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Prof. Dr. Ing. PUSZTAI Kalman

ATM networks

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ATM overview

• Promising technology in early 90s (why?)

• Connection-oriented (virtual circuits)

• Signalling protocol: Q.2931

• Support for QoS (reserve bandwidth and buffer space for each VC at switches)

• Fixed-size packets (cells): 53 bytes

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Cells and cell size

• Pros and cons of variable-sized packets– Bandwidth efficiency (no zero padding)– Hardware complexity

• Benefits of fixed-size packets (cells) – Switch processing is same for all cells– Small cells reduce “preemption delay”

• Trade-offs in choosing cell size– Bandwidth efficiency & cell processing speed– ATM: 48B, 5B header, 53B total

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ATM cell format

• VPI & VCI: identify virtual-circuit

• Type bits:– Explicit Forward Congestion Indication– User signalling bit (used by AAL5: shows last

cell of a packet)

• Cell Loss Priority bit

GFC HEC (CRC-8)

4 16 3 18

VPI VCI CLPType Payload

384 (48 bytes)8

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Segmentation And Reassembly (SAR)

• Also referred to as “fragmentation and reassembly” (in the IP context)

• SAR is performed by ATM Adaptation Layer (AAL)– AAL attaches an additional header to the packet needed for

reassembly at the receiver– Five different AAL protocols were defined; AAL5 was the most

popular in data networks

AAL

ATM

AAL

ATM

… …

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AAL5 SAR protocol

CRC-32

< 64 KB 0– 47 bytes 16 16

ReservedPad Len

32

Data

CS-PDUtrailer

User data

48 bytes 48 bytes 48 bytes

ATM header Cell payload

Padding

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VPIs and VCIs

• ATM connection identifiers: 8b VPI + 16b VCI • Two-level hierarchy:

– Virtual paths are used between major network nodes– Virtual circuits are multiplexed in same virtual paths

• Objective: backbone switches need smaller VC table

Public network

Network BNetwork A

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ATM in the LAN environment

• A bit of history: ATM vs Ethernet

• Major issue: how to implement broadcast and multicast in an ATM LAN?

E1

H5

H6

H7

H1E3

H2

H4

H3E2

ATM linksEthernet links

Ethernet switch

ATM switchATM-attachedhost

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ATL LAN Emulation (LANE)• Objective: make ATM network look like shared-media

LAN (e.g., Ethernet) to higher-layer protocols

Higher-layerprotocols

(IP, ARP, . . .)

Signalling+ LANE

AAL5

ATM

PHY

ATM

PHY PHY

Higher-layerprotocols

(IP, ARP, . . .)

Signalling+ LANE

AAL5

ATM

Host Switch Host

PHY

Ethernet-likeinterface

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LANE servers

• LECS: LAN Emulation Configuration Server– Used to first connect an ATM host to a LANE network

• LES: LAN Emulation Server– Keeps track of ATM and MAC addresses for each host

• BUS: Broadcast & Unknown Server– Maintains point-to-multipoint VC to all registered clients

H2H1

BUSLESATM network

Point-to-point VC

Point-to-multipoint VC