DATA (1-4500 bytes) (Frame Relay)

Protocol: Frame Relay Frame Relay PCI

Field: data

Length: 1-4500 bytes

Contents: upper layer protocoll data / user data.

Frame Relay had been standardized by ANSI which also had specified FDDI. As a result, FDDI and Frame Relay both support up to 4500 of data bytes.

Connection establishment and connection release are both done by the LMI protocol. LMI is a funtion of Frame Relay.

DLCI (10-24 bits) (Frame Relay)

LMI - Local Management Interface

LMI uses the first four bytes of Frame Relay's data field:

- Byte 1: Unnumbered Information Indicator (UII)

- Byte 2: Protocol Discriminator

- Byte 2: Call Reference

- Byte 3: Message Type

LMI - Byte 1: Unnumbered Information Indicator (UII)

This field is structured the same way as LAPB's unnumbered information with P/F bit set to zero.

a

LMI - Byte 2: Protocol Discriminator

The Protocol Discriminator indicates the serviced upper layer protocol.

LMI - Byte 3: Call Reference

This octet is set to '00000000' when being used with PVCs.

LMI - Byte 4: Message Type

Indicates the LMI message type:

- status message

- status enquiry message

Status enquiry message is used to detect the link status; status message is used for replies.