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Chapter 4: ATM Solutions
Foundation Topics
ATM Protocols and Specifications
For most people, ATM is short for Automatic Teller Machine. ATMs can be used at many
locations throughout the world.
To some Ethernet lovers who have seen this highly touted technology protocol rise and fall in
popularity, ATM stands for A Tragic Mistake.
To a CCDP, ATM stands for
Asynchronous Transfer Mode
. ATM is a technology that allows
voice, video, and data transmission at gigabit speeds. Until the advent of Gigabit Ethernet, ATM
was the only technology that could integrate all types of network traffic at higher speeds. ATM
is still the only technology that provides multiservice integration with quality of service.
The ATM Forum is responsible for creating standards and specifications that ensure that ATM
remains responsive to user needs. The ATM Forum was founded in October 1991 by Nortel
Networks, Sprint Communications, Sun Microsystems, and Digital Equipment Corporation.
ATM switches small units of data called
cells
. The cell has a fixed length of 53 bytes--48 for
data and 5 for the header. The small size of the cell makes ATM ideal for handling delay-
sensitive traffic, such as voice and video. With the fixed length, the ATM switch doesn't have to
be notified when the transmission is done. The switch knows when each cell ends and can
process the data rapidly. ATM switches switch cells in hardware. An ATM switch doesn't waste
overhead looking for information in software. ATM can move data
fast!
See Figure 4-2.
Figure 4-2
ATM Data Frame
Header
Payload
5 bytes
48 bytes
GFC
VPI
VCI
PT
C
L
P
HEC
GFCÑGeneric Flow Control
VPIÑVirtual Path Identifier
VCIÑVirtual Channel Identifier
PTÑPayload Type
CLPÑCell Loss Priority
HECÑHeader Error Check
ATM Cell FormatÑHeader and Payload
4
8
16
3
1
8
bits
CH04.fm Page 116 Wednesday, August 22, 2001 3:20 PM