKI).............................................................................................75

8. Transport Operability..................................................................................................................76 8.1 Ethernet OAM......................................................................................................................76 8.2 Transport PlugnPlay (SON)................................................................................................77

8.2.1 Auto-Connection...........................................................................................................77 8.2.2 Auto-configuration.........................................................................................................79

9. Flexi Transport Sub-Modules for LTE.........................................................................................80 10. List of Features....................................................................................................................83 11. References ..........................................................................................................................84 12. Change History ....................................................................................................................86


How to Read this Document This document is intended to support LTE pre-sales activities. It describes standardized techniques as well as NSN concepts, recommendations and solutions. Any statements regarding release planning in this document are of informative character only and are subject to change. This document will be further revised as product planning progresses.


Terminology

Control Plane The protocols, functions and interactions that are related to the control of the system

DiffServ The Differentiated Services protocol which provides a mechanism for applying Quality of Service in IP [RFC2474], [RFC2475]

eNodeB A logical node responsible for radio transmission/reception in one or more cells to/from the User Equipment. It terminates the S1 interface towards the MME and Serving GW. This is usually abbreviated to eNB.

EPS Bearer An EPS bearer is the level of granularity for bearer level QoS control in the EPC/E-UTRAN. In the case of GTP-based S5/S8 an EPS bearer runs between a UE and a PDN GW; in the case of PMIP-based S5/S8 it runs between a UE and a Serving GW.

Long Term Evolution The name given to the work on the evolution of the UTRAN for the next 10 years and beyond working towards a high data rate, low latency and packet-optimized radio-access technology.

Mobility Management Entity The C-plane functional element in the EPC that manages and stores UE context (UE/user identities, UE mobility state, user security parameters). It is responsible for the management of identities, checking authorization, bearer management, mobility management and NAS termination.

Network Domain Security The security mechanism employed to protect IP traffic in 3GPP and fixed broadband core networks.

Packet Data Network Gateway The Packet Data Network (PDN) Gateway (P-GW) is one of the gateways that provide the U-plane gateway functionality for the EPC. The P-GW terminates the SGi interface to the PDN. Its functionality includes policy enforcement and charging support.

Quality of Service The concept of providing varying levels of service depending on a variety of factors such as resources available, priority of data, subscription paid.

Serving GW The Serving Gateway (S-GW) is one of the gateways that provide the U-plane gateway functionality for the EPC. The S-GW is the gateway to the E-UTRAN and serves as a mobility anchor point.

System Architecture Evolution This is name given to the work on the evolution of the core network system architecture for the next 10 years and beyond towards a high data rate, low latency and packet-switched system supporting multiple radio access networks.

Transport Plane The protocols, functions and interactions related to transport of C-plane and U-plane data over the interfaces between network nodes of the E-UTRAN and EPC (and within the E-UTRAN and EPC).

User Plane The protocols, functions and interactions related to the transport and manipulation of user data in the system.


Glossary of Acronyms

AAA Authentication, Authorization and Accounting AMBR Aggregate Maximum Bit Rate ANR Automatic Neighbor Relation ARP Allocation and Retention Priority BTS Base Transceiver Station BTSEM BTS Element Management application BTSOM BTS OAM protocol BTSSM BTS Site Manager CA Certificate Authority CAC Connection Admission Control CAPEX Capital Expenditure CBS Committed Burst Size CDN Content Distribution Network CESoPSN Circuit Emulation Service over Packet Switched Network CIR Committed Information Rate CoMP Cooperative Multipoint DL Downlink DSCP DiffServ Code Point EBS Excess Burst Size EIR Excess Information Rate EMS Element Management System eNB Evolved Node B (also abbreviated as eNodeB) EPC Evolved Packet Core EPS Evolved Packet System E-UTRAN Evolved Universal Terrestrial Radio Access Network EVC Ethernet Virtual Connection FD Frame Delay FDD Frequency Division Duplex FDV Frame Delay Variation FFS For Further Study FLR Frame Loss Ratio FM Fault Management FTM Flexi Transport sub-Modules GBR Guaranteed Bit Rate


GGSN Gateway GPRS Support Node GPS Global Positioning System GTP GPRS Tunneling Protocol HO Handover IDU Indoor Unit IETF Internet Engineering Task Force IKE Internet Key Exchange L2 Layer 2 LTE Long Term Evolution MAC Medium Access Control MBH Mobile Backhaul MBMS Multimedia Broadcast/Multicast Service MBR Maximum Bit Rate MEF Metro Ethernet Forum MEG Maintenance Entity Group MEP Maintenance End Point MIMO Multiple Input Multiple Output MLO Multi-Layer Optimization MME Mobility Management Entity MTU Maximum Transmission Unit MWR Microwave Radio NDS Network Domain Security NE Network Element NGMN Next Generation Mobile Networks NMS Network Management System NRT Non Real Time NTP Network Time Protocol O&M Operations and Maintenance OAM Operation, Administration, Maintenance ODU Outdoor Unit OPEX Operating Expenses PD Packet Delay PDCP Packet Data Convergence Protocol PDN Packet Data Network PDN-GW Packet Data Network (PDN) Gateway PDU Protocol Data Unit


PDV Packet Delay Variation PKI Public Key Infrastructure PLR Packet Loss Ratio PRC Primary Reference Clock PS Packet Switched PSK Pre-Shared Key PW Pseudo-Wire QoS Quality of Service RLC Radio Link Control RNC Radio Network Controller RNL Radio Network Layer RRM Radio Resource Management RT Real Time RTT Round-Trip Time SA Security Association SAE System Architecture Evolution SAE-GW System Architecture Evolution Gateway SCTP Stream Control Transmission Protocol SDF Service Data Flow SDU Service Data Unit SEG Security Gateway SFN Single Frequency Network S-GW Serving Gateway SGSN Serving GPRS Support Node SLA Service Level Agreement SLS Service Level Specification SPQ Strict Priority Queuing SW Software TDD Time Division Duplex TNL Transport Network Layer UE User Equipment UL Uplink UNI User Network Interface VPLS-TE Virtual Private Line Services Transport Equipment VPN Virtual Private Network WFQ Weighted Fair Queuing


1. INTRODUCTION

LTE refers to the Long Term Evolution of the 3GPP radio access technology and is considered the successor of the current UMTS system with the rollout anticipated to begin with trials in 2009. The work in 3GPP is closely aligned to the 3GPP System Architecture Evolution (SAE) study which examines the overall evolved 3GPP architecture and its operation in conjunction with the Evolved UTRAN (E-UTRAN). SAE work in 3GPP has been renamed into Evolved Packet Core (EPC). Together EPC and LTE form Evolved Packet System (EPS). Whereas the Radio Network Layer (RNL) is being specified by 3GPP, the Transport Network Layer (TNL) for LTE is nowhere defined consistently. Contributing standardization bodies are:

IETF IEEE ITU-T Metro Ethernet Forum (MEF)

TransportNetwork

Layer

RadioNetwork

Layer MMESAE-GW

O&MeNB

UE

Figure 1 Standardization landscape

To some extent, the Next Generation Mobile Networks (NGMN) consortium of mobile operators aims at closing the gap. Naturally, there is a lot of room for innovation and a need for education.


The following figures indicate a number of Frequently Asked Questions and corresponding misconceptions which deserve clarification.

eNB

What about performance

impact with IPSec?

Ethernet / IP

What transport capacity is required? What transport

delay is acceptable?

What are the synchronization requirements?

Point-to-pointor

Multipoint-to-Multipoint Ethernet?

How to connect automatically(plugnplay)?

How to implement

QoS?

How to share transport with a co-sited base station?

O&M

MME

SAE-GW

How should IP addresses and VLANs be used

Ethernet switch

orIP router?

Is Transport security needed?

Figure 2 FAQs

eNB

Ethernet / IP

Transport capacity has to be >500Mbit/s

per eNB Transport delay has to be


The NSN LTE FDD product roadmap comprises the following stages: RL09 provides basic connectivity functions (LTE protocol stack, IP/Ethernet interface) RL10 supports in particular transport QoS, transport security and a variety of synchronization

options RL20 provides support for base station co-location and operability enhancements RL30 and beyond will focus on transport resilience, IPv6 and other optimizations

Features are also linked to TDD releases (RLxxTD) following a similar logic. This document is focused on features planned until RL20 / RL15TD. However, some functionality intended for later releases is also included. In general, the functionality of a next release extends the functionality of the previous release rather than changing it. Where this is not the case it will be highlighted.


2. LTE BASICS

2.1 Network Architecture Evolution

Two general trends can be observed which have a great impact on the architecture of mobile networks: First, circuit switched (CS) connectivity services will be gradually replaced by packet switched (PS) services. This implies in particular that legacy voice services will sooner or later disappear and be replaced by Voice-over-IP (VoIP) service. Secondly, mobile networks evolve towards flat architectures, leading to more favorable economies with the ever-increasing amount of user traffic. With 3GPP Rel.6 (WCDMA), both the Radio Access Network (RAN) and the Core Network (CN) were built hierarchically. The RAN was composed of the base station (NodeB, NB) and the Radio Network Controller (RNC) and the CN of the Serving GPRS Support Node (SGSN) and the Gateway GPRS Support Node (GGSN). In a first evolution step (3GPP Rel.7), the concept of direct tunnel between RNC and GGSN has been introduced, so that user plane (U-plane) traffic can bypass the SGSN (flat CN). In a next step, pioneered by NSN, RNC functions have been merged into the I-HSPA base station (iNB, flat RAN).

NB RNC SGSN GGSNInternet

3GPP Rel. 6 / HSPA

Direct tunnel

3GPP Rel. 7 / HSPA

Internet

Direct tunnel

3GPP Rel. 7 / Internet HSPA

Internet

NB

iNB

Figure 4 Evolution towards flat network architecture

The LTE network architecture (Figure 5) introduced with 3GPP Rel.8 is built upon the same principles. Consequently, the CS part has been eliminated completely.


Direct tunnel

Internet

eNB SAE-GW

MME

3GPP Rel 8 / LTE/SAE

Figure 5 LTE network architecture

The LTE network elements are as follows: The Mobility Management Entity (MME) is the control plane (C-plane) functional element in

EPC. The MME manages and stores the UE context, generates temporary identities and allocates them to UEs, authenticates the user, manages mobility and bearers, and is the termination point for Non-Access Stratum (NAS) signaling.

The Serving Gateway (S-GW) is the user plane (U-plane) gateway to the E-UTRAN. The S-GW serves as an anchor point both for inter eNodeB (eNB) handover and for intra 3GPP mobility, i.e. handover to and from 2G or 3G.

The Packet Data Network Gateway (P-GW) is the U-plane gateway to the PDN, e.g. the Internet or the operators IP Multimedia Subsystem (IMS). The P-GW is responsible for policy enforcement, charging support, and users IP address allocation. It also serves as a mobility anchor point for non-3GPP access.

Most often, the S-GW and P-GW functions are combined in one network element, often called SAE Gateway (SAE-GW).

The E-UTRAN contains only the base station, eNB, which provides the air interface to the UE.

Evolved Packet System (EPS) reference points are specified in [TS36.300], [TS23.401] and [TS23.402]. The following ones apply to the E-UTRAN:

S1-MME Control plane reference point between eNB and MME S1-U User plane reference point between eNB and the S-GW X2 Control & user plane reference point between two eNBs

eNBs can be connected to each other via the X2 reference point and connected to MMEs and S-GWs via the S1-MME/S1-U reference points. A single eNB can be connected to multiple MMEs and multiple S-GWs. O&M traffic, a.k.a. Management plane (M-plane) is not specified by 3GPP, but yet to be considered. A fourth functional plane shall be introduced here: The Synchronization plane (S-plane). Terminated by respective applications in the network (synchronization grandmaster) and in the base station (synchronization slave), this concept can easily be understood since synchronization traffic flowing over the IP transport network has its own set of QoS requirements and should be distinguished from U/C/M-plane traffic correspondingly (see chapter 6.2).


Inter-eNBconnection

(X2) IP

NMS

X2U/C-Plane

M-Plane

S1 U-Plane

S1 C-Plane

MME

SAE-GW

eNB 2

eNB1

S-PlaneSynchronization

Grandmaster(optional)

Figure 6 LTE traffic types


2.2 Protocol Stacks

All protocol stacks for user (U), control (C), management (M) and (optionally) synchronization (S) planes are based on IP. This section outlines the protocol stacks of the S1 and X2 interfaces.

2.2.1 User Plane

The U-plane service layer is IP. UE mobility is achieved through tunnels which originate from the PDN-GW and are anchored at the S-GW. Within the E-UTRAN, the GPRS Tunneling Protocol (GTP-U) is used as tunneling protocol.

PDCP

RLC

MAC

PHY

User IP

PDCP

RLC

MAC

PHY

Uu

UE eNB

S1-U

UDP

IP

L1/L2

GTP-U

S-GW

UDP

IP

L1/L2

GTP-U

eNB

X2

UDP

IP

L1/L2

GTP-U

eNB

UDP

IP

L1/L2

GTP-U

Figure 7 User-plane protocol stacks

2.2.2 Control Plane

Within the E-UTRAN, SCTP is used to carry the control plane protocols (Application Protocols) S1-AP and X2-AP. SCTP has been used previously to carry NBAP over IP (Iub/IP).

PDCP

RLC

MAC

PHY

RRC

PDCP

RLC

MAC

PHY

Uu

UE eNB

S1-MME

IP

L1/L2

MME

SCTP

IP

L1/L2

NAS

RRCS1-AP

SCTP

S1-AP

eNB

X2

IP

L1/L2

eNB

SCTP

IP

L1/L2

SCTP

X2-AP X2-AP

Figure 8 Control-plane protocol stacks


2.3 Intra-LTE Handover

Intra-LTE handover (HO) can be performed over the S1 interface (S1 handover) or X2 interface (X2 handover). S1 handover is always required for MME relocation. With X2 handover, DL data forwarding between Source eNB and Target eNB will be optimized. Furthermore, most of the C-plane messages are exchanged directly between the involved eNBs, so the MME processing load will be reduced significantly. Implications on the Mobile Backhaul Network will be elaborated in the following. In principle, the considerations apply similarly to both S1 and X2 handover. Since the X2 interface is expected to be present in many cases (X2 is required for ANR), this will be analyzed in more detail. During the handover procedure the radio link is interrupted for a short moment (3050ms). DL packets arriving at the Source eNB will be forwarded to the Target eNB via the (logical) X2 interface until the S1 path has been switched to the Target eNB (Hard Handover).

S1-uS-GW

Target eNB

Source eNB

Before

S1-u

S-GW

Target eNB

Source eNB

After

S1-uS-GW

Target eNB

Source eNB

During

X2-u

Figure 9 Handover over X2 - Principle

In the first phase (1) of the handover (see Figure 10, left side), the DL traffic destined to the Source eNB fills the normally lightly used Source eNB UL path (asymmetric user traffic and symmetric backhaul capacity DL/UL assumed). The duration of phase (1) T1 is determined by C-plane processing (including transport latency) and may be significantly longer than the air interface interruption time. In the second phase (2) (see Figure 10, right side), the Target eNB connection may be temporarily congested if many handovers are taking place simultaneously and DL packets are already arriving through the S1 path to the Target eNB while X2 packets are still in transit. This overlap time T2 is mainly given by the X2 transport latency, assuming S1 latency toward Source eNB and Target eNB are almost equal. If congestion occurs packets may be queued (and potentially QoS aware dropped) at the affected transport node(s). But even if the X2 traffic goes completely along with S1 traffic (T2 ~ S1 UL latency + S1 DL latency), the probability and duration of such bursts will be very limited. As a conclusion, the capacity need for X2 is very small compared to S1. Analysis of the mobility model resulted in 2%3% extra capacity.


S1-uS-GW

Target eNB

Source eNB

During HO (1)

S1-uS-GW

Target eNB

Source eNB

During HO (2)

X2-u

Figure 10 Handover over X2 - Capacity


2.4 Transport Performance Requirements

2.4.1 Throughput (Capacity) Cell peak rates will increase dramatically with LTE. This is mainly due to larger available bandwidth (max 20MHz per cell compared to 5MHz with WCDMA). It should be noted, however, that HSPA evolution will also bring multi-carrier modes, leveling this advantage at least partly. Maximum cell peak rates of 150Mbit/s downlink (64QAM 2x2 MIMO) and 75Mbit/s uplink (64QAM single stream) are available for a 20MHz configuration (ref. [LTE for UMTS], chapter 9.2, Table 9.3 and 9.4). DL/UL peak rates of 172Mbit/s / 58Mbit/s have often been used for marketing purposes, but are actually not achievable since the code rate 1/1 is not defined by 3GPP. It should also be noted that the cell peak rates are achievable only under ideal air interface conditions, i.e. very close to the base station (up to 20% of cell range, corresponding to 4% of cell area) and without interference from other cells.

Max. peak data rate

Mbp

s

LTE 2x20 MHz (2x2 MIMO

LTE 2x20 MHz (4x4 MIMO

Downlink

Uplink

350

300

250

200

150

100

50

0

Figure 11 Cell peak capacity


As important as the cell peak rate is the cell average capacity (Figure 12). Average downlink cell spectral efficiency has been determined as ~1.51.7 bit/s/Hz in macro cell deployment with three sectors and 2.42.9 bit/s/Hz in micro cells with one sector (based on 3GPP simulations, 10MHz bandwidth, ref. [LTE for UMTS], Figure 9.12). It should be noted that this describes the cell peak capacity under average conditions, not an averaging over time. The spectral efficiency also varies with the available bandwidth. For further consideration in this chapter, macro cell configurations with a spectral efficiency of 1.7 bit/s/Hz downlink and 0.7 bit/s/Hz uplink are assumed which is a good approximation for 10MHz and 20MHz bandwidth.

Average throughput (macro cell, 20 MHz)

Mbp

s

LTE (2x2 MIMO), 20 MHz carrier

LTE 4x4 MIMO, 20 MHz carrier

60

50

40

30

20

10

0

Downlink

Uplink

Figure 12 Cell average capacity

The required transport capacity can be dimensioned with two different approaches: 1) Dimensioning based on user traffic profile (recommended) 2) Dimensioning based on air interface capabilities

Dimensioning based on the user traffic profile requires planning data from the operator. It can be tailored to the actual needs and allows QoS parameters to be taken into account. On the other hand, dimensioning based on air interface capabilities provides a simple and straightforward alternative if the traffic profile is not available and should be sufficient for initial planning considerations. This approach will be elaborated further in this chapter. Note that the considerations herein refer to the eNB backhaul interface only. Depending on the traffic profile, statistical multiplexing gain could reduce the total capacity requirements at aggregation points. For translating the capacity at the air interface into an equivalent capacity need at the transport (backhaul) interface of the eNB, air interface overhead has to be subtracted, while transport overhead has to be added.


GTP-U (without header extension) 8 bytesUDP 8 bytesIPv4 (transport) 20 bytesIPSec ESP Header (SPI/Sequence Number) 8 bytes AES Initialisation Vector 16 bytes ESP Trailer (2-17 bytes, incl. 0-15 padding bytes, average 8 bytes) 10 bytesIPSec Authentication (HMAC-SHA-1-96) 12 bytesIPSec Tunnel mode IP header 20 bytesEthernet higher layer (incl. 4 bytes for VLAN) 22 bytesEth. Inter Frame Gap, Preamble/SFD 20 bytesTotal transport overhead 144 bytes

Table 1 Transport overhead (with IPv4 and IPsec)

For a typical traffic profile with 50% small (60 bytes), 25% medium-size (600 bytes) and 25% large (1500 bytes) packets, the transport overhead can be estimated as ~25% with IPv4 and IPsec (~15% without IPsec). After subtracting the air interface overhead (PDCP/RLC), 20% (with IPsec) has to be added to the data rate at the air interfaces to calculate the corresponding transport capacity (15% without IPsec).

If N is the number of cells of one eNB site and Cpeak and Cavrg denote the peak and average capacity of one cell, the following main concepts can be distinguished (example in Figure 13 with 3 cells):

All-Average The backhaul connection supports the aggregated average capacity of all cells. Ctrs = N x Cavrg

All-Average/Single-Peak The backhaul connection supports the aggregated average capacity of all cells and the peak capacity of one cell, whatever value is greater. Ctrs = MAX (N x Cavrg ; Cpeak)

All-Peak The backhaul connection supports the aggregated peak capacity of all cells (non-blocking). Ctrs = N x Cpeak


Cell

pea

k

Cell average eNB

tran

spo

rt

All-Average

All-Average/Single-Peak

PeakRate!

All-Peak

Ove

rbo

okin

g

Figure 13 Transport capacity dimensioning scenarios

In most cases, All-Average/Single-Peak" is a reasonable trade-off between user service capabilities and transport capacity requirements, which may have a great impact on the operating costs. With that approach, advertised user service peak rates up to the cell peak rate can be (momentarily) supported in one cell. The advertised maximum user service (average) rate will be, however, only a fraction of the cell peak rate. It is mainly driven by fixed broadband user experience (xDSL service rates) and applications and will be enforced by LTE QoS parameters. This also means that multiple users can be supported with that rate simultaneously. With service differentiation, different maximum rates could be applied to different users, i.e. premium users could be served with highest rates. If the number of users is (initially) low, transport costs could be reduced even further if the maximum user service (average) rate will be applied for dimensioning instead of the cell peak rate. In summary, All-Average/Single Peak is a good trade-off in general, but it may be under-dimensioned for hot spots where users are located close to the antenna in multiple cells of the same eNB, and it may be over-dimensioned for sites with low utilization. The All-Peak concept will always lead to over-dimensioning, thus usually extra costs. However, if fiber is available at eNB sites, the incremental cost impact may be tolerable. So, there will be cases where the operator requires that the transport sub-system shall be no bottleneck at all.

As an example for the All-Average/Single-Peak concept, a 1+1+1 10MHz 2x2MIMO configuration is shown in Figure 14.


AirInterface eNB

90

30

Ethernet layer, with IPsec

TransportInterface

3 cells, 10MHzDL 17 Mbit/s net PHY average rate per cellUL 7 Mbit/s net PHY average rate per cellDL 75 Mbit/s net PHY peak rate per cell (64QAM 2x2 MIMO) UL 25 Mbit/s net PHY peak rate per cell (16QAM)

75

25

+20%

Figure 14 Example: All-Average/Single-Peak 1+1+1 10MHz

The maximum configuration with Flexi Multiradio BTS is the All-Peak scenario with a 2+2+2 20MHz 2x2MIMO air interface as shown below. It should be noted that with a single Gigabit Ethernet interface at the eNB the DL transport capacity is limited to 1Gbit/s, so the theoretically possible air interface capacity cannot fully be utilized.

AirInterface eNB

1000

360

830

300

Ethernet layer, with IPsec

TransportInterface

+20%

6 cells, 20MHzDL 150 Mbit/s net PHY peak rate per cell (64QAM 2x2 MIMO) transport capacity limited UL 50 Mbit/s net PHY peak rate per cell (16QAM)

Figure 15 Example: All-Peak 2+2+2 20MHz

As stated in chapter 2.3, the capacity need for Intra-LTE handover (over S1 or X2 U-plane) is very small compared to S1 U-plane traffic (2%3%). Also, M-plane (~1Mbit/s) and C-plane (~0.3Mbit/s) capacity requirements are negligible compared to S1 U-plane.


2.4.2 Delay (Latency), Delay Variation (Jitter) Delay requirements originate from both user services (U-plane end-to-end delay / delay variation for VoIP, web surfing, file transfer, gaming, streaming, email, etc.), radio network layer protocols (C-plane) and synchronization (S-plane).

U-plane U-plane delay requirements are determined by user service quality expectations. Obviously, delay requirements are important for interactive services (impact on response time, see Figure 16). However, there is also a relationship between U-plane delay and throughput performance if the user service is based on TCP (see chapter 2.4.3).

Figure 16 U-plane latency requirements

With respect to LTE system performance, less than 15ms round-trip time (RTT) can be achieved with pre-allocated resources, ~20ms including scheduling (ref. [LTE for UMTS], chapter 9.7). This can be verified with ping delay measurements starting from the UE, where the server is located near the SAE-GW (ignoring processing delays in the IP stacks and ping application). In contrast to GSM and WCDMA, the relative contribution of the mobile backhaul delay to the RTT has become significant and thus should be treated with special care. It is important to note that each 500km one-way distance between a base station and a usually centralized SAE-GW contribute 5ms to the RTT. This is only taking into account the unavoidable fiber delay (~200,000km/s is the speed of light in glass). Including the delay introduced by intermediate transport nodes, the transport part of the RTT can easily exceed the radio part.

In addition to delay (latency), also delay variation (jitter) has to be considered. Both the LTE air interface (scheduler) and the transport network contribute to the end-to-end jitter. Real-time end user applications (e.g. VoIP, audio/video streaming) are usually designed so that they can tolerate jitter in the order of 1020ms, using a properly dimensioned jitter buffer. Network dimensioning and in-built QoS


mechanisms have to assure that the end-to-end delay and delay variation budget of supported applications are not exceeded. In principle, the user service related considerations above apply to both S1 and X2. As described in chapter 2.3, DL packets during handover arriving at the Source eNB will be forwarded to the Target eNB, i.e. will take a longer route. Basically, implementing X2 latency significantly less than the radio link interruption time (3050ms) would have no benefit since those packets would have to wait at the Target eNB anyway. Most user applications will tolerate such a very short-term delay increase.

C/M-plane In contrast to WCDMA, where RNL related latency requirements are imposed by a number of RAN functions over Iub/Iur (e.g. Macro-Diversity Combining, Outer Loop Power Control, Frame Synchronization, Packet Scheduler), LTE transport latency requirements are mainly driven by user services (U-plane). In particular, most of the handover messaging is performed in the latency-insensitive preparation phase. C-plane protocol timers give implicitly an upper bound for the S1/X2 transport RTT (50ms default, configurable 102000ms).

S-plane A specific requirement for delay variation (jitter) applies if Precision Time Protocol (PTP) based on IEEE1588-2008, a.k.a. Timing-over-Packet, is used for eNB synchronization (see chapter 6.2.1).

NSN recommendations on network performance can be found in chapter 3.3.2.


2.4.3 TCP Issues

The Transmission Control Protocol (TCP) is designed to provide reliable transport for packet data. Today TCP is used for 80-90% of all packet traffic in the internet. TCP is typically used for applications and their respective protocols where reliability is important, e.g. web browsing (HTTP), Email (SMTP) and file transfer (FTP). Due to its acknowledgement mechanism, a single TCP connection is rate limited by the ratio between the TCP window size and the RTT. Given a standard 64KB TCP receive window size (RFC793), 20ms RTT would limit the achievable service data rate at ~25Mbit/s for a single TCP connection in the steady state (TCP flow control). This limitation could be mitigated with multiple concurrent TCP connections (typically the case for web browsing) and/or enlarging the receive window through TCP Window Scaling (RFC1323) which is supported by many web servers today. In particular large file transfers (music, video, SW download) will benefit from these measures, if high service rates would be offered by the operator.

20

25

Figure 17 TCP data rate in steady state vs. RTT (without Window Scaling)

Apart from the steady state, also TCP connection start-up (TCP slow start) and behavior after packet loss (TCP congestion avoidance) is affected by the RTT. With todays browsers and web sites (average page size


3. MOBILE BACKHAUL ARCHITECTURE FOR LTE

3.1 X2 Connectivity Requirements

Mobile networks are commonly structured with respect to user densities (metropolitan areas vs. rural areas), topological requirements (highways, railways, etc.) and capacities of network elements. Respectively, eNBs should be assigned to groups (clusters) where the handover probability between eNBs belonging to the same group is high and the handover probability between eNBs belonging to the different groups is zero or low. Since all LTE protocols are built upon IP, all network elements which need to communicate with each other must be IP reachable (logical view). This includes in particular eNBs within the same group which have to be able to reach their adjacent eNBs via the (logical) X2 interface (Figure 18).

BackboneNetwork

Mobile BackhaulNetwork

eNB GroupeNB 11

eNB 12

eNBs withhigh HO

probabilitywithin one

group

eNB 21

eNB 22

Small or no HO probabilitybetween eNB

groups

X2-u/c

S1-u/c

Edge router

MME

SAE-GW

Figure 18 End-to-end architecture and connectivity

X2 handover principles are explained in chapter 2.3. There is some common misconception with respect to the requirements on the transport network, which shall be analyzed here. At a first glance, it seems obvious that it would be beneficial if the X2 turning point would be located close to the base stations (Near-End X2 connectivity, Figure 19). This can be implemented with Ethernet switching or IP routing functions, which implies additional CAPEX and OPEX in many cases. X2 latency would be low and X2 traffic wouldnt load higher parts of the Mobile Backhaul Network. On the other hand, the S1 transport path should be optimized for low latency anyhow (see chapter 2.4.2). X2 latency significantly less than radio link interruption time (30...50ms) doesnt add value. Furthermore, the amount of X2 traffic is marginal compared to S1 traffic, so the potential for savings is very limited.


Mobile Backhaul Network Backbone NetworkeNB

eNB

MME SAE-GW

X2-u/c S1-u/c

Figure 19 Near-End X2 connectivity

Far-End X2 connectivity (Figure 20), on the other hand, can be built on existing topologies (hub & spoke). It requires IP routing at the Far-End (Edge Router or Security Gateway) in order to limit the size of L2 broadcast domains. Also this architecture meets the latency and capacity requirements analyzed earlier.

Mobile Backhaul Network Backbone NetworkeNB

eNB

MME SAE-GW

X2-u/c S1-u/c

Figure 20 Far-End X2 connectivity

It can be concluded that Near-End X2 connectivity is not advantageous for 3GPP Rel.8/9/10 LTE network architectures. In particular, direct physical connections between adjacent eNBs (fully meshed network) are not required. This conclusion has to be reviewed again if functional requirements change with the architectural evolution. This might be the case when inter-site Cooperative Multipoint (CoMP) technology will be introduced (not expected before 3GPP Rel.11).


3.2 Implementation Options

3.2.1 L2 vs. L3

There is a common misconception that the mobile backhaul network for LTE requires an IP router at every transport node. As a matter of fact, most LTE backbone networks will be IP/MPLS based. The borderline between backbone network and Mobile Backhaul Network, implemented by an Edge Router, deserves further consideration with respect to network security (Security Gateway, see chapter 7). If IPsec is applied, the Security Gateway (SEG) terminates the IP layer with the eNB at the other end, thus would be a natural termination point of the Mobile Backhaul Network.

AggregationNetwork

Mobile Backhaul Network

BackboneNetwork

eNB

eNBMME SAE-

GW

L2 or L3 L3

eNB

eNBMME SAE-

GW

AccessNetwork

L2

Figure 21 L2 vs. L3

Since the eNB supports an IP-over-Ethernet interface, both Ethernet (L2 VPN) and IP (L3 VPN) transport services would be adequate in the Mobile Backhaul Network. In the following subchapter, an Ethernet based backhaul network implementation option shall be elaborated further.


3.2.2 Ethernet Based Backhaul Network

An Ethernet transport service can be delivered by the mobile operator itself (self-built transport network) or by another operator (a.k.a. leased line service). In both cases, service attributes and parameters are usually stated in a Service Level Specification (SLS), which in the latter case is part of a Service Level Agreement (SLA). While this makes a big difference with respect to the operator business case, it will be treated here conceptually as equal. The following Ethernet services types will be distinguished as defined by Metro Ethernet Forum [MEF6.1]:

E-Line E-LAN E-Tree


3.2.2.1 E-Line

The mobile backhaul network can be purely based on L2 (Ethernet) transport. Point-to-point Ethernet Virtual Connections (EVC) between a User Network Interface (UNI) at the eNBs and another UNI at the (redundant) edge router are the straightforward solution. EVC attributes can be well controlled (see [MEF10.1]) and commercial services are commercially available in many markets, known as E-Line services (Ethernet leased lines). At the UNI between Mobile Backhaul and Backbone Network, the VLAN ID identifies a specific eNB. The Edge Router (or Security Gateway) performs routing of S1 traffic between eNBs and the core nodes as well as routing of X2 traffic between eNBs within one group and between eNBs belonging to different groups.

eNB GroupeNB 11

eNB 12

eNB 21

eNB 22

UNI

UNI

UNI

UNI

EVC 11 (E-Line)

EVC 12 (E-Line)

EVC 21 (E-Line)

EVC 22 (E-Line)

UNI

UNI

UNI

UNIFar-End X2 routing

between eNBs within one group and

between eNB groups

Figure 22 L2 mobile backhaul network with E-Line

Advantages Security (impact of DoS attacks is limited to one eNB) QoS assurance (simple traffic engineering) Operability (simple troubleshooting)

Disadvantages Scalability (high number of EVCs)


3.2.2.2 E-LAN

As an alternative, the Mobile Backhaul Network could be built based on multipoint-to-multipoint EVCs (E-LAN service). Most appropriately, eNBs belonging to one group should be assigned to one E-LAN (Figure 23). At the UNI between Mobile Backhaul and Backbone Network, the VLAN ID identifies an eNB group. Inside the eNB group, an eNB is identified by its MAC address. The Edge Router (or Security Gateway) performs routing of S1 traffic between eNBs and the core nodes as well as routing of X2 traffic between eNBs belonging to different groups, but traffic between eNBs within one group will be switched inside the (virtual) LAN.

eNB GroupeNB 11

eNB 12

eNB 21

eNB 22

UNI

UNI

UNI

UNI

UNIEVC 1 (E-LAN)

EVC 2 (E-LAN) UNI

Far-End X2 routing

between eNBgroups

Near-End X2 switching

between eNBsof one group

Figure 23 L2 mobile backhaul network with E-LAN

Advantages Scalability (low number of EVCs)

Disadvantages Security (adjacent eNBs can be attacked) QoS assurance (traffic engineering is more difficult) Operability (troubleshooting is more complex)


3.2.2.3 E-Tree

Using an E-Tree service with its root at the Edge Router means in a way a restricted E-LAN service, where inter-eNB connectivity would not be possible. So, X2 traffic would go through the Edge Router. From that perspective, it corresponds to the E-Line service.

eNB GroupeNB 11

eNB 12

eNB 21

eNB 22

UNI

UNI

UNI

UNI

EVC 2 (E-Tree)

EVC 1 (E-Tree)UNI

UNI

Far-End X2 routing between eNBs within

one group and between eNB groups

Figure 24 L2 mobile backhaul network with E-Tree

Advantages Scalability (low number of EVCs) Security (impact of DoS attacks is limited to one eNB)


3.2.2.4 Comparison

In the table below, E-Line, E-Tree and E-LAN services are compared.

Advantages Disadvantages QoS assurance (simple traffic engineering) Operability (simple troubleshooting)

E-Line

Security (impact of DoS attacks is limited to one eNB)

E-Tree

Scalability (high number of EVCs)

Security (adjacent eNBs can be attacked) QoS assurance (Traffic engineering is more difficult)

E-LAN Scalability (low number of EVCs)

Operability (Troubleshooting is more complex)

Table 2 Comparison of Ethernet Services

In summary, If physical security is considered not sufficient

o E-Line or E-Tree type of Ethernet service are most suitable. o E-LAN: The scalability advantage has to be balanced against the security disadvantage.

If physical security is considered sufficient o E-Line or E-LAN type of Ethernet service is recommended. o E-Tree is possible but would require configuration of host routes for each eNB at the Edge

Router.


3.2.3 Implementation Examples

Figure 25 below illustrates the physical view where the Mobile Backhaul Network is further sub-divided into the Access Network and the Aggregation Network. In many cases, the Access Network is built upon a microwave radio infrastructure, whereas the Aggregation Network uses fiber rings. The Access Network may be purely implemented on L2 (Ethernet). For scalability reasons (limiting broadcast domains), the network should be partitioned into different VLANs where bridging takes place at intermediate nodes (e.g. microwave radio hubs). The Aggregation Network could also be built upon L2 (Figure 25). As one solution, VPLS-TE with one VPLS instance per VLAN can be provisioned. The logical point-to-point structure allows for sophisticated traffic engineering while high availability can be supported through physical ring or mesh topologies. This concept is also known as Multi-Layer Optimization (MLO). It aims at saving network CAPEX and OPEX through optimized application of different types of network elements, working on L1, L2 or L3 at different locations in the network topology. NSN Carrier Ethernet Transport (CET) supports this approach completely.

BackboneNetwork

Access Network AggregationNetwork

eNB 11

eNB 12

eNB 21

eNB 22

Carrier Ethernet Transport

MME

SAE-GW

Ethernet switch at MWR hub

location

Fiber connection

L2 Access & Aggregation

IP/MPLS

L3 Backbone

Figure 25 Implementation example: Carrier Ethernet Transport for Access & Aggregation, IP/MPLS for Backbone


Alternatively, the Aggregation Network could be built upon L3 (Figure 26). It should be noted, however, that the IP layers in the Aggregation and Backbone Network will be separated by a Security Gateway in case IPsec is used.

BackboneNetwork

Access Network AggregationNetwork

eNB 11

eNB 12

eNB 21

eNB 22

CET

MME

SAE-GW

Ethernet switch at MWR hub

location

Fiber connection

IP/MPLS

L2 Access L3 Aggregation L3 Backbone

Figure 26 Implementation example: Carrier Ethernet Transport for Access, IP/MPLS for Aggregation & Backbone

If an IP router is present at the base station site it can be used to provide routed connections between the eNB and the core nodes as well as between neighboring eNBs for X2 traffic. Alternatively, the router can be used for providing VPLS services. In this case the topology from the eNB point of view is similar to the E-Line architecture presented earlier.


3.3 Transport Service Attributes

Transport service attributes are used to describe the requirements on the transport service for eNB backhaul. It should be noted that these attributes are defined end-to-end (between eNB and core network elements) on the IP layer (L3) and have to be allocated in general to multiple parts of the network. In particular, if the transport service is (partially) provided by Ethernet Virtual Connections (EVC), these attributes have to be translated into their L2 equivalents. Ethernet service attributes are defined by Metro Ethernet Forum [MEF6.1]. Note that these attributes are optional and a subset can be fully sufficient to define an LTE transport service.

3.3.1 Capacity

Ethernet capacity requirements are described by the service attributes Committed Information Rate (CIR) / Committed Burst Size (CBS) and Excess Information Rate (EIR) / Excess Burst Size (EBS) (optional).

Dimensioning of eNB backhaul capacity (last mile) is described in chapter 2.4.1.

3.3.2 Performance

As explained in chapter 2.4, performance requirements on the transport service are primarily driven by user service aspects.

NSN recommends the following performance values:

Service Attribute Value Comments L3: Packet Delay (PD) L2: Frame Delay (FD)

10 ms TCP based applications require low transport latency

L3: Packet Delay Variation (PDV) L2: Frame Delay Variation

+/- 5.0 ms If Timing-over-Packet (ToP) based on IEEE1588-2008) is applied (could be relaxed otherwise)

L3: Packet Loss Ratio (PLR) L2: Frame Loss Ratio

10e-4 (0.01%)

May be different depending on user service

Table 3 Transport Service Attributes (Recommendation)


3.4 Traffic Differentiation with VLAN and IP Addressing

LTE132 VLAN based traffic differentiation (RL10, RL15TD) LTE875 Different IP addresses for U/C/M/S-plane (RL10, RL15TD)

The traffic differentiation capability is tightly coupled with VLAN support and IP addressing. So, these topics will be handled here together. Traffic differentiation can be used for various purposes. First, there may be a need to separate traffic of different planes (eNB applications) from each other for network planning reasons. E.g. M-plane traffic may need to be separated from U/C-plane traffic. S-plane traffic may require further separation. In another network scenario, traffic may be split into multiple paths which have different QoS capabilities (path selection). The destination IP address is used as differentiation criteria. Traffic differentiation could be performed on multiple layers:

L3: Using different IP subnets L2: Using different VLANs (Note: Each VLAN is terminated with a dedicated IP address at the

eNB) L1: Using different physical paths (Note: External equipment required. Support for multiple

physical interfaces is an RL30 study item)

First, the generic eNB IP addressing model shall be explained. Based on that, typical network scenarios will be elaborated.

3.4.1 Generic eNB IP Addressing Model

The model is based on two basic components: 1. Binding of eNB applications to IP addresses 2. Assignment of interface IP addresses to eNB interfaces


Binding of eNB applications to IP addresses eNB applications (S1/X2 U-plane, S1/X2 C-plane, M-plane, S-plane) may be arbitrarily bound to

eNB interface address(es) or eNB virtual address(es)

as illustrated in Figure 27 below. Interface addresses are directly visible at eNB interfaces, whereas virtual addresses appear behind an eNB internal IP router.

S1/X2 U-plane application

S1/X2 C-plane application

S-plane application

M-plane application

U

C

M

S

Binding to virtual address

Binding to interface address

Virtual IP address

eNB

Interface IP address

Figure 27 Binding of eNB applications to IP addresses

Configurations where eNB applications are bound to virtual addresses are typically used in scenarios where the transport link (VLAN, IPsec tunnel) shall be terminated with one IP address (interface IP address) while application separation on L3 is also required. Address sharing, i.e. configuration with the same IP address, is possible. In the simplest configuration, the eNB features a single IP address for U-plane, C-plane, M-plane and S-plane.


Assignment of interface IP addresses to eNB interfaces eNB interface IP address(es) may be assigned to different types of data link layer interfaces, which are provided by

physical interface(s) or logical interface(s)

as illustrated in Figure 28. A physical interface is provided by an Ethernet port, whereas a logical interface is provided by a VLAN termination. RL10 supports one physical interface and max 4 logical interfaces. Different interfaces belong to different IP subnets.

Physical interface

(Ethernet)

VLAN(optional)

Interface address assigned to physical

interfaces

eNBInterface addresses assigned to logical

interfaces

eNB

VLAN2

VLAN3

VLAN4

VLAN1

Physical interface

(Ethernet)Logical

interface (VLAN)

Figure 28 Assignment of interface IP addresses to eNB interfaces


Figure 29 and Figure 30 below illustrate the configuration flexibility.

Application(s) bound to interface address Application(s) bound to virtual address(es)

VLAN(optional)

U

C

M

S

No or single VLAN per eNB

VLAN2

VLAN3

VLAN4

VLAN1

M

S

U

C

Multiple VLANsper eNB

VLAN(optional)

No or single VLAN per eNB

U

C

M

S

Figure 29 IP addressing with VLAN examples (RL10)

Application(s) bound to virtual address(es)

VLAN2

VLAN3

VLAN4

VLAN1

Multiple VLANsper eNB

U

C

M

S

Figure 30 IP addressing with VLAN examples (RL20)


3.4.2 Network Reference Configurations

The figures on the next pages illustrate the eNB addressing options with some typical network scenarios (Network Reference Configurations). Those are different with respect to the underlying transport services. Ethernet transport services may be implemented as E-Line or E-LAN. As per MEF definition, E-Line denotes an Ethernet transport service based on point-to-point Ethernet Virtual Connection (EVC), whereas E-LAN denotes an Ethernet transport service based on multipoint-to-multipoint Ethernet Virtual Connection (EVC). The IP address concept is also related to the IPsec architecture (see chapter 7). Configurations where eNB applications are bound to virtual addresses are typically used in a scenario where the IP transport layer is separately administered (IPsec VPN). In that case, the IPsec tunnel is terminated with separate interface addresses at the eNB and SEG (Figure 34).

E-Line with single IP Address Simplest configuration for E-Line service 1 VLAN per eNB

VLAN(optional)

M

S

U

C

Ethernet

eNB

eNB

SAE-GW

~

ToPMaster

O&M

IP Router

MME

Figure 31 E-Line with single IP address


E-LAN with single IP Address Simplest configuration for E-LAN service 1 VLAN per eNB group

VLAN(optional)

M

S

U

C

eNB

Ethernet

eNB

SAE-GW

~

ToPMaster

O&M

IP Router

MME

Figure 32 E-LAN with single IP address

E-Line with L2 Differentiation Supporting virtually separate networks for different planes 2 VLANs per eNB (max 4 VLANs possible)

VLAN2

VLAN1

M

S

U

C

Example configuration

eNB

eNB

Ethernet

SAE-GW

~

ToPMaster

O&M

MME

IP Router

EVC/VLAN 11, 12

EVC/VLAN 21, 22

Figure 33 E-Line with L2 differentiation

L2 differentiation can be supported in the same way with E-LAN.


IPsec VPN with L3 separation Single interface IP address for IPsec tunnel termination

IPsec Tunnel

S VLAN

InterfaceIP address

Application IP address

U

C

M

IP

eNB

eNB

MME

SEG

SAE-GW

~

ToPMaster

O&M

IPsec VPN

Ethe

rnet

IP Router

Figure 34 IPsec VPN (separate interface IP address)

S-plane binding to interface address is recommended to avoid additional jitter due to IPsec processing. It is required for on-path support to enable accurate phase synchronization (see chapter 6.2.1).


3.5 Transport Redundancy

3.5.1 General

Transport redundancy can be applied on different layers. As a basic principle, if multiple redundancy features are present, the redundancy mechanism on the lower layer needs to be faster than another mechanism on an upper layer.

Layer 1 (physical layer) based redundancy An example is 1+1 link redundancy with MWR equipment. L1 based redundancy doesnt require any specific support from Flexi Multiradio BTS.

Layer 2 (data link layer) based redundancy Ethernet switching procedures apply in case of L1 failure. One of the following concepts may be utilized:

Rapid Spanning Tree Protocol (RSTP), Multiple Spanning Tree Protocol (MSTP) IEEE802.3ad (Ethernet Link Aggregation) ITU-T G.8031 (Ethernet Protection Switching) ITU-T G.8032 (Ethernet Ring Protection) Ethernet APS (RFC3619)

L2 based redundancy is a study item for RL30.

Layer 3 (network layer) based redundancy IP re-routing, optionally supported by a routing protocol such as OSPF, applies in case of L2 link failure. IP based load balancing mechanisms, e.g. Equal Cost Multi-Path (ECMP) routing, could be utilized if supported by a corresponding edge router. L3 based redundancy is a study item for RL30.

Layer 4 (transport layer) based redundancy SCTP multi-homing for the C-plane is supported with RL20. See chapter 3.5.2 below.

Layer 7 (application layer) based redundancy Examples are:

U-plane: Multiple S-GWs available per eNB C-plane: Multiple MMEs available per eNB (S1 Flex) S-plane: Multiple clock sources available per eNB, e.g.

o Multiple IEEE1588 (ToP) Grandmasters o Multiple Ethernet links supporting SyncE o Simultaneous provision of multiple synchronization technologies (e.g. GPS and ToP)


3.5.2 SCTP Multi-homing

LTE775 SCTP multi-homing (MME) (RL20, RL15TD)

Multi-homed nodes can be reached under several IP addresses. With this feature, the eNB supports two IP addresses at the MME for S1 C-plane termination. SCTP (RFC4960) supports multi-homing for failover (no load-sharing). Associations become tolerant against physical interface and network failures. At the set-up of an SCTP association, one of the several destination MME IP addresses is selected as initial primary path. Data is transmitted over this primary transmission path by default. Retransmissions happen on other available paths. Path status and reachability is monitored at the SCTP layer (via data chunks and heartbeats). The feature is primarily meant for MME interface redundancy, not path redundancy. So, C-plane termination with a single IP address at the eNB is sufficient.

eNB

IP

Dual interfaceDual IP address

MME

Single interfaceSingle IP address

Figure 35 SCTP Multi-homing


3.6 Base Station Co-location

LTE649 QoS aware Ethernet switching (RL20, RL15TD)

At least initially, existing (macro) 2G/3G base stations sites will be reused for LTE in order to keep deployment costs reasonably low. As a consequence, there is a clear demand for 2G/3G/LTE common backhaul solutions. The most straightforward way is to upgrade legacy base stations with an Ethernet backhaul option and aggregate the traffic using an Ethernet switch. With the eNB integrated Ethernet switching function, Flexi Multiradio BTS eliminates the need for an external device. FTLB supports three Ethernet interfaces, while two Ethernet interfaces are usable with FTIB. The aggregated Ethernet traffic is shaped according to a configurable level with respect to the available uplink capacity.

eNB

UNI(1)

UNI(2)

UNI

QoS awareshaping

Ethernetintegrated Ethernet switch

highlow

other base station

EVChigh

high

lowlow

highlow

Figure 36 QoS aware Ethernet switching

Synchronization solutions with co-located base stations are described in chapter 6.3

If a co-located 2G base station doesnt support Ethernet, the Flexi Multiradio BTS integrated CESoPSN feature can be applied (RL30 study item).


4. LTE TRANSPORT INTERFACES AND PROTOCOLS

4.1 Ethernet

LTE118 Fast Ethernet (FE) / Gigabit Ethernet (GE) electrical interface (RL09, RL05TD) LTE119 Gigabit Ethernet (GE) optical interface (RL09, RL05TD) LTE491 FlexiPacket Radio Connectivity (RL20, RL15TD)

Ethernet based on the [IEEE 802.3] standard will be the main eNB backhaul technology. In many cases, a network terminating device (leased line termination equipment, MWR IDU, xDSL CPE, etc.) is present at the eNB site. Electrical connection is most economical in that case which connects to the eNB via Ethernet. Flexi Multiradio BTS supports suitable indoor and outdoor site solutions. No such device is needed in case FlexiPacket Radio is used. This MWR ODU can be directly connected and managed through the Ethernet interface of the Flexi Multiradio BTS (zero footprint" solution). Also, if fiber access is available at the site, it can be connected directly. The following interface types are supported (see chapter 9 for details of different Flexi Transport sub-modules):

Gigabit Ethernet (GE) 100/1000Base-T, electrical Gigabit Ethernet (GE) 1000Base-SX/LX/ZX through optional SFP module, optical

The Ethernet interfaces, based on the [IEEE 802.3] standard (with type interpretation of the type length field, Ethernet II/DIX frame), support the following features:

Full-duplex transmission mode Auto-negotiation and forced mode for the duplex mode (only full duplex advertised) Auto-negotiation and forced mode for the data rate Auto-negotiation and forced mode for MDI/MDIX VLAN tagging according to [IEEE 802.1q] Ethernet priority bits (p-bits) according to [IEEE 802.1p] IPv4 over Ethernet [RFC894] Address Resolution Protocol [RFC826] (ARP) MEF UNI Type 1 [MEF11]

It should be noted that U-plane IP packets may exceed the maximum SDU size of the Ethernet backhaul link due to additional packet overhead (GTP-U, UDP, IP/IPsec). See chapter 4.2.2.


4.2 IP

LTE664 LTE transport protocol stack (RL09, RL05TD)

4.2.1 General

All eNB applications (U/C/M/S-plane) are running on top of IP. Protocol stacks are explained in chapter 2.2. User applications are hooked upon the user IP layer which is independent from the LTE application IP layer between eNB and SAE-GW. GTP-U is used as a tunneling mechanism in between. This traffic may optionally be put into yet another tunnel if IPsec (tunnel mode) is used for transport security purposes (see chapter 7). So, there may be up to three IP layers which are all independent from each other (Figure 37):

User IP layer, terminated by user application at the UE and corresponding peer LTE application IP layer, terminated by LTE/SAE network element applications IPsec tunnel IP layer (optional), terminated by SEG (integrated in the eNB, external device at the

other end) The eNB terminates the LTE application IP layer (and optionally the IPsec tunnel IP layer). Thus, the U/C/M/S-plane address (application address) type is in principle independent from the IPsec tunnel termination address (interface address). Note: The term Transport IP layer is ambiguous. It refers to the IP layer used by the transport network. If IPsec is not used, this would coincide with the LTE application IP layer; with IPsec it would coincide with the IPsec tunnel IP layer.

FlexiMultiradio

BTS

Eth MAC

Eth PHY

UDP

G TP-U

SAE-GW ServerInternet Operator

Services

Eth MAC

Eth PHY

User App

Eth MAC

Eth PHY

UDP

G TP-U

LTE PHY

User App

LTE PHY

UE

LTE MAC

PDC P

Eth MAC

Eth PHY

Security GW(optional)

Eth MAC

Eth PHY

Eth MAC

Eth PHY

LTE MAC

PDC P

IP

IP

IP

User IPUser IP

IPsec Tunnel IP

LTE App IP LTE App IP

IPsec Tunnel IP

Figure 37 IP layers


4.2.2 MTU Size Issues

Both GTP/UDP/IP and (optional) IPsec tunneling add overhead to the user IP packet. If this has been generated with an MTU size of 1500 bytes, corresponding to the maximum SDU size of a standard Ethernet frame between Internet routers, it will not fit to the Ethernet interface SDU of the mobile backhaul network. In principle, there are three ways to solve this problem:

1. Enforcement of lower MTU size at the user IP layer 2. IP fragmentation & reassembly at the LTE application IP layer 3. Enlargement of Ethernet SDU using Jumbo Frames under the LTE application IP layer (or IPsec

tunnel IP layer, respectively)

All solutions have their pros and cons. Lower MTU size cannot be enforced in all circumstances. Ethernet Jumbo Frames would solve the problem nicely but are not available everywhere. IP fragmentation & reassembly would always work but has performance issues. In the following, the options will be elaborated more in detail.


4.2.2.1 Enforcement of lower MTU size

The MTU size used for packets at the user IP layer can be determined through static or dynamic configuration of the IP protocol stack SW of the UE. Unfortunately, this approach will not solve the problem completely. With one approach, the MTU size of the IP protocol stack in the UE could be configured to a lower value. There are UE vendor specific settings (e.g. 1400 bytes in Symbian) which support this idea, but this is not mandated by any standard. In case of IPv6, the MTU size at the UE could be set with the Router Advertisement message that the PDN-GW sends after PDN connection establishment ([RFC4861], section 4.6.4). Alternatively, the user IP layer MTU could also be enforced through Path MTU discovery with a lower MTU properly set in the PDN-GW. This approach has been adopted with [TS29.275 v8.4.0] where a default MTU of 1280 bytes is proposed (equivalent to the minimum MTU size defined for IPv6). Unfortunately, Path MTU discovery (IPv4: RFC1191, MTU 68, IPv6: RFC1981, MTU 1280) is not mandated for neither IPv4 nor IPv6 (though strongly recommended). Its usually performed for TCP/SCTP, but not for UDP based applications. The impact of a lower MTU size on the transport efficiency is negligible (


4.2.2.2 IP Fragmentation and Reassembly

Without using IPsec, Fragmentation and Reassembly of LTE application IP layer packets is terminated at the eNB at one end and the SAE-GW at the other end. With IPsec, the same approach should be applied in order to avoid multiple fragmentation. In this case IPsec packets are carrying fragments, but are not fragmented themselves. The MTU size (S1-GW_MTU) would need to be set up to a maximum of 1438 bytes in order not to exceed a value of 1500 for the Ethernet SDU at the eNB (example with IPv4 S1-U over IPv4 IPsec tunnel, see Figure 39). While this is a well standardized and generic solution, there are potential drawbacks, such as additional processing delay and throughput degradation due to processing load at the end points. Anyway, the performance impact will be limited due to the countermeasures taken in the user IP layer as described above. The impact on transport efficiency is minor (~2.5%). Large packets (~25%) will be fragmented, i.e. cause double transport overhead (~10% per fragment).

GTP-UUDP

168 1394

ESPIPsecTunnel

IP

2020

User IP Payload PL/NH AuthPaddingRAN

IP

24 0 2 12

1496 1500

90 x 16 = 1440

GTP-UUDP

168 150020

User IP PayloadRANIP

1544

GTP-UUDP

168 139420


1438(1st fragment)

1438S-GW_MTU1500eNB_MTU1500UE_MTU

eNB_MTU S-GW_MTU

SAE-GWSEGeNB

UE_MTU

Figure 39 Fragmentation & Reassembly with IPv4

Flexi Multiradio BTS supports Fragmentation and Reassembly at the LTE application IP layer.


4.2.2.3 Ethernet Jumbo Frames

This solution has no impact on user IP layer and a slightly higher transport efficiency. On the other hand, it is not standardized (vendor specific SDU sizes up to 9000 bytes are possible) and therefore not generally supported by Ethernet equipment and Ethernet transport services.

1544S-GW_MTU1608eNB_MTU1500UE_MTU

GTP-UUDP

168 1500

ESPIPsec

TunnelIP

2020

User IP Payload PL/NH AuthPaddingRAN

IP

24 6 2 12

1608

97 x 16 = 1552

GTP-UUDP

168 150020


1544

eNB_MTU S-GW_MTU

UE_MTU

eNB SAE-GWSEG

Figure 40 Ethernet Jumbo Frames with IPv4

Support for Ethernet Jumbo Frames is a study item for RL30.


4.2.3 IPv6

The GTP-U tunneling concept allows for using private IP addresses in the LTE transport network (eNB backhaul) while user applications require a public IP address. Instead of assigning a public address directly to the UE, mobile operators often use NAPT for saving public IPv4 addresses. There are more than 16M private IPv4 addresses available which are even reusable in distinct parts of the operator network if routing in between is not needed. This is particularly the case with respect to the transport network (LTE application IP layer, optionally IPsec tunnel IP layer). Nevertheless, some operators claim to face a shortage of private IP addresses already now. Migration to IPv6 in the transport network is seen as a solution. Sooner or later, IPv6 in the transport network will become an operational cost optimization topic for network elements that anyway have to have IPv6 addresses due to support for IPv6 in the User IP layer (PDN-GW). It can be expected that these network elements would be simpler to manage if the number of IPv4 addresses is minimized. The network elements are typically connected to a separate O&M network which is logically or even physically separated. Private IPv4 addressing can be used here and there is no technical reason to transport O&M traffic over IPv6. Migration to IPv6 is a major step since several O&M system components (incl. NetAct, iOMS, DHCP server, CMP server, etc.) are typically also used for legacy mobile networks. Anyway, IPv6 for M-plane transport will probably become an operational cost optimization topic later when all other networks are running on IPv6. However, NetAct will support IPv6 in LTE M-plane applications, i.e. where IP addresses are handled (NetAct Top level UI, Object browser, NetAct Configurator, NetAct topology database, etc.). QoS (DiffServ) and security (IPsec) mechanisms are basically the same for IPv4 and IPv6. In the following the relevant scenarios will be analyzed.

User IPv4 over LTE transport IPv4 This is the basic scenario, supported already with RL09 (IPsec option with RL10).

FlexiMultiradio

BTS

Eth MAC

Eth PHY

UDP

G TP-U

SAE-GW Server

Eth MAC

Eth PHY

User IP

User App

Eth MAC

Eth PHY

UDP

G TP-U

LTE PHY

User App

LTE PHY

User IP

UE

LTE MAC

PDC P

Eth MAC

Eth PHY


Eth MAC

Eth PHY

Eth MAC

Eth PHY

LTE MAC

PDC P IPv4

IPv4

IPv4

IPsec Tunnel IP


IPsec Tunnel IP

Figure 41 User IPv4 over LTE transport IPv4


User IPv4/IPv6 over LTE transport IPv4 Some user services are running on IPv6. This scenario is completely transparent to the eNB, thus also available with RL09 (if supported by the SAE-GW).

FlexiMultiradio

BTS

Eth MAC

Eth PHY

UDP

G TP-U

SAE-GW Server

Eth MAC

Eth PHY

User App

Eth MAC

Eth PHY

UDP

G TP-U

LTE PHY

User App

LTE PHY

UE

LTE MAC

PDC P

Eth MAC

Eth PHY


Eth MAC

Eth PHY

Eth MAC

Eth PHY

LTE MAC

PDC P IPv4

IPv4

IPv4/IPv6 User IPUser IP

IPsec Tunnel IP


IPsec Tunnel IP

Figure 42 User IPv4/IPv6 over LTE transport IPv4

User IPv4/IPv6 over LTE transport IPv4/IPv6 The transport network is (partially) based on IPv6 (RL30 study item). If both LTE application IP layer and IPsec tunnel IP layer are owned by the mobile operator, it is possible to use the same address for tunnel termination(s) and LTE applications (binding to interface addresses, collapsed inner and outer addresses, see chapter 3.4.1 and 7.4) in order to save IP addresses and reduce complexity.

FlexiMultiradio

BTS

Eth MAC

Eth PHY

UDP

G TP-U

SAE-GW Server

Eth MAC

Eth PHY

User App

Eth MAC

Eth PHY

UDP

G TP-U

LTE PHY

User App

LTE PHY

UE

LTE MAC

PDC P

Eth MAC

Eth PHY


Eth MAC

Eth PHY

Eth MAC

Eth PHY

LTE MAC

PDC P IPv4/IPv6

IPv4/IPv6

IPv4/IPv6 User IPUser IP

IPsec Tunnel IP


IPsec Tunnel IP

Figure 43 User IPv4/IPv6 over LTE transport IPv4/IPv6


5. QOS

5.1 End-to-End QoS

In order to facilitate a certain end-to-end QoS for a user service, radio and transport QoS have to be considered consistently. Radio interface resources are scarce, so strict control has to be applied. This means, capacity has to be reserved for Guaranteed Bit Rate (GBR) type of services through Connection Admission Control (CAC). Controlled overbooking with prioritization is the solution for non-GBR type of services. The resource situation in the access section (last mile) of the mobile backhaul network is similar, so the solution principles are similar: Committed Information Rate (CIR) guarantees continuation of GBR services, whereas Extended Information Rate (EIR) with overbooking and prioritization can be applied for non-GBR traffic. Nevertheless, it is recommended to reserve also a minimum capacity using CIR for non-GBR traffic. In the aggregation section of the network the situation is more relaxed, so less strict control is required. Still, there might be a need for capacity reservation and traffic prioritization. If sufficient resources are available, this section may be ignored by the CAC. The mobile network up to the SAE-GW is operator controlled, so with proper network dimensioning the service quality can be guaranteed. Naturally, CAC makes most sense here. This is not the case if the actual user service is provided by a server located anywhere in the Internet. Only Best Effort can be expected. If the user service is based on TCP (8090% of the traffic), TCP congestion control mechanisms will assure a fair distribution of capacity.

eNB

EthernetSAE-GW Internet Server

IPIP Router

InternetAggregationCore

Access

Best effort

Guaranteed

Serv

ice

qualit

y

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IP

No control Scarce resources Strict control (RRM)

Reservation for GBR Controlled overbooking

with prioritization for non-GBR

Scarce resources Strict control (Transport)

Reservation (CIR) Controlled overbooking with

prioritization (802.1p)

Fair resources Less strict control

Reservation (MPLS TE) Overbooking with

prioritization (DiffServ)

UE

Radio & Transport CAC(CIR for GBR, EIR for non-GBR)

Radio scheduling per UE (UL+DL) Transport UL policing and

shaping per eNB DL policing and shaping

per UEQoS

featu

res

Flow & congestion control (TCP)

Traffic marking Flow &

congestion control (TCP)

DL policing and shaping per eNB

Figure 44 End-to-End QoS


5.2 Transport QoS Features

5.2.1 Traffic Prioritization on IP Layer

LTE131 Traffic prioritization on IP layer (RL10, RL05TD)

In order to avoid over-dimensioning in the backhaul network, Flexi Multiradio BTS supports QoS differentiation between user, control and management plane traffic as outlined in Figure 45. DiffServ is the most common way of traffic prioritization on IP layer [RFC2474], [RFC2475]. Traffic end points (eNB, S-GW, MME, O&M/NetAct) will set DSCP values, while intermediate network elements have to handle the packets accordingly. DSCP values for different U/C/M-plane traffic types are configurable.

Flexi Multiradio

BTS

Eth

L2

L1

Eth MAC

Eth PHY

UDP

IP

L2

L1

L2

L1

IP

BTSOM

SCTP

S1AP / X2AP

M-planeC-plane

GTP-U

(User) IP

U-plane

TCP

IP

Eth MAC

Eth PHY

IP

Eth MAC

Eth PHY

IP

UDP

GTP-U

(User) IP

M-planeC-planeU-plane

BTSOM

TCPSCTP

S1AP

IP

IEEE1588

UDP

S-plane

Eth MAC

Eth PHY

IP

IEEE1588

UDP

S-planeQoS marking(DSCP)

NetActSAE-GW

MME

IP Router(Security GW)

~

ToP Server

DSCP aware

Figure 45 Traffic prioritization on IP layer


5.2.2 Traffic Prioritization on Ethernet Layer

LTE129 Traffic prioritization on Ethernet layer (RL10, RL15TD)

If traffic aggregation is performed by Ethernet switching, not IP routing, the transport network may not be IP QoS (DiffServ) aware. In this case, Flexi Multiradio BTS supports traffic prioritization on the Ethernet layer, using frame marking methods (code points) applicable to Ethernet as illustrated in Figure 46. Ethernet priority bits (IEEE 802.1p) are set based on the DiffServ Code Point (DSCP). The mapping table is configurable.

Flexi Multiradio

BTS

Eth

Eth PHYEth PHY

Ethernet

NetAct

Ethermet Switching

SAE-GW

MME

IP Router(Security GW)

UDP

IP

L2

L1

BTSOM

SCTP

S1AP / X2AP

M-planeC-plane

GTP-U

(User) IP

U-plane

TCP

IEEE1588

UDP

S-plane

802.1p marking

Eth MAC

Eth PHY

IP

Eth MAC

Eth PHY

IP

Eth MAC

Eth PHY

IP

UDP

GTP-U

(User) IP

M-planeC-planeU-plane

BTSOM

TCPSCTP

S1AP

Eth MAC

Eth PHY

IP

IEEE1588

UDP

S-plane

~

ToP Ser

152880920 lte transport tutorial

Documents

lte basics

lte handover

network architecture

delay latency

user plane

control plane

delay variation jitter

x2 connectivity requirements