ATM Protocol
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
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
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 network. 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.
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