FREE ESSAY ON ASYNCHRONOUS TRANSFER MODE

ASYNCHRONOUS TRANSFER MODE

Asynchronous Transfer Mode:
Asynchronous Transfer Mode
By Gene Bandy
State Technical Institute
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 
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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 64 byte payload which would provide efficient data transfer
but poor quality voice and the 32 byte payload which could
Asynchronous Transfer Mode:
transmit voice without echo but provided inefficient data transfer. The 48 byte payload
size allows ATM to carry multiple forms of traffic. Both time-sensitive traffic (voice)
and time-insensitive traffic can be carried with the best possible balance between
efficiency and packetization delay.
ATM Advantages:
1. ATM supports voice, video and data allowing multimedia and mixed services over a
single network. 
2. High evolution potential, works with existing, legacy technologies
3. Provides the best multiple service support 
4. Supports delay close to that of dedicated services 
5. QoS (Quality of Service)classes 
6. Provides the capability to support both connection-oriented and connectionless traffic
using AALs(ATM Adaptation Layers)
7. Able to use all common physical transmission paths (DS1,SONET) 
8. Cable can be twisted-pair, coaxial or fiber-optic 
9. Ability to connect LAN to WAN 
10. Legacy LAN emulation 
11. Efficient bandwidth use by statistical multiplexing 
12. Scalability 
13. Higher aggregate bandwidth 
14. High speed Mbps and possibly Gbps 
Asynchronous Transfer Mode:
ATM disadvantages:
1. Flexible to efficiency's expense, at present, for any one application it is usually
possible to find a more optimized
2. Technology
3. Cost, although it will decrease with time 
4. New customer premises hardware and software are required
5. Competition from other technologies -100 Mbps FDDI, 100 Mbps Ethernet and fast
ethernet
6. Presently the applications that can benefit from ATM such as multimedia are rare
7. The wait, with all the promise of ATM's capabilities many details are still in the
standards process 
Asynchronous Transfer Mode:
Reference:
1. Freeman, Roger L. ((1996). Telecommunication System Engineering: Third Edition. City:
New York, John Wiley & Sons, INC.
2. Spohn, Darren L. (1997). Data Network Design. City: McGraw-Hill Company.
3. Taylor, D. Edgar (1995). The McGraw-Hill Internetworking Handbook. City: New York,
McGraw-Hill Company.
Internet:
1. Quigley, David (1997). A Technical View of ATMs. [online], Available:
http://www.mathcs.carleton.edu/students/quigleyd/atmtech.html.