Outline: ATM

• standardization process
• cells
• virtual paths and virtual channels (VPI/VCI)
• signaling and QoS
• AAL: segmentation and reassembly
• physical layer

ATM Standards -- History

• standards process -- consensus building, slow
• '80s: international (phone company) standards committees very formal
• ATM Forum -- "voluntary", less formal association of telephone, networking, computer companies -- to "suggest" standards ('90s)
• from phone companies: cell format, general signaling protocol, physical standards, idea of QoS
• from ATM Forum: AAL 5, LANE, CLIP, ATMARP, IP multicast over ATM, Multiprotocol over ATM (MPOA), practical details of QoS
• UNI, NNI, P-NNI
• telephone-like, not shared-medium

ATM Cells

• packet switching usually has variable-sized packets
• ATM uses fixed-size cells:
  • 53 bytes (424 bits): 5 bytes of header and 48 bytes of payload
  • fixed size is easier to handle in hardware
  • small size gives:
    • higher partial utilization
    • shorter real-time delay
    • finer control over delay variation (jitter)
    • header is larger percentage of total cell, gives worse utilization
    • most packets have to be segmented into multiple cells, then reassembled

Cell Format

• 5-byte header, 48-byte payload
• header (User-Network Interface, UNI):
  1. 4-bit Generic Flow Control (GFC) -- not present for NNI
  2. 8-bit Virtual Path Identifier (VPI) -- 12 bits for NNI
  3. 16-bit Virtual Channel Identifier (VCI) -- VPI + VCI are the virtual circuit identifier (whose acronym would also be "VCI"!)
  4. type-start: 1-bit user/management cell (fields "type-start"-"type-end" make up the Type field)
  5. 1-bit forward congestion indicator
  6. type-end: 1-bit user signalling (last cell marker)
  7. 1-bit Cell Loss Priority (CLP)
  8. 8-bit CRC

VPI/VCI

• VPI/VCI together are 24 bits on UNI, 28 bits on NNI
• can establish single connections by setting both VPI and VCI
• by agreeing to switch based only on VPI, can establish many connections at once
• example: trunk line (with intermediate switches) has many connections from location A to location B, as well as between other locations
• open single VP to carry all circuits between A and B
• then establishing a new channel between A and B does not require any negotiation with the intermediate switches

Signaling

• telephony: exchanging information about connections
• ATM: protocol to establish and remove connections
• connection establishment is a negotiation involving:
  • path determination (routing)
  • resource allocation
  • confirmation by all components involved in the route (except for channels carried in existing virtual paths)
  • request/reply (RPC)
• ATM signaling protocol is Q.2931

In-class exercise

• groups of 4 (A, B, C, D: A talks to B, B talks to A and C, C talks to B and D, D talks to C)
• 5 minutes
• exchange pieces of paper (messages) of three types:
• request specifies source, destination, and a request ID (chosen by source)
• reply specifies source, destination, request ID, whether successful, and if successful: VPI/VCI
• data specifies source, destination, contents
• establish a connection to each person in your group, and send a message to them.

Resources

• ATM guarantees in-order cell-delivery
• signaling lets ATM guarantee such properties as:
  • connection bandwidth (leaky bucket)
  • delay, delay variation
  • cell loss ratio
  These properties are called Quality of Service, QoS

Segmentation and Reassembly

• ATM Adaptation Layer (AAL)
• AAL-1, AAL-2: real-time traffic
• AAL-3/4, time-insensitive traffic (4 bytes per cell):
  • 2-bit type (first, intermediate, last, single cell)
  • 4-bit sequence
  • 10-bit frame ID
  • payload (1-44 bytes)
  • 6-bit length
  • 10-bit CRC

AAL 5

• time-insensitive traffic
• 8 bytes per PDU:
  • payload (1-64KB)
  • padding (0-47 bytes)
  • 16-bit reserved
  • 16-bit length
  • 32-bit CRC
• much less per-cell overhead than AAL-3/4
• reassembly checks CRC at end, cannot do early discard
• "user bit" in ATM header used to decide when to check CRC

ATM Physical Layer

• SONET (T1: 1.5Mb/s, T3: 45 Mb/s)
• OC-3: 155 Mb/s (fiber, CAT-5)
• OC-12: 622 Mb/s fiber
• OC-48: 2.4 Gb/s
• 25 Mb/s (CAT-5)