Call Setup - Funkwerk bintec R4100 User Manual

1
X.25 Menu
Multiple connections between two end points can be achieved by multiplexing
logical connections onto data links. Different logical connections (or "Virtual Cir-
cuits") are identified by assigning each logical connection a virtual circuit num-
ber. This number is included in the header of each X.25 data packet.
Packet-Switched X.25 is a packet-switched network, which means that user data is subdivided
and placed into X.25 packets of a predefined maximum length (usually 128
bytes).
Each packet is assigned a virtual circuit number and is transmitted over the data
link. With a 128 byte packet size, user data will be fragmented into many pack-
ets. The X.25 frame format defines a special field, M-bit (from more data pack-
et), which is used to allow fragmented packets to be reunited at the receiving
station.
Reliable X.25 connections are reliable connections, which means that all data packets
sent are confirmed by the receiving station. This is achieved using either special
packets (Receiver Ready Packets) or by having the receiving station "piggy-
back" confirmation messages onto other packets. Also, in X.25, packets always
arrive in sequence at the receiving station.
1.1

Call Setup

Before data can be exchanged among X.25 partners an X.25 call must be set
up. An X.25 CALL packet is sent by the calling partner to the called partner who
can accept/refuse the connection. Once a call has been established, a unique
Virtual Circuit (VC) number is assigned to the connection which is used during
the whole connection time.
4 bintec User's Guide X.25