Cabletron Systems BRIM-A6 manual Figure B-1 ATM Cells

Appendix B: ATM Overview

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.)

Header

Data

5 Bytes	48 Bytes
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.

Page B-2 BRIM-A6 User’s Guide