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ATM

Class 6Cell Switching

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Topics

• Network Architecture• ATM Protocol Architecture• Logical connections• ATM cell structure• Service levels/categories• ATM Adaptation Layer (AAL)

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IntroductionMoving from LAN to WAN need

– Higher speed– Traffic aggregation– Multi-protocol handling– Multiple-vendor support

Various Solutions– Frame Relay– Asynchronous Transfer Mode – aka Cell Relay– Connection oriented cell-based

• vs. packet switching• vs. frame switching• vs. telephone

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Background • ATM evolved from B-ISDN development efforts

– Frame Relay: high-speed WAN (1.5+ Mbps)

– ATM: very high speed WAN (155 Mbps and 622Mbps)

• ATM, like Frame Relay, was built on the assumption that the underlying physical media was reliable and flexible– minimal error and flow control capabilities

– even more streamlined, therefore faster, than Frame Relay

• Specifications developed by ITU-T and ATM Forum

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ATM Network Architecture

Public UNI connects ATM end-systems to a public ATM networkPrivate UNI connects ATM end-systems to a private ATM networkResidential UNI which is a variant of the public UNI for residences or small businesses

Public NNI interconnects ATM switches in the public network of the same providerPrivate NNI (PNNI) interconnects ATM switches in a private networkBroadband inter-carrier interface (B-ICI) – connects two public networks of different providers

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Network Interface Points

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Datagram (Switching & Forwarding) –Connectionless Routing

0

132

01 3

2

013

2

Switch 3 Host B

Switch 2

Host A

Switch 1

Host C

Host D

Host EHost F

Host G

Host H

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Connectionless Routing

Host can send a packet anywhere any time– Host does not know network congestion– Host does not know states of intervening

routers/switches– Packets are forwarded independently of each

other– Packets contain routing information– Link failures have minimal impact on packet

delivery

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Virtual Circuit Switching

0

13

2

01 3

2

0

13

2

Switch 3

Host B

Switch 2

Host A

Switch 1

5 11

4

7

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Connection-oriented RoutingHost requests to send a packet somewhere

– Host requests bandwidth which may or may not be available

– All intervening routers/switches must comply with the request

– Packets are forwarded sequentially and do not (should not!) go out of order

– Packets contain VCI information– Link failures have an impact on packet delivery

– must be torn-down & explicitly rebuiltKey difference signaling must be used to setup the

connection before traffic is admitted

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ATM Protocol

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ATM Protocol Architecture

• Fixed-size packets called cells– “cell switching” like packet switching

• 2 primary protocol layers relate to ATM functions:– Common layer providing packet transfers,

logical connections (ATM)– Service dependent ATM adaptation layer (AAL)

• AAL maps other protocols to ATM– like IP (AAL5)

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ATM Protocol Reference Model

Various data rates (155.52 Mbps, 622.08 Mbps) over various physical media types (Fiber Optic, SONET, UTP, etc.)

Framing, cell structure& Logical Connections

Map data to the ATM cellstructure

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Protocol Model3 Application Function Planes

– User – provides user (station) information transfer and associated controls (flow control, congestion control) – traffic

– Control – performs call control and connectioncontrol functions – signaling

– Management – provides plane management and layer management and coordination functions

• ATM Adaptation Layer (AAL)• ATM Layer• Physical Layer

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User Plane Layers

AAL

ATM

User information

User information

AAL

ATM

PHYPHY

ATM

PHY

ATM

PHY

…

End system End systemNetwork

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Virtual Circuits

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Logical Connections• VCC (Virtual Channel Connection)

– a logical connection analogous to a virtual circuit in X.25, or Frame Relay data link connection

• full-duplex flow between end users• user-network control signaling• network-network management/routing

• VPC (Virtual Path Connection)– a bundle of VCCs that

• Have the same end points (not necessarily same end-users)

• Are switched along the same virtual path(es)

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ATM Connection Relationships

Physical Transmission Medium

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ATM VPC/VCC

c ATMSw1

ATMSw4

ATMSw2

ATMSw3

ab

de

VP3 VP5

VP2

VP1

a

bc

de

ATMDCC

VP6

Sw = switch

DCC = Cross-connect switch

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VCC (logical connection) Uses

• Exchange between end users– user data– control signaling (more later)

• Exchange between an end user and a network entity– control signaling (more later)

• Exchange between 2 network entities– traffic management– routing functions

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Advantages of Virtual Paths• Simplified network architecture

– allows separation of functionality into into individual logical connections and related groups of logical connections

• Increased network performance and reliability– network consists of fewer aggregated entities

• Reduced processing and short connection setup time– complex setup tasks are in virtual paths– simplifies setup of new virtual channels over existing virtual path

• Enhanced network services – supports user-specified closed groups/networks of VC bundles

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Cell-based Switching within VCs

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Cell Format and Switching• Virtual Path Identifier (VPI)• Virtual Channel Identifier (VCI)

– ATM LANs do all the switching using VPs and VCs are used for multiplexing/de-multiplexing

• 5 byte header and 48 byte payload• Cell loss priority, Header error checksum

4 16 3 18

Payload

384848 bytes5 bytes

GFC HEC (CRC-8)VPI VCI CLPType

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ATM Cell Format

UNI NNI

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Header Format

• Generic flow control (more )

• Virtual path identifier (VPI)• Virtual channel identifier (VCI)• Payload type

– (3 bits: identifies cell as user data or network management cell, presence of congestion, SDU type)

• Cell loss priority (0: high don’t lose; 1: low)• Header error control (more )

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Generic Flow Control

• Controls traffic flow at UNI– alleviates short-term overload conditions– Note: not employed in network core

• When GFC is enabled at the UNI, two procedures are used:– Uncontrolled transmission

• not subject to flow control– Controlled transmission

• flow control constraints (using GFC) are in force

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Header Error Control• 8-bit CRC• error detection• in some cases, error correction of single-

bit errors in header• 2 modes:

– Error detection– Error correction

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HEC Operation at Receiver

Based on recognition of fact that bit errors occur in bursts

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Effect of Error at Cell Header

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ATM Adaptation Layer (AAL)

• Support higher-level protocols– e.g., PCM voice, LAPF, IP

• AAL Services– Handle transmission errors– Segmentation/reassembly (SAR)– Handle lost and misinserted cell

conditions– Flow control and timing control

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Applications of AAL and ATM

• Circuit Emulation (e.g., T-1 synchronous TDM circuits)

• VBR voice and video• General data services• IP over ATM• Multiprotocol encapsulation over ATM

(MPOA)• LAN Emulation (LANE)

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Segmentation and Reassembly

AAL

ATM

AAL

ATM

… …

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ATM Adaptation Layer Types

AAL provides different services according to:– Time relationship between source and

destination – Bit rate - fixed vs. variable– Connection mode – connection oriented

vs. connectionless• Types

– AAL1, AAL2, AAL3/4, AAL5

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AAL Protocol and Services

Basis for classification:• requirement for a timing relationship between source and destination

• requirement for a constant bit rate data flow• connection or connectionless transfer

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AAL Protocols

• AAL layer has 2 sublayers:– Convergence Sublayer (CS)

• Supports specific applications using AAL• Users attach via the Service Access Point

(like port number)• Common part (CPCS) and application service-

specific part (SSCS)– Segmentation and Reassembly Sublayer

(SAR)• Packages data from CS into ATM cells and

unpacks at other end

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AAL Protocols and PDUs

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AAL Protocol Types

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Segmentation and Reassembly PDUs

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AAL Type 1

• Constant-bit-rate source• SAR simply packs bits into cells and

unpacks them at destination• One-octet header contains 3-bit SC

field to provide an 8-cell frame structure

• No CS PDU structure is defined since CS sublayer primarily for clocking and synchronization

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AAL Type 2• Intended for use with applications with low

variable bit-rate service on multiple channels (multiplexing)– Mini-cells

• Initial specification has been withdrawn

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AAL Type 3/4

• Intended for applications that generate bursty data and demand low loss

• Originally delineated as– Connectionless (AAL4) or – Connection (AAL3) oriented

• May be message mode or streaming mode

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AAL 3/4 Example (message mode)

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Adaptation Layer 3/4CS PDU

CPI Btag BASize Pad 0 Etag Len

8 16 0– 24 8 8 16< 64 KB8

User data

ATM Cell

2 44 25

ATM header Length CRC-10

40 2 4

SEQ MIDType Payload

352 (44 bytes)10 6 10

AAL 3/4

53

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Encapsulation and Segmentation

CS-PDUheader

CS-PDUtrailerUser data

44 bytes 44 bytes 44 bytes ≤ 44 bytes

ATM headerAAL header

Cell payloadAAL trailer

Padding

244

2

5

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AAL Type 5

• Streamlined transport for connection oriented protocols– Reduce protocol processing overhead– Reduce transmission overhead– Ensure adaptability to existing transport

protocols– primary function is segmentation and

reassembly of higher-level PDUs

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AAL5 Example

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CPCS PDUs: AAL 3/4 , AAL5

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AAL5CRC-32

< 64 KB 0– 47 bytes 16 16

ReservedPad Len

32

Data

CS-PDUtrailerUser data

48 bytes 48 bytes 48 bytes

ATM headerCell payload

Padding

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ATM Layer

• Assigns header to the segment streams generated by AAL

• Routing and multiplexing of cells over VCs

• Maintains VCI table

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Topics for Next Time

• ATM Switch design• ATM LAN emulation