ATM Overview

The cornerstone of ATM is the cell. the information flow, with different and varying bit rate, is uniformly into cell which  consist of a cell header of five octets and a user part of 48 octets , ATM is a packet mode technique, but the delay in the work can be kept to a minimum because the cell have a fixed length

structure of ATM

format header of ATM

function and Concept

• ATM layer

Asynchronous Transfer Mode (ATM) is a standardized method for exchanging information between different nodes in a transport network

 

                                    Adding/Release header in ATM layer

• ATM Adaptation Layer (AAL)

the different standardized ATM adaptation Layer (AAL) enhance the service provide by the ATM layer to sopport function by the next higher layer

• AAL2 signaling (Q.2630)

The AAL type 2 (AAL2) signaling protocol (Q.2630) in standardized inter-node protocol and node function that control AAL type 2 point-to-point connections

• TDM-related function

The Time Division Multiplexing (TDM) Termination, TDM cross-connection Circuit Emulation and Fractional ATM functions are provided in order to enable TDM related interoperability

1. ATM Layer

the ATM layer Provide the transport capability for exchanging of information between node in the a network by establishment between ATM connections between ATM nodes

  the ATM connection type are virtual channels (VC) and virtual Path (VP) a VC a logical part of a VP, a VC connection (VCC) consist one or more VC link (VCL), a VP connection (VCC) consist of one or more VP link

2.Class specification of quality

The ITU-T has standardized a protocol reference model that shows similarity whit the OSI model .. the three lowest layer in the protocol reference model are :

• Layer 1 – the physical layer
• Layer 2 – the ATM layer
• Layer 3 – the AAL layer

to enable transfer of both data ad isochronous service, the information must be adapted to the network in different ways, ATM has been divided into four service classes (A,B,C and D) on the basic of three parameters , the following protocol are defined for each pn of the classes

• AAL 1
• AAL 2
• AAL 3/4
• AAL 5

where AAL is not part of the cell header

the following three parameters are used for classification

• Isochronous  or asynchronous service 
• constant and variable bit rate
• Connection oriented or Connection less Oriented transfer

3. Inverse multiplexing for ATM

Inverse multiplexing for ATM is a method that makes it possible  for several physical link to carry a single ATM stream, the main advantages from this method increased robustness the traffic distributed on all physical links and in the case failure on one physical link, the traffic is distributed over remaining physical link, no traffic will be lost if the remaining capacity is sufficient, another important factor is that larger link result in a increased potential for statistical multiplexing gain

                          IMA Links

summary of ATM description

1. carries all type of network traffic
2. provides the theoretically unlimited bandwidth
3. guarantee high data rate and short queues
4. know no limitation as to the maximum distance
5. is not limited by the requirement of the several port and interface   
6. does not depend of high  protocol in contras in router base network
7. is standard accepted all over the world