ATM (asynchronous transmission mode)

Asynchronous transmission mode, also known as cell relay. An ATM adopts a connection-oriented exchange mode, in which the Unit is credit. Each cell is 53 bytes long. The header occupies 5 bytes. ATM is ideal for implementing various QoS, supporting both connected businesses and unconnected businesses. Is a model of broadband ISDNB-ISDN) technology.

Asynchronous transmission mode (ATM)ProtocolSet to establish a mechanism for transmitting all communication traffic over a fixed 53-byte packet cell. Fixed-size packages ensure fast and easy exchange and multiplexing. ATM is a connection-oriented technology. In other words, to establish communication between two network systems, it is necessary to notify the intermediate media service requirements and traffic parameters.

The ATM reference mode is divided into three layers: the ATM Adaptation Layer AAL), the ATM layer and the physical layer. Higher Level of AAL connectionProtocolTo the ATM layer, it is mainly responsible for exchanging ATM cells between the upper layer and the ATM layer. After receiving the information from the upper layer, AAL splits the data into ATM cells. After receiving the information from the ATM layer, AAL must recombine the data to form a format that can be recognized by the upper layer, the above process is called segmentation and restructuring SAR ). Different AAL types are used to support different traffic or service types used on an ATM network.

The ATM layer is mainly responsible for forwarding cells from AAL to the physical layer for easy transmission and forwarding of cells from the physical layer to AAL for ease of use in the terminal system. The ATM layer can determine where incoming cells should be forwarded; reconfigure the corresponding connection identifier and forward the cells to the next link, buffer cells, and handle various traffic management functions, such as cell loss priority mark, congestion mark, and universal traffic control access. In addition, the ATM layer monitors the transmission rate and the traffic policies agreed by the Service ).

The physical layer of ATM defines bitwise timing and other features, and encodes and decodes data into appropriate radio waves or light waves for transmission and receiving on specific physical media. In addition, it provides frame adaptation functions, including cell profiling, header error verification HEC) Generation and processing, performance monitoring, and load rate matching for different transmission formats. The physical layer usually uses SONET, DS3, optical fiber, and twisted pair cables.

ProtocolStructure

ProtocolStructure
ATM Cell format:

HEADER	GFC or VPI	VPI
	VPI	VCI
	VCI
	VCI/td>	PT3 Bit)	CLP
	HEC
IE	Cell Payload48 Bytes)

Header-5 bytes) General Traffic Control VPI/VCI and other control headers. IE-48 bytes) cell payload. Physical Layer specifications-dedicated 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/194.4 Mbaud	Multimode Fiber, STP
STS-3c STM-1	155.52 Mbps	UTP-3
STS-12, STM-4	622.08 Mbps	SMF, MMF

Physical Layer specifications-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

RelatedProtocol	SONET, AAL0-AAL5, LAN Emulation, CES, PNNI, MPOA, Q.2931
Organization Source	ATMProtocolThe reference model is based on the ITU Standard. Http://www.atm forum.com/standards/approved.html: ATM Forum approved specifications
Related Links	ITU-T Recommendation I .363, B-ISDN ATM Adaptation Layer (AAL) Specification Http://www.atm forum.com/standards/approved.html?uni: ATM User-Network Interface Specification Http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/atm.htm: ATM Overview