understanding and managing ip/mpls mobile backbone · pdf fileunderstanding and managing...

WHITE PAPER
Understanding and Managing IP/MPLS Mobile Backbone and Backhaul Networks

Copyright 2015, Packet DesignPage 2 of 18
Table of Contents
Introduction 3
Evolved Packet System (EPS) 4
Transport Requirements 5
IP/MPLS Transport 6
VPWS Backhaul and L3VPN Backbone Transport 7
L3VPN Backhaul and Backbone Transport 8
Route Analytics 9
Visualizing Layer 3 VPNs 10
Visualizing Layer 2 VPNs (VPWS) 12
Visualizing RSVP-TE Tunnels 13
Visualizing EPS Traffic 15
Concluding Remarks 17

IP/MPLS Backbone and Backhaul Transport Networks
IntroductionWith the advent of the smart phones, tablets and other connected devices, traffic has grown exponentially and created congestion in mobile networks. Circuit switching 2G and 3G mobile networks allocate bandwidth statically (as they require TDM circuits) to each cell site and do not take advantage of the statistical multiplexing found in packet switching networks. Wired service providers, however, have adopted packet-switching IP/MPLS-based network architectures to take advantage of the bandwidth efficiency and higher resiliency, while choosing Ethernet for framing due to significant cost per port reductions.
To meet ever-increasing user traffic demands, mobile orators have embraced Long Term Evolution (LTE) radio access. LTE Advanced can provide up to 1 Gbps bandwidth to each user. If unchecked, this LTE traffic would further burden the mobile network. To address these challenges, the Third Generation Partnership Project (3GPP) has defined System Architecture Evolution (SAE), the core network architecture for the non-radio access part of the network. The main component of the SAE is a new IP-based Evolved Packet Core (EPC), shown in Figure 1. Together, SAE and LTE form the Evolved Packet System (EPS). Mobile operators have been deploying EPS networks, often marketed simply as LTE networks.
Figure 1. All IP-based Evolved Packet Core
Because the EPS is all IP based, IP/MPLS is used for the backhaul and backbone transport networks that connect various end-points. With EPS, 3G mobile operators, who are already running an IP/MPLS-based backbone network, are extending IP/MPLS to their backhaul networks.
An IP/MPLS-based network presents new and unique challenges. With statically-configured circuits, mobile operators enjoy predictable performance. For example, the propagation delay of a circuit is known during provisioning. When that circuit fails, a protection circuit takes over which is also pre-provisioned and has predictable performance. (Note that this leads to over-provisioning bandwidth.) With IP/MPLS, the paths between EPS end-points are dynamic and extremely resilient to failures; IP/MPLS will find a path as long as one exists, regardless of the number and locations of failures in the network. However, one of many consequences of this is that the delay of an IP path can vary significantly, especially under failure conditions. Many LTE applications, such as voice, video, and real-time gaming, require strict quality
IP IP Evolved Packet Core
Backhaul and Backbone Transport

of service with delay and packet loss budgets across the EPS. Not meeting these requirements can be detrimental to the user experience and may lead to subscriber churn.
Statistical multiplexing of IP packets, even though much more efficient, also makes capacity planning more challenging as it can introduce congestion during peak use periods or under link or router failures.
The dynamic IP/MPLS control plane needs to be managed carefully to ensure EPS end-point reachability is not compromised and a path always exists between end-points that need to communicate with each other. As we will see later, several IP/MPLS control plane protocols are in use here. These protocols interact with each other in complex ways. When reachability between two end-points becomes compromised, understanding the protocol interactions will help in finding the root cause of the failure. For that, it is necessary to collect, analyze, and monitor the protocol messages and behaviors.
In this white paper, we first give a brief overview of EPS. We then illustrate how mobile operators deploy IP/MPLS in their backhaul as well as backbone networks. We then illustrate how route analytics technology can address the challenges of running IP/MPLS backbone and backhaul transport networks.
Evolved Packet System (EPS)
An EPS typically encompasses the following logical elements:
eNodeB: This is the LTE evolved base station. eNodeBs are the radio towers to which user equipment (UE), such as cell phones and tablets, connect.
Serving Gateway (S-GW): The S-GW is a data plane element. S-GWs are typically placed at the demarcation point between the radio access network (RAN) and core network. The S-GWs main purpose is to track the users mobility and to send its traffic to the appropriate eNodeB as the user moves. All user packets are carried inside bearers logical pipelines connecting two or more points. The S-GW redirects these bearers as a UE moves from one eNodeB to the next. It also maintains state for the bearers when the UE enters low power mode and un-associates its bearers.
Packet Data Network (PDN) Gateway (P-GW): The P-GW is also a data plane element. P-GWs are placed at the demarcation of the PDN. The P-GW assigns IP addresses to UEs, enforces QoS, filters packets, collects charging information, and forwards UE IP packets to/from the PDN, including the Internet.
Mobility Management Entity (MME): The MME is a control plane element. It manages the UE, including access to the network, assignment of resources, and management of mobility (i.e., tracking, paging, roaming and hand-over). For example, when new packets arrive at an S-GW for a UE that is in low power mode, MME pages that UE so that it can reestablish its bearers and receive the packets S-GW had been buffering.
Policy and Charging Rules Function (PCRF). The PCRF is a control plane element. As the name implies, it is the policy and charging brain for the network. However, enforcement is done at the P-GW. The PCRF tells the P-GW how to handle packets. For example, for a user who has exceeded their quota, the PCTF may instruct the P-GW to rate limit the users packets.

Home Subscriber Server (HSS): The HSS is a control plane element. It contains a users subscribed services, such as whether or not the user is allowed to roam and the QoS treatments to which they have subscribed.
The above description is overly simplified, but sufficient to serve this white paper. Since this white paper is about providing an IP/MPLS transport to the EPS, the most relevant elements are eNodeBs, S-GWs, P-GWs and MMEs. Also, we purposefully illustrated them as logical elements. They may be appliances on their own, may be running in a blade in some other appliance such as an IP/MPLS router, or may be part of a combo-device, for example, one with IP/MPLS routing and forwarding functionality.
Transport Requirements
There are two main kinds of packets the network carries: data (including voice) and control packets. UE data packets are IP packets. At this IP layer, UE is one hop away from the P-GW. That is, the next IP hop from the UE is the P-GW. This is because these packets are relayed through the eNodeB and S-GW using GPRS Tunneling Protocol (GTP). GTP itself rides on UDP that rides on the networks true IP layer. The control packets between the MME and UE are also relayed over IP (but use a different set of protocols).
eNodeBs communicate with S-GWs and MMEs. S-GWs also communicate with P-GWs. In addition, as a UE moves between neighboring eNodeBs, the hand over is done directly between the involved eNodeBs. Hence, eNodeBs also communicate with neighboring eNodeBs. This is best illustrated by Figure 2. The GTP connections between eNodeBs are referred to as X2 connections and the GTP connection between an eNodeB and an S-GW or an MME is referred to as an S1 connection. eNodeBs, S-GWs, MMEs and the S1 and X2 connections between them forms the RAN (E-UTRAN). The network that transports these S1 and X2 connections is often called the backhaul network. The network that connects S-GWs to P-GWs and other EPS elements and the Internet is called the backbone network.
Figure 2. eNodeB Communication Patterns
UE eNodeB S-GW P-GW
PCRFMME
HSS
OperatorsIP services (e.g. IMS)
MME/S-GW MME/S-GW

IP/MPLS TransportA typical mobile operator will have a national backbone network and for each of its regions it will have a backhaul network. Already with 3G, backbone networks transported mobile traffic using IP/MPLS control plane, more specifically using IP/MPLS BGP VPNs (often referred to as L3VPNs). Because now the payload on the backhaul network is also IP, IP/MPLS can be used in the backhaul network as well.
There are many possible IP/MPLS transport architectures for backhaul and backbone networks [See Cisco UMMT at https://communities.cisco.com/docs/DOC-30621]. In this paper, we focus on two scenarios that are most widely deployed by the mobile operators. In both scenarios, L3VPNs are used in the backbone network, just like with 3G networks. In the first scenario, L3VPNs are extended to the backhaul networks. In the second scenario, L2VPNs, specifically Virtual Private Wire Service (VPWS), are used in the backhau

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