Introduction to Data Communications
Previous 45o. IEEE 802.5 Frame Fields (cont'd) Next
Source Address (SA)

The Source Address is identical to the Destination Address field except that the Individual/Group bit is always set to Individual (0) for IEEE 802.5. For IBM Token Ring, the first bit is the Routing Bit which instructs bridges to pass the Token Frame (1) or to ignore it (0).

IEEE 802.5

IBM Token Ring

This difference is very important. Only IBM Token Ring allows Source Route Bridging.

Functional Addresses

Token Ring provides MAC addresses that are reserved for special functions:

	Null Address:			0000 0000 0000

	Broadcast Address:		FFFF FFFF FFFF or C000 FFFF FFFF

	Active Monitor Address:		C000 0000 0001

	Ring Parameter Server:		C000 0000 0002

	Ring Error Monitor		C000 0000 0008

	Configuration Report Server	C000 0000 0010

	Netbios Address:		C000 0000 0080

	Bridge Address:			C000 0000 0100

	Lan Manager Address:		C000 0000 2000

	User Defined addresses:		C000 0008 0000 to C000 4000 0000

Information Field (INFO)

The Information Field is used to:

The Frame Control bits FF determine whether the information is for the MAC (FF=00) or LLC (FF=01) layer. The LLC information is processed identical to Ethernet as covered earlier under IEEE 802.2 and will not be covered here.

The Routing Bit determines whether the frame uses normal MAC layer communication or if Source Routing is used. Only, IBM Token Ring uses Source Routing, IEEE 802.5 does not.

If the Routing Bit is set, then the MAC Frame INFO field contains routing information used during Source Routing. The Routing Bit instructs the bridge to pass the Token Frame or ignore.

If the Routing Bit is not set than normal MAC layer local ring communication is active. This includes specific subcommands called MVID (Major Vector IDs) that work with the Frame Control control bits to: Beacon, Claim Token , Ring Purge, etc.. (See Token Ring MVID Table)

Introduction to Data Communications
Previous Table of Contents Next