ATM (Asynchronous Transfer Mode)

• Adopted by CCITT in 1988 as the basis for the development of B-ISDN. So often the terms ATM Networks and B-ISDN are used synonymously.

• The following table illustrates the ATM layers below the Applcation Layer (adapted from Fig. 1-31 [Tanenbaum 3rd ed.]):

OSI    ATM     ATM  
Layer  Layer  Sublayer     Functionality
-------------------------------------------------------------------------------------
                CS        Interface (convergence), services to different applications
 3/4    AAL     - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 
                SAR       cell/packet segmentation and reassembly 
-------------------------------------------------------------------------------------
                          Flow control
 2/3    ATM               Cell header generation/extraction
                          Virtual circuit / virtual path management
                          Cell multiplexing/demultiplexing
-------------------------------------------------------------------------------------
                          Cell rate decoupling
                          Header error checking (checksum) 
  2             TC        Cell generation
                          Packing/unpacking cells
                          Frame generation
 - -  Physical  - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
                          Bit timing
  1             PMD
                          Physical network access
-------------------------------------------------------------------------------------
   
   AAL -- ATM Adaptation Layer
   CS  -- Convergence Sublayer
   SAR -- Segmentation And Reassembly sublayer
   TC  -- Transmission Convergence sublayer
   PMD -- Physical Medium Dependent sublayer

Cell networks

• Cell Networks send data packets in very small units (cells) (Fig. 25.10)
  

  Cell Networks

  Chips are available to segment and reassemble (SAR chips)

• Cell size - most common 53 byte cells (5 bytes control, 48 bytes data)

Advantages of cell networks

• Efficiency: very little wasted bandwidth

• Lower latency. Serialization delay in a packet network is illustrated in Fig. 25.11.
  

  Cell Network Latency

• Serialisation delay in a cell network is illustrated in Fig. 25.12.
  

  Cell Network Serialisation

• Makes network better for delivering voice, video, and data.

  Increases speed of store-and-forward networks (it takes about 1 microsecond to receive a 128 byte cell on a 1 Gbit network).

• Original ISDN provided for fixed 64 kbps channels. Fixed size channels are never the right size.

Cell Format

• 53 bytes - 5 bytes header, 48 bytes payload (Fig. 25.13)
  

  Cell Format

• Header has 2 formats - one for inside the network (network-network-interface, or NNI) and one for entering/leaving network (user-network-interface, or UNI).

ATM NNI Cell header

ATM NNI Cell header

• VPI = Virtual Path Identifier, VCI = Virtual Channel Identifier

  VCI + VPI = unique routing indicator for the cell.

  Like phone number, VPI is like area code (607), VCI is like phone number (555-1212) (Fig. 25.14).

  Routers can use hop-by-hop routing, may route only on VPI in the network backbone.

• Payload type (Fig. 25.15)
  

  Payload type

  CLP = Cell Loss Priority. 1=low priority, 0=high priority (a hint for switches as to which cell to drop)

  CRC = Header Error correction code

• ATM UNI is still poorly defined