IEEE 802.5 Token Ring

Type:	Physical and Data Link Layers.
SNMP MIBs:	iso.org.dod.internet.mgmt.mib-2.dot5SrMIB (1.3.6.1.2.1.42).

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.

Token:

00	01	02	03	04	05	06	07
Start Delimiter
Access Control
End Delimiter

Abort:

00	01	02	03	04	05	06	07
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

Priority bit. The first three bits are reserved to indicate the priority of the token.
Token Indicator bit. This bit indicates whether a token or packet.
Monitor Count bit. Monitors for packets that continually loop through the network.
Priority Reservation bits. The last three bits reserves a token on a priority basis.

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

Frame Type bits. The first two bits indicates a data or command frame type. The data frames contain upper-layer protocol information. The command frames contain control information with no upper-layer protocol information. This control information is: 00=MAC, 01=non-MAC, or 1x=reserved.
Physical Control Field bits. This is only for MAC frames.

Value Physical Control Field

0 Normal Buffer.

1 Express Buffer.

2 Purge.

3 Claim Token.

4 Beacon.

5 Active Monitor.

6 Standby Monitor.

Value	Physical Control Field
0	Normal 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 :::

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.

Group addresses when the group/specific bit is set to 1. Broadcast addresses go to entire group on a network. Multicast addresses go to a subset group.

Functional/nonfunctional addresses.

Global/Local assigned addresses.

Source Addresses. 6 bytes.
Physical address.

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

Token Ring Hold Time. The maximum time a station may hold a token before transmitting the data.

Adapter Driver Limit.

Packet Size Limit.

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
ARI	FCI	0	0	ARI	FCI	0	0

ARI, Address Recognition Indicator. 1 bit.
FCI, Frame Copied Indicator. 1 bit.

Glossary:

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.

RFCs:

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