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 3-3 helps make the usefulness of this feature apparent. The figure shows a PC that uses both TCP/IP to talk to a web server and Novell IPX to talk to a Novell NetWare server.

Figure 3-3 Multiplexing Using Data-Link Type and Protocol Fields

Novell Server

Client

IPX

ip

PC1

Data Link

_LL

802.3

802.2

Data

802.3

802.2

Data

802.3

When PC1 receives data, should it give the data to the TCP/IP software or the NetWare client software? Of course, that depends on what is inside the Data field. If the data came from the Novell server, PC1 hands off the data to the NetWare client code. If the data comes from the web server, PC1 hands it off to the TCP/IP code. But how does PC1 make this decision? Well, IEEE Ethernet 802.2 Logical Link Control (LLC) uses a field in its header to identify the type of data in the Data field. PC1 examines that field in the received frame to decide whether the packet is an IP packet or an IPX packet.

Each data-link header has a field, generically with a name that has the word Type in it, to identify the type of protocol that sits inside the frame's data field. In each case, the Type field has a code that means IP, IPX, or some other designation, defining the type of protocol header that follows.

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