ATM Protocol
BRIM-A6 User’s Guide Page B-7
ATM provides the function of the first three layers of the OSI model, but
performs all the information routing (cell switching) in only the Physical
and Data Link Layer.
Performing the information routing functions in the Physical and Data
Link layers essentially bypasses the Network Layer. This allows ATM to
process cells much quicker and more efficiently than packet-based
routing.
Since ATM differs with the OSI model in only its bottom three layers, a
direct comparison of each layer provides the best method for
understanding where these changes exist.
Layer 1: Physical Layer
In both models, the Physical Layer defines how information is transported
over a network. This information includes all the physical connectors,
pinouts, voltage levels, media type, and so on, that are considered valid
for the network.
The major difference between each model in the Physical Layer, is in the
type of media considered suitable for transmission. While PTM
transmission protocols specify certain media types, ATM is not limited to
any particular media. Due to its fixed-cell construction, ATM is only
limited to the media type it uses. For example, if ATM is operating over
coaxial cable, it only performs at the level that coaxial cable allows.
Layer 2: The Data Link Layer vs. The ATM Layer
In the OSI model, the Data Link Layer is involved with data transmission,
error detection, and flow control. The data link layer acts as a shield for
higher network model layers by controlling transmission and reception.
In the ATM model, the ATM layer defines the format of the ATM cell, and
how it is transmitted over an ATM netw ork. In addition, this layer defines
the negotiated quality of service (QOS) and Traffic Management. This
information is contained within the Header field of the cell.
book Page 7 Thursday, April 18, 1996 2:18 PM