Introduction to Tandem X.25 Capabilities

Packet Types
What is X.25?
065307 Tandem Computers Incorporated 1–21
Data Packets Data packets contain user data. During the data transfer stage, both DTEs can
exchange data. If a DTE does not have any data to send, it replies with an RR packet
(see Figure 1-15). The RR packet acknowledges the packets received thus far and
indicates the readiness of the DTE to receive additional packets.
Figure 1-15. Data Packets
Network
Data Packet
or
RR Packet
Data Packet
Local
DTE
Remote
DTE
054
Local
DCE
Remote
DCE
Data Packet
Data Packet
or
RR Packet
Packet-Layer Modulos and Window Size
Data packets contain sequencing numbers for packets sent and packets received:
P(S) identifies the sequence number of the data packet sent.
P(R) identifies the sequence number of the next data packet that is expected to be
received
The sequencing numbers make it possible to ensure that all the packets sent for a
particular call 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
0.
The number of packets that one DTE can send before receiving an acknowledgement
from the other DTE depends on the packet-layer window size. The most common
packet-layer window size is 2, but other window sizes may be made available by
some networks. With a window size of 2, the calling DTE must receive an
acknowledgement with a P(R) of 2 before it can send a third and fourth data packet.
Once the acknowledgement has been received, the lower and upper edges of the
window move by a factor of 2, so that the next P(S) and P(R) values become valid.
This process is referred to as window rotation. Figure 1-16 provides a simplified
illustration of how this window rotation works. In this figure, the data packets are
only travelling in one direction. In reality, data packets can travel in both directions.