25-03-2011, 02:20 PM
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Asynchronous Transfer Mode (ATM)
An Overview of ATM
• ATM is Asynchronous Transfer Mode.
• ATM is originally the transfer mode for implementing Broadband ISDN (B-ISDN).
• In 1988, CCITT (from ITU) issued the first two recommendations relating to B-ISDN:
– I.113 Vocabulary of Terms for Broadband Aspects of ISDN
– I.121 Broadband Aspects of ISDN
• In 1911, the ATM Forum was created with the goal of accelerating the development of ATM standards.
• Connection-oriented packet-switched network
• Used in both WAN and LAN settings
• Signaling (connection setup) Protocol:
– Q.2931
• Packets are called cells (53 bytes)
– 5-byte header + 48-byte payload
• Commonly transmitted over SONET
– other physical layers possible
• Connections can be switched (SVC), or permanent (PVC).
Variable vs. Fixed-Length Packets
• No Optimal Length
– if small: high header-to-data overhead
– if large: low utilization for small messages
• Fixed-Length Easier to Switch in Hardware
– simpler
– enables parallelism
Big vs. Small Packets
• Small Improves Queue behavior
– finer-grained pre-emption point for scheduling link
• maximum packet = 4KB
• link speed = 100Mbps
• transmission time = 4096 x 8/100 = 327.68us
• high priority packet may sit in the queue 327.68us
• in contrast, 53 x 8/100 = 4.24us for ATM
– near cut-through behavior
• two 4KB packets arrive at same time
• link idle for 327.68us while both arrive
• at end of 327.68us, still have 8KB to transmit
• in contrast, can transmit first cell after 4.24us
• at end of 327.68us, just over 4KB left in queue
• Small Improves Latency (for voice)
– voice digitally encoded at 64KBps (8-bit samples at 8KHz)
– need full cell’s worth of samples before sending cell
– example: 1000-byte cells implies 125ms per cell (too long)
– smaller latency implies no need for echo cancellors
• ATM Compromise: 48 bytes = (32+64)/2
• ATM operates on a best effort basis.
• ATM guarantees that cells will not be disordered.
• Two types of connections:
– Point-to-point
– Multipoint (Multicast)
• Four Types of Services:
– CBR (Constant Bit Rate)
– VBR (Variable Bit Rate)
– ABR (Available Bit Rate) Flow Control, Rate-based, Credit- based
– UBR (Unspecific Bit Rate) No Flow control.
• Aggregate Bandwidth vs. Shared Medium (FDDI, Fast Ethernet).
ATM Characteristics
• No error protection or flow control on a link-by-link basis.
• ATM operates in a connection-oriented mode.
• The header functionality is reduced.
The information field length is relatively small and fixed
ATM Layer Service
• Transparent transfer of 48-octet data unit
• Deliver data in sequence on a connection
• Two levels of multiplexing
• Three types of connections
– Point-to-point
– Point-to-Multipoint
– Multipoint-to-Multipoint
• Transport is best-effort
• Network QoS negotiation
• Traffic control and congestion control
Asynchronous Transfer Mode (ATM)
An Overview of ATM
• ATM is Asynchronous Transfer Mode.
• ATM is originally the transfer mode for implementing Broadband ISDN (B-ISDN).
• In 1988, CCITT (from ITU) issued the first two recommendations relating to B-ISDN:
– I.113 Vocabulary of Terms for Broadband Aspects of ISDN
– I.121 Broadband Aspects of ISDN
• In 1911, the ATM Forum was created with the goal of accelerating the development of ATM standards.
• Connection-oriented packet-switched network
• Used in both WAN and LAN settings
• Signaling (connection setup) Protocol:
– Q.2931
• Packets are called cells (53 bytes)
– 5-byte header + 48-byte payload
• Commonly transmitted over SONET
– other physical layers possible
• Connections can be switched (SVC), or permanent (PVC).
Variable vs. Fixed-Length Packets
• No Optimal Length
– if small: high header-to-data overhead
– if large: low utilization for small messages
• Fixed-Length Easier to Switch in Hardware
– simpler
– enables parallelism
Big vs. Small Packets
• Small Improves Queue behavior
– finer-grained pre-emption point for scheduling link
• maximum packet = 4KB
• link speed = 100Mbps
• transmission time = 4096 x 8/100 = 327.68us
• high priority packet may sit in the queue 327.68us
• in contrast, 53 x 8/100 = 4.24us for ATM
– near cut-through behavior
• two 4KB packets arrive at same time
• link idle for 327.68us while both arrive
• at end of 327.68us, still have 8KB to transmit
• in contrast, can transmit first cell after 4.24us
• at end of 327.68us, just over 4KB left in queue
• Small Improves Latency (for voice)
– voice digitally encoded at 64KBps (8-bit samples at 8KHz)
– need full cell’s worth of samples before sending cell
– example: 1000-byte cells implies 125ms per cell (too long)
– smaller latency implies no need for echo cancellors
• ATM Compromise: 48 bytes = (32+64)/2
• ATM operates on a best effort basis.
• ATM guarantees that cells will not be disordered.
• Two types of connections:
– Point-to-point
– Multipoint (Multicast)
• Four Types of Services:
– CBR (Constant Bit Rate)
– VBR (Variable Bit Rate)
– ABR (Available Bit Rate) Flow Control, Rate-based, Credit- based
– UBR (Unspecific Bit Rate) No Flow control.
• Aggregate Bandwidth vs. Shared Medium (FDDI, Fast Ethernet).
ATM Characteristics
• No error protection or flow control on a link-by-link basis.
• ATM operates in a connection-oriented mode.
• The header functionality is reduced.
The information field length is relatively small and fixed
ATM Layer Service
• Transparent transfer of 48-octet data unit
• Deliver data in sequence on a connection
• Two levels of multiplexing
• Three types of connections
– Point-to-point
– Point-to-Multipoint
– Multipoint-to-Multipoint
• Transport is best-effort
• Network QoS negotiation
• Traffic control and congestion control
