IEEE 802.5 Token Ring

Description Glossary RFCs Publications Obsolete RFCs


Type:Physical and Data Link Layers.

In IEEE 802.5, the token passing scheme is used in place of Carrier Sense Multiple Access with Collision Detection (CSMA/CD) on a ring topology local area network (LAN). A token is circulated around a network. The computer that has possession of the token has the right to transmit packets for a certain period of time. If that computer has no packets to transmit then the token is passed to the next computer. Only one computer at a time can transmit packets so this helps to avoid collision problems.


Start Delimiter
Access Control
End Delimiter


Start Delimiter
End Delimiter

IEEE 802.5 Frame:

Start Frame Delimiter and Control IEEE 802.5 packet FCS Frame Delimiter and Status

SFD, Start Frame Delimiter. 8 bits.
Js and Ks are infractions. Js of ones. Ks of zeros. Signals the start of a token or packet.

00 01 02 03 04 05 06 07
J K 0 J K 0 0 0

Access Control. 8 bits.

00 01 02 03 04 05 06 07
P2 P1 P0 TI M PR2 PR1 PR0

Frame Control. 8 bits.
The frame contains data or control information.

00 01 02 03 04 05 06 07
F1 F0 0 0 PC3 PC2 PC1 PC0
ValuePhysical Control Field
0Normal Buffer.
1 Express Buffer.
2 Purge.
3 Claim Token.
4 Beacon.
5 Active Monitor.
6 Standby Monitor.

IEEE 802.5. Packet for token networks:

00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15

Destination Address

Source Address
Data :::

Destination Addresses. 6 bytes.
Some specific format variations in MAC addressing scheme.

Source Addresses. 6 bytes.

Physical address.

Data Field. Variable length.
MAC or non-MAC information about the frames.

FCS, Frame Check Sequence. 4 bytes.
Cyclic redundancy check (CRC) value.

EDEL, End Frame Delimiter. 10 bits.
Signals the end of a token or packet.

00 01 02 03 04 05 08 09
J K 1 J K 1 IF ED

IF, Intermediate Frame.
If more frames follow in logical sequence, then this bit is set to 1. Otherwise it is cleared to zero.

ED, Error Detection Indicator.
Set to 1 if a CRC error is detected in the frame.

Frame Status Field. 1 byte.

00 01 02 03 04 05 06 07


Good Frame.
(RFC 1513, page 5) The 802.5 specification [Token Ring Access Method and Physical Layer Specifications, Institute of Electrical and Electronic Engineers, IEEE Standard 802.5-1989, 1989.] defines the term "good frame" as a frame that is bounded by a valid SD and ED, is an integral number of octets in length, is composed of only 0 and 1 bits between the SD and the ED, has the FF bits of the GC field equal to 00 or 01, has a valid FCS, and has a minimum of 18 octets between the SD and the ED.


[RFC 1042] A Standard for the Transmission of IP Datagrams over IEEE 802 Networks.

[RFC 1239] Reassignment of Experimental MIBs to Standard MIBs.

[RFC 1271] Remote Network Monitoring Management Information Base.

[RFC 1469] IP Multicast over Token-Ring Local Area Networks.

[RFC 1513] Token Ring Extensions to the Remote Network Monitoring MIB.

[RFC 1748] IEEE 802.5 MIB using SMIv2.

[RFC 1749] IEEE 802.5 Station Source Routing MIB using SMIv2.

Obsolete RFCs:


[RFC 1231] IEEE 802.5 Token Ring MIB.

[RFC 1743] IEEE 802.5 MIB using SMIv2.

Description Glossary RFCs Publications Obsolete RFCs