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B.2 the atm cell – Cabletron Systems BRIM-A6 User Manual

Page 40

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Appendix B: ATM Overview

Page B-2

BRIM-A6 User’s Guide

B.2 THE ATM CELL

One major difference between ATM and current transmission methods is
that instead of using variable-length packets, ATM transmits fixed-size
cells. (See Figure B-1.)

Figure B-1

ATM Cells

ATM uses three types of cells – Assigned, Unassigned, and Idle. Assigned
cells carry actual traffic. Unassigned cells are free and waiting to be
assigned traffic. Idle cells are used to separate cells, provide receiver
synchronization (byte alignment), and reserve bandwidth for another
channel (i.e., occupy bandwidth between transmissions). Idle cells do not
receive traffic.

ATM cells total 53 bytes long and consist of two general parts: the Header
field and the Data field. The Header field consists of 5 bytes and contains
management/control information for the cell. The Data field contains 48
bytes of information. Figure B-2 shows the additional components of the
Header field.

Header

Data

5 Bytes

48 Bytes

book Page 2 Thursday, April 18, 1996 2:18 PM