Verilink 9000 Series (34-00271) Product Manual - Manual (Page 245)

13
X.25
X.25 Basics The concepts of X.25 are rather straightforward. Through a series of requests and
acknowledgments, a logical connection or pathway is established between two
devices in a network. Since X.25 is a bit-oriented protocol and relies on data
placement within a frame, as opposed to having a special character to indicate that
data is starting, it can carry any form of information desired. X.25 is called a
transport protocol because it can carry information from multiple protocols
simultaneously since it does not interact with the actual data. It merely transports
data from one point in the network to another without changing it. The X.25
protocol breaks information into smaller, more manageable segments, but it
contains mechanisms to reassemble those segments into their original form once
they reach their destination.
An X.25 network also ensures data delivery. Once an X.25 cloud acknowledges
receipt of a message, it is responsible for delivering it. The integrity of the
message is checked at each switch as the message travels through the network. It is
also possible for the network to notify the sender that the message was actually
delivered to the intended destination. The key description of an X.25 network is
integrity, though this comes at a cost. The act of constantly checking and
rechecking the integrity of the message places extra overhead on the transition
time through the network.
Prior to any data transfer, a logical channel must be established between the two
locations. In X.25, there are two types of logical channels: Permanent (PVC) and
Switched (SVC). There is no signaling mechanism implied with a PVC. The
channel can be used for data transfer. For the PVC, flow control and error recovery
apply like the SVC. The difference with the SVC is that prior to data transfer, the
channel should be established by signaling the network.
The basic handshaking to establish a connection between two points begins with
one of the endpoints sending a call request packet. This packet contains the
address of the destination, the address of the sender, a user data field, and other
minor information (see Figure 13-1). The MultiPro platform utilizes the free-form
nature of the user data field to carry information about the requesting station. This
field consists of three bytes of miscellaneous MultiPro unit data followed by four
bytes representing the name of the final destination (this is actually the
hexadecimal representation of the 16-bit CRC that is calculated for each device
name. This is followed by an additional four bytes representing the address (in
hexadecimal) of the requesting station. If the call request reaches its final
destination and is accepted, a call accept packet is generated to the initiating
station and a logical pathway is established. Data packets may now flow to and
from the connected endpoints.