Asynchronous Transfer Mode
Asynchronous Transfer Mode (ATM) is a "high-speed transmission protocol in
which data blocks are broken into small cells that are transmitted individually
and possibly via different routes in a manner similar to packet-switching
technology". In other words, it is a form of data transmission that allows
voice, video and data to be sent along the same network. In the past, voice,
video and data were transferred using separate networks: voice traffic over the
phone, video over cable networks and data over an internetwork. ATM is a cell-
switching and multiplexing technology designed to be a fast, general purpose
transfer mode for multiple services. It is asynchronous because cells are not
transferred periodically. Cells are given time slots on demand. What seperates

ATMs is its capability to support multimedia and integrate these services along
with data over a signal type of transmission method. The ATM cell is the data
unit used to transmit the data. The data is broken into 48-byte data packets for
transmission. Five bytes of control data are appended to the 48-byte data
packets, forming a 53-byte transmission frame. These frames are then transmitted
to the recipient, where the 5-byte control data (or Header) is removed and the
message is put back together for use by the system In an ATM network, all data
is switched and multiplexed in these cells. Each ATM cell sent into the network
contains addressing information that achieves a virtual connection from
origination to destination. All cells are then transferred, in sequence, over
this virtual connection. Asynchronous Transfer Mode: The header includes
information about the contents of the payload and about the method of
transmission. The header contains only 5 octets. It was shortened as much as
possible, containing the minimum address and control functions for a working
system. The sections in the header are a series of bits which are recognized and
processed by the ATM layer. Sections included in the header are Generic Flow

Control (GFC), Cell Loss Priority (CLP), Payload Type, Header Error Control, the

Virtual Path Identifier and the Virtual Channel Identifier. The Header is the
information field that contains the revenue bearing payload. A GFC is a 4-bit
field intended to support simple implementations of multiplexing. The GFC is
intended to support flow control. The CLP bit is a 1-bit field that indicates
the loss priority of an individual cell. Cells are assigned a binary code to
indicate either high or low priority. A cell loss priority value of zero
indicates that the cell contents are of high priority. High priority cells are
least likely to be discarded during periods of congestion. Those cells with a
high priority will only be discarded after all low priority cells have been
discarded. Cell loss is more detrimental to data transmission than it is to
voice or video transmission. Cell loss in data transmission results in corrupted
files. The Payload Type section is a 3-bit field that discriminates between a
cell payload carrying user data or one carrying management information. User
data is data of any traffic type that has been packaged into an ATM cell. An
example of management Asynchronous Transfer Mode: information is information
involved in call set-up. This section also notes whether the cell experienced
congestion. The Header Error Control field consists of error checking bits. The

Header Error Control field is an 8-bit Cyclic Redundancy Code to check for
single bit and some multi-bit errors. It provides error checking of the header
for use by the Transmission Convergence (TC) sublayer of the Physical layer. The

Virtual Path Identifier in the cell header identifies a bundle of one or more

VCs(virtual channels).The Birtual Channel Identifier (VCI) in the cel header
identifies a single VC on a paricular Virtual Path. The path is divided into
channels. The choice of the 48 byte payload was made as a compromise to
accommodate multiple forms of traffic. The two candidate payload sizes were
initially 32 and 64 bytes. The size of the cell has and effect on both
transmission efficiency and packetization delay. A long payload is more
efficient than a small payload since, with a large payload, more data can be
transmitted per cell with the same amount of overhead (header). For data
transmission alone, a large payload is desirable. The longer the payload is,
however, the more time is spent packaging. Certain traffic types are sensitive
to time such as voice. If packaging time is too long, and the cells are not sent
off quickly, the quality of the voice transmission will decrease. The 48 byte
payload size was the result of a compromise that had to be reached between the

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