ATM

1
ATM

• Module 7

2
ATM Defined

• Asynchronous Transfer Mode.
• One of a class of packet switching technologies
  that relays traffic via an address contained
  within the packet
• To overcome overhead and latency, a fixed size
  cell of 53 bytes is used.
• ATM is defined as a transport and switching
  method in which information does not occur
  periodically with some reference, hence the name
  Asynchronous

3
Standards and Applications

• ATM defined by ANSI and ITU
• PBX to PBX tie lines and trunks
• Host to Host Computer Links
• Video Conferencing circuits
• LAN to LAN bridged or routed traffic
• Multimedia networking services
• Workstations Routers Bridges Gateways
• up to 622 Mbits per second

4
Cells

• Standard fixed cell size for ATM is 53 bytes
• 5 bytes of overhead and 48 bytes of payload
• The format of the 48 bytes of data depends on the
  type of data being carried.
• Some types of data use another 4 bytes for
  control, leaving 44 bytes for payload

5
CELL Overhead

• GFC 4 bits, General Flow Control
• VPI 8 bits, Virtual Path Identifier, part of the
  address, a grouping of channels between network
  node.
• VCI 16 bits, Virtual Channel Identifier. The
  other part of the datalink address
• PT 3 bits, Payload Type
• CLP 1 bit, Cell-Loss Priority
• HEC 8 bits, correct single bit errors and detect
  double bit errors on first 4 bytes of header

6
The Adaptation Layer

• The ATM Adaptation Layer
• provides the flexibility of a single
  communication process to carry multiple types of
  traffic such as data, video voice and multimedia.
• AAL convergence sublayer
• SAR segmentation and reassembly sublayer

7
5 Adaptation types

• CBR Constant Bit Rate services
• VBR Variable Bit Rate time sensitive services
• Connection Oriented VBR Data Transfer
• Connectionless VBR data
• SEAL Simple and Efficient Adaptation Layer

8
ATM Forum

• The ATM forum is an international consortium of
  members that is chartered to accelerate the
  acceptance of ATM products and services in local
  and wide area networks
• Forum works with official standards groups
• First specification on ATM UNI defines interface
  between router and private ATM switch

9
Equipment

• Bridges
• CSU/DSU
• Gateways
• Multiplexers
• Routers
• Servers