| Asynchronous Transfer Mode Essay, Research Paper
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 catchword 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 catchword 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 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
Bibliography
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.
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