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D-20 COMMUNICATIONS 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.
SLDH message: 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.
L3 packet: 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.
L2 frame: 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.