ATM:
Asynchronous Transfer Mode Protocol
The Asynchronous Transfer Mode (ATM) composes a protocol suite
which establishes a mechanism to carry all traffic on a stream of fixed 53-byte
packets (cells). A fixed-size packet can ensure that the switching and
multiplexing function could be carried out quickly and easily. ATM is a
connection-oriented technology, i.e.; two systems on the network should inform
all intermediate switches about their service requirements and traffic
parameters in order to establish communication.
The ATM reference model, which has two forms - one for the
user-to-network interface (UNI) and the other for the network-to-node interface
(NNI), is divided into three layers: the ATM adaptation layer (AAL), the ATM
layer, and the physical layer. The AAL interfaces the higher layer protocols to
the ATM Layer, which relays ATM cells both from the upper layers to the ATM
Layer and vice versa. When relaying information received from the higher
layers, the AAL segments the data into ATM cells. When relaying information
received from the ATM Layer, the AAL must reassemble the payloads into a format
the higher layers can understand. This is called Segmentation and Reassembly
(SAR). Different AALs are defined in supporting different types of traffic or
service expected to be used on ATM networks.
The ATM layer is responsible for relaying cells from the AAL to
the physical layer for transmission and from the physical layer to the AAL for
use at the end systems, it determines where the incoming cells should be
forwarded to, resets the corresponding connection identifiers and forwards the
cells to the next link, as well as buffers cells, and handles various traffic
management functions such as cell loss priority marking, congestion indication,
and generic flow control access. It also monitors the transmission rate and
conformance to the service contract (traffic policing).
The
physical layer of ATM defines the bit timing and other characteristics for
encoding and decoding the data into suitable electrical/optical waveforms for
transmission and reception on the specific physical media used. In addition, it
also provides frame adaptation function, which includes cell delineation,
header error check (HEC) generation and processing, performance monitoring, and
payload rate matching of the different transport formats used at this layer. SONET , DS3, Fiber, twisted-pair are few media often used
at the physical layer.
Protocol Structure - ATM: Asynchronous
Transfer Mode Protocol
ATM Cell Format:
|
H
E
A
D
E
R
|
GFC or VPI
|
VPI
|
|
|
VPI
|
VCI
|
||
|
VCI
|
|||
|
VCI
|
PT (3 Bit)
|
CLP
|
|
|
HEC
|
|||
|
IE
|
Cell Payload (48 Bytes)
|
||
- Header — (5 Bytes) Generic flow
control, VPI/VCI, and other control header.
- IE — (48 Bytes) Cell Payload.
Physical Layer Specification – Private UNI:
|
Frame Format
|
Bit Rate/Line Rate
|
Media
|
|
Cell Stream
|
25.6 Mbps/ 32 Mbaud
|
UTP-3
|
|
STS-1
|
51.84 Mbps
|
UTP-3
|
|
FDDI
|
100 Mbps/ 125 Mbaud
|
Multimode Fiber
|
|
STS-3c, STM-1
|
155.52 Mbps
|
UTP-5
|
|
STS-3c, STM-1
|
155.52 Mbps
|
Single-Mode Fiber,
Multimode Fiber, Coax pair
|
|
Cell Stream
|
155.52 Mbps/
194.4Mbaud
|
Multimode Fiber, STP
|
|
STS-3c, STM-1
|
155.52 Mbps
|
UTP-3
|
|
STS-12, STM-4
|
622.08 Mbps
|
SMF, MMF
|
Physical Layer Specification – Public UNI:
|
Frame Format
|
Bit Rate/Line Rate
|
Media
|
|
DS1
|
1.544 Mbps
|
Twisted pair
|
|
DS3
|
44.736 Mbps
|
Coax pair
|
|
STS-3c, STM-1
|
155.520 Mbps
|
Single-mode Fiber
|
|
E1
|
2.048 Mbps
|
Twisted pair, Coax pair
|
|
E3
|
34.368 Mbps
|
Coax pair
|
|
J2
|
6.312 Mbps
|
Coax pair
|
|
N × T1
|
N × 1.544 Mbps
|
Twisted pair
|
The ATM protocol reference model is based on standards developed by the ITU.
