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98 Chapter 3: OSI Reference Model & Layered Communication

Figure 3-12

Frame Relay Network

Data Link Function 3: Error Detection

Error detection is simply the process of learning whether bit errors occurred during the
transmission of the frame. To do this, most data links include a frame check sequence (FCS) or
cyclical redundancy check (CRC) field in the data link trailer. This field contains a value that is
the result of a mathematical formula applied to the data in the frame. The FCS value calculated
and sent by the sender should match the value calculated by the receiver. All four data links
discussed in this section contain an FCS field in the frame trailer.

Error detection does not imply recovery; most data links, including 802.5 Token Ring and 802.3
Ethernet, do not provide error recovery. In these two cases, however, an option in the 802.2
protocol, called LLC type 2, does perform error recovery. (SNA and NetBIOS are the typical
higher-layer protocols in use that request the services of LLC2.)

Data Link Function 4: Identifying the Encapsulated Data

Finally, the fourth part of a data link identifies the contents of the data field in the frame. Figure
3-13 helps make the usefulness of this feature apparent.

Timbuktu

Kalamazoo

East Egypt

ch03.fm Page 98 Monday, March 20, 2000 4:58 PM