3Com Router User Manual

X.25 and LAPB Protocols Overview197
Once a virtual circuit is established between a pair of DTEs, it is assigned with a 
unique virtual circuit number. When one DTE is to send a packet to the other, it 
numbers this packet (with virtual circuit number) and sends it to DCE. According 
to the number on the packet, DCE determines the method to switch this packet 
within the switching network, so that this packet can reach the destination. Since 
the X.25 layer 3 multiplexes the link established between DTE...

198CHAPTER 16: CONFIGURING LAPB AND X.25
Figure 61   Relations among LAPB, X.25 and X.25 switching
Configure LAPB LAPB configuration includes:
■Configure the link protocol of the interface to LAPB
■Configure LAPB protocol parameters
1Configure the Link Layer Protocol of the Interface to LAPB
Perform the following command in the interface view.
Ta b l e 229   Configure the link layer protocol of the interface to LAPB
If not specified, the working mode of LAPB is DTE by default.
2Configure LAPB Protocol...

Configure X.25199
N1 value represents the maximum number bits of I frame that DCE or DTE 
wants to receive from DTE or DCE.
N2 value represents the maximum number of times that DCE or DTE tries to 
successfully send a frame to DTE or DCE.
Ta b l e 232   Configure LAPB N1, N2
By default, n1 is 12032, and n2 is 10.
dConfigure LAPB T1, T2, T3
LAPB T1, the retransmission timer (T1) determines how long a frame that is 
already sent, can remain unacknowledged before retransmission. The value of 
T1 should be...

200CHAPTER 16: CONFIGURING LAPB AND X.25
Configure X.25 
InterfaceThe configuration of X.25 interface includes:
■Configure X.121 address
■Configure X.25 working mode
■Configure X.25virtual circuit range
■Configure X.25 modulo
■Configure X.25 default flow control parameter
Only when configured as an X.25 interface, can an interface transmit data with 
X.25 protocol.
In the following configuration commands, only “Configure X.25 working mode” is 
mandatory, and other configuration items are optional,...

Configure X.25201
X.25 protocol can multiplex multiple virtual connection over a real physical link 
between DTE and DCE, also called virtual circuit (VC) or logical channel (LC). X.25 
can establish up to 4095 virtual connections numbered from 1 to 4095. The 
number that can be employed to identify each virtual circuit (or logical channel) is 
called logical channel identifier (LCI) or virtual circuit number (VCN).
Strictly speaking, virtual circuit and logical channel are two different concepts....

202CHAPTER 16: CONFIGURING LAPB AND X.25
Ta b l e 237   Set/cancel X.25 virtual circuit range
The above shows that each section (except the permanent virtual circuit section) is 
defined by two parameters: upper limit and lower limit, the value of which ranges 
between 1 and 4095 (including 1 and 4095). Correct configuration must satisfy 
the following conditions:
■In strict numerically increasing order, i.e. 1lichic

Configure X.25203
end-to-end is affected, that is, the efficiency between two sets of communicating 
DTE increases.
5Configure X.25 flow control parameter
It is essential to set correct default flow control parameters (window size and 
packet size) for the operation of the link because X.25 protocol is good at traffic 
control. However, most public X.25 packet networks use the default window size 
and maximum packet size specified in ITU-T X.25 Recommendation, which is also 
true for 3Com Router series....

204CHAPTER 16: CONFIGURING LAPB AND X.25
■Prohibit the restart of X.25 layer 3
It is necessary to configure certain supplementary X.25 parameters in some special 
network environments. 
1Configure the delay of X.25 layer 3 timer
X.25 protocol defines a series of timers to facilitate its procedure. After X.25 sends 
a control packet, if it does not receive the response before the timeout of the 
corresponding timer, X.25 protocol will take corresponding measure to handle this 
abnormal event. The names...

Configure X.25205
of a call that reaches X.25 interface may be inconsistent with X.121 address of the 
destination interface (because the destination address of this call is modified 
within the network), still the interface will accept this call. At this time, one or 
multiple aliases should be specified for this interface by performing the following 
tasks in the interface view:
Ta b l e 242   Specify/Cancel an alias for the interface
To satisfy the requirements of different networks, nine matching...

206CHAPTER 16: CONFIGURING LAPB AND X.25
Ta b l e 244   Configure/Cancel the attributes related to the address code block in the call 
packet or call accept packet
4Configure default upper layer protocol
X.25 call request packet includes a CUD field (Call User Data), which shows the 
upper layer protocol type X.25 protocol carries. When receiving X.25 calls, the 
router will check packet CUD field; when receiving calls carrying the CUD fields 
that cannot be identified, it will reject them. But an upper...