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WAN_Technologies - 1 Copyright: see page 2

WAN Technologies(to interconnect IP routers)

Mario Baldiwww.baldi.info


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The Context

Wide Area

IP host

IP router

IP network


Why Not Just Cables? Expensive! Time consuming

To lay To modify


Use a Network Cables are laid once Interconnectivity can be configured Resources (and their costs) shared

Interconnecting a new site implies just laying a “short” cable

New “virtual” interconnectionscan be easily and quickly created


And before building new ones Reuse existing technologies Specifically, existing networksThey are “Dead”: there is no new development Old: designed tens of years ago with

various purposes in mind Not for interconnecting IP routers

With one exception (frame relay) … but huge investments


Commonly Deployed Technologies Circuit switching

ISDN PDH SONET/SDH

Frame Relay ATM

Dead and old, but still heavily used→ relevant!

[also optical networks, which are a (relatively)new technology]


Circuit Switching Technologies


TDM: Time Division Multiplexing

Switch Switch

BANDWIDTH


Interconnections based on TDM TDM (Time Division Multiplexer)

divide the total bandwidth in sub-bands Each network user (e.g., router) can “see”

only its assigned sub-band as asynchronous channel with fixed bit rate

Routers can use a channel as the equivalentof a physical link

In reality at any given time the link carriestraffic of only one user


TDM: Time Division MultiplexingTake turns

at predefined, recurring time instants → time rules

Switch Switch

BANDWIDTH


Good service without flexibility (by design) Deterministic performance

Delay Jitter Bandwidth

Fixed rate Bandwidth not actually used by a user (in a

certain interval of time) cannot be used byother services and it is wasted Higher costs Particularly for bursty traffic

E.g., data traffic

Designed for voice traffic


Standard Channel Hierarchies Standardized channel speeds Standardized transmission speeds

And corresponding equipment Transmitters Receivers Cables

Copper Fibers

How to combine multiple low speedchannels into a high speed signaltransmitted on a linkMultiplexing and demultiplexing


PDH: Plesiochronous Digital Hierarchy Looser synchronization requirements

Limited transmission speeds High overhead

USA

T1 - DS1 1.544 Mbps

T3 - DS3 44.736 Mbps

Europe

E1 2.048 Mbps

E3 34.368 Mbps

E4 139.26 Mbps

32 x 64 kb/s24 x 64 kb/s

2816

4


Why a hierarchy? Hierarchy: how to combine multiple lower

rate frames into higher rate frame Higher capacity links in the backbone, lower

capacity at the accessGrooming and degrooming


SDH: Synchronous Digital HierarchySONET: Syncronous Optical Network

USA: SONET Europe: SDH

OC-3c / STS-3c 155.52 Mbps

STM-1155.52 Mbps

STM-4622.08 Mbps

OC-12c / STS-12c 622.08 Mbps

STM-122.4 Gbps

OC-48c / STS-48c 2.4 Gbps


Higher com

plexity → costs

Mor

e st

ringe

nt s

ynch

roni

zatio

n

Higher transmission speeds Lower overhead


InteroperabilityPDH

hierarchies

USA

T1 - DS1 1.544 Mbps

T3 - DS3 44.736 Mbps

SDH hierarchies

USA: SONET Europe: SDH


STM-1155.52 Mbps

STM-4622.08 Mbps

OC-12c / STS-12c 622.08 Mbps

STM-122.4 Gbps

OC-48c / STS-48c 2.4 Gbps


Europe

E1 2.048 Mbps

E3 34.368 Mbps

E4 139.26 Mbps


Physical architecture Section: fiber optic between repeaters Line: sequence of sections between devices

oeprating at line level Path: end-to-end connection

T

SON

ET/S

DH

M

UXT

TA

dd/D

rop

MU

X

SON

ET/S

DH

M

UX

T

T

T

Section Section Section SectionLine Line

Path

Repeater Repeater


Protocol architecture Multiple layers

Complex protocol architecture

Monitoring functionalites Fast fault recovery

50 ms Ring topology

Photonic Layer

Section Layer

Line Layer

Path Layer

Frame

Photonic Layer

Section Layer

Photonic Layer

Section Layer

Line Layer

Photonic Layer

Section Layer

Line Layer

Path Layer

Higher Layer Higher Layer

Light

STS-N / STM-N Block

Envelope

Terminal TerminalRepeater Multiplexer


Frame formatSTS-1: 810 bytes transmitted in 125 s 51.84 Mbps

A1 A2 C1 J1

B1

D1

H1

B2

D4

D7

D10

Z1

D2

H2

K1

D5

D8

D11

Z2

F1

D3

H3

K2

D6

D9

D12

E2

B3

C2

G1

E1

F2

H4

Z3

Z4

Z5

Payload Payload

90 octets

9 oc

tets

s

sSection Overhead Line Overhead Path Overhead

Synchronous Payload Environment (SPE) 87 octets3 octets


Frames In packet switching

Frame contains bytes of a single communication Destination (source) identified in the header Frames are transmitted asynchronously

Whenever there is data to transmit Whenever a link is free

Statistical multiplexing In circuit switching

Frame contains bytes of multiple communications Destination (source) identified by the position Frames are transmitted synchronously

Back-to-back Independently of whether there is data to transmit


What do we do with them? Same duration (125 µs) at any bit rate

1 byte carries one 64kb/s channel More bytes per frame at higher rates Hierarchy: how to combine multiple lower

rate frames into higher rate frame Higher capacity links in the backbone, lower

capacity at the accessGrooming and degrooming


How do we connect routers? One channel between two routers

All bytes in a frame A fraction of a channel

Multiple bytes in a frame


But this is not the way it was meant to be used

Voice

Integrated services


ISDN -Integrated Service Digital Network

TELEPHONE

COMPUTER

FACSIMILEGROUP 4

VIDEOTELEPHONE

MULTISERVICETERMINAL

TERMINAL/TELEX

ISDN

So what?Now it is normal, but back then it was a big step forward


ISDN Access Interface

ISDNExchange

(Multiplexer)NT1

ISDN phone

PC with ISDN

Customer Premises Operator

Bus S

Fax G.4

Videoconference


ISDN Access Interface Data + voice The user terminal becomes digital 2B + D or base access

2 data channels at 64 kbps 1 signaling channel at 16 kbps total 144 kbps up to user's premises

30B + D or primary access 30 data channels at 64 kbps 1 signaling channel at 64 kbps total 2 Mbps up to user's premises


Packet Switching Technologies


Packet-based multiplexing/switching If a source has no traffic, bandwidth is

not wastedStatistical multiplexing

The same network infrastructure canstatistically accommodate morecommunicationsCost of communicating is lowerService is not deterministic


Statistical Multiplexing

Switch Switch

BANDWIDTH


Statistical MultiplexingTake turns

opportunistically, i.e., as soon as link isavailable

Switch Switch

BANDWIDTH


Statistical MultiplexingIf one is using less, others can use more

Switch Switch

BANDWIDTH


Building packet switching on top of circuit switching

Use circuits through a circuit network to interconnect packet/frame

switches/routers

PDH SDH

Packet switching Cell switching

X.25 Frame Relay ATM


Frame Relay


Frame Relay Network Architecture

DTE

DTEDTE

DTE

DCE

DCE

DCE

DCE

Frame RelayNetwork

VirtualCircuits

Data Communication Equipment

Data Terminal Equipment


Frame Relay standard

Standard for DCE-DTE interfacesMultiple logical connections through a single

access link Similarly to X.25

Layer 2 only Switches do not need to process 2 headers (layer

2 and layer 3) like in the previous X.25 X.25 does have a layer 3


Core-Edge Approach Error correction: re-transmission only at the

network edge, not in the intermediate nodes(core) X.25 corrects errors at each hop

Requires reliable links Takes advantage of fast links Small latency:

2 ms per frame relay node from 5 to 20 ms per X.25 node


Frame Relay applications

Interconnection of Intermediate Systems(router, bridge, gateway) through WANs All commercial devices offer FR interfaces Physical layer is common (PDH) Data-link layer is implemented in software

It is possible to specify the bandwidthrequired by/provided to the customer

Variable transit times Problematic with voice/video transmission


CIR: Committed Information Rate Bc: committed burst size

Maximum burstiness Tc=Bc/CIR

Interval of time where CIR is applied It is possible to transmit up to Bc bit at wire speed

of the access link in each time interval Tc

Wire speed

CIR

Tc

Bc

Guaranteedservice

Non guaranteedservice


Asynchronous Transfer Mode (ATM)


Relationships among differenttechnologies

analog telephony leased analog

digital telephony leased digital

Switching

+

Frame Relay

+X.25

TDMISDN

B-ISDN ATM

SMDS

DQDB

Digital conversion

voice + data

wideband

frame

core-edge

cells

packet

core-edge

cells

cells


Distinguishing Factors

Protocol Architecture

Core & EdgeCells

System design choices

Asynchronous interconnectionservces

ATM

Enabling technlogies

Optical fibersVLSI (CMOS)


General features Switching of small, fixed length data units:

cells 53 bytes

Fast links (with low bit error rate) 150 Mb/s

Low latencyGood for data, voice and video

For deployment in both LAN and WAN Selected for implementing the B-ISDN


Sophisticated signaling:Multiparty or point-to-point connections

Sophisticated mechanisms for flow control Sliding window is not efficient on long, fat pipes

Dynamic bandwidth allocation Bandwidth management

Fine granularity in bandwidth allocation Support for “bursty” traffic Adaptability for applications that are

sensible to time or data loss

General features

Anything else?!?!


Virtual channels

4 Mbps (3 party conference)50 Mbps (Image transfer)

ATM net

ATM Switch


Cell switching

Cell = 53 bytes

485

ATMSwitch


Statistical multiplexing

A C B C A

A A

C C

B MultiplexATM


ATM technology

Switch Switch

BANDWIDTH

ATMSwitch

ATMSwitch


1

2

3

4

5

6

53

.

.

Hea

der

Payl

oad

GFC VPIVPI VCI

VCIVCI PT CLP

HEC

User Data

(48 octets)

7 6 5 4 3 2 1 0

UNI Cell

ATM Cell


Header Field Names GFC: General Flow Control VPI: Virtual Path Identifier VCI: Virtual Channel Identifier PT: Payload Type CLP: Congestion Loss Priority HEC: Header Error Control


A couple more details Cells are transmitted back-to-back,

poossibly inserting empty ones Each cell carries an indentifier of the circuit

VCI/VPI: Virtual Channel/Path Identifier Error correction:

Core-edge approach as in frame relay Flow control more sophisticated than

sliding windows, to take into account: Different types of traffic The“memory” of the channel


Cell Routing

VCI/VPI changes each time an ATM switch is traversed

C

D

B A 2

n

2

mi

1 1

Port Label

Look-upport #2

i D

1 C


Core-Edge Principle Nodes execute only basic functionalities

(switching and multiplexing) ATM Layer (L1-L2 OSI stack)

Additional functionalities for the differentservices are implemented at the edge

PHY PHY PHY

ATM ATM (core)AAL

PHYATM

AALError control (only for some services and upon request)

Userterminal

ATMswitch

Upper layer

protocols

Upper layer

protocols

Userterminal


B-ISDN/ATM Reference Model

Control Plane User Plane

Management Plane

Laye

r Man

agem

ent

PlM

t

Physical Layer

ATM Layer

ATM Adaption Layer (AAL)

Higher Layer ProtocolsHigher Layer Protocols


AAL 5 Segmentation and Reassembly

USER BITS

0 to 65,000 Bytes

L3-PDU

REASSEMBLYSEGMENTATION

PAD Ctrl/Length CRC

0-47 4 4Bytes


The Vision

PrivateNNI

Bridge /Router

PublicNNI

PublicUNI

Private ATM Network

Public ATM Network

PublicUNI

Bridge /Router

PrivateUNI

PrivateUNI

PublicUNI

PrivateUNI

PublicUNI

PublicUNI

PrivateUNI


Reality

ATM

IP host

IP router

IP network


A problematic solution How can R know that Q is

the next hop towards D? How can R know the ATM

address of Q?

ATM

D

R

Q


B

ATM SWITCH

Ethernet

Access device

workstation

PHYATMAAL

HigherLayerLLC

LAN em

PHY

MAC

HigherLayerLLC

PHY PHYATMAAL MAC

LAN emBridging

PHYATMAAL

Signalling

ATMWorkstation

Access devices operate as bridges

ATM LAN Emulation


LES

BUS

SWITCH ATM Access device

ATMWorkstation

Access device

Server

ATM LAN Emulation


IP over ATM: Classical model Direct communication within a subnet Use router across subnets Emulate ARP for address resolution within

the subnet Find ATM address of destination or router

It does not use ATM potential (performance)IP Subnet BIP Subnet A

ARP Server B

H3

ARP Server AH1

H2

R


Advanced Solutions:Next Hop Resolution Protocol Source finds the ATM address of the best

hop to a destination Destination itself if it is on the ATM network A router connected out of the ATM network if the

destination is not on the ATM network Complex

Logical Address Group BLogical Address Group ANHRP Server B

H3

NHRP Server AH1

H2


Only one good way outRouters and ATM switches “speak the same language”

It’s the MPLS solution! Same control plane

Routers and ATM/MPLSswitches exchange routinginformation

ATM

D

R

Q

Destinations are identified with IP addresses

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