AT&T 7400 series, 7500 series, 7100 series, 2500 series manual Message, Packet, and Frame, Sldh

D-20COMMUNICATIONS PROTOCOLS

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Message, Packet, and Frame

The units of information in BX.25 are the message, the packet, and the frame. Starting with the data that an application (e.g., on the 501CC) wishes to send to its remote peer (on the AP), the session layer entity (DIP) serving the local application prefaces the application data with a SLDH (session layer data header), thereby forming a message. The SLDH describes the length of the application message and defines a message sequence number used by the remote session layer entity to ensure that messages are received in the same order they were sent.

message:

SLDH

application message

Layer 3 (in the DCIU) then prefaces the session layer message with another header to form a packet. If the message exceeds the maximum packet size, layer 3 will break the message into multiple packets in a way which allows later reconstruction of the original message. The layer 3 header defines packet sequence numbers used for flow control and delivery confirmation; it also identifies the logical channel to which this packet belongs so that different logical channels can be multiplexed onto the same layer 2 link.

packet:

L3

SLDH

application message

Layer 2 (also in the DCIU) envelopes the packet with a layer 2 header and a layer 2 trailer to form a frame. The header and trailer have unique bit patterns to delimit the frame. The trailer also carries the cyclic redundancy check, while the header defines frame sequence numbers which govern retransmission in the event that a bit error is detected by the remote receiver.

frame:

L2

L3

SLDH

application message

L2

In addition to the packet and frame formats used for packaging application data, BX.25 provides for control packets and control frames which generally have a more abbreviated format and do not contain application data or SLDHs.