Introduction to Tandem X.25 Capabilities

Data-Link-Layer Components

What is X.25?

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Frame-Layer Modulos and Window Size

Information frames (I-frames) contain sequencing numbers for I-frames sent and

I-frames received:

N(S) identifies the sequence number of the I-frame sent.

N(R) identifies the sequence number of the I-frame that is expected to be received

The sequencing numbers make it possible to ensure that all the I-frames that are sent

are received by the other end. The maximum sequence numbers are determined by

the modulo format:

Modulo-8 format sequence numbers rotate from 0 to 7, then start over again at 0.

Modulo-128 format sequence numbers rotate from 0 to 127, then start over again at

The frame-layer (layer-2) window size determines the number of I-frames that can be

outstanding before the transmitter must cease the transmission of information frames.

The window size must be no larger than the modulo count minus 1.

Figure 1-20 shows how the N(S) and N(R) variables are used to keep track of I-frames

that have been sent and received. The DCE sends the first I-frame, indicating a N(S)

of zero and and N(R) of zero. The DTE acknowledges receipt of the I-frame by

incrementing its N(R) variable to 1. At the same time, the DTE sends an I-frame to the

DCE. When the DCE receives the I-frame sent by the DTE, the DCE acknowledges the

receipt by incrementing its N(R) variable to 1. By this time, the DCE has already sent

two I-frames in previous transmissions to the DTE, so it increments the N(S) variable

to 2. Note that the DCE continues sending I-frames without receiving a reply from the

DTE. When the DTE sends an I-frame with N(R) = 5, it implicitly acknowledges all the

I-frames that the DCE has sent to that point (DCE I-frames, N(S) = 2, N(S) = 3, and

N(S) =4).

When data transfer has been completed, the DCE issues an RR command frame (see

the following subsection on“Frame Types”for more information on RR frames) with

the P bit set to 1 as a check-pointing procedure to make sure that all the data has been

successfully transferred.

The DTE notifies the DCE that all data has been accepted by sending an RR response

frame with the F-bit set to 1. The N(R) variable in this frame indicates that the DTE

has correctly received all I-frames up to and including N(R)-1. In Figure 1-20, the final

N(S) of the DCE is 5, which equals the final N(R)-1 transmitted by the DTE. After this

acknowledgment, the link can be disconnected.