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 and DCE by the 
X.25 layer 2 (LAPB), what finally viewed by the user will be multiple usable virtual 
circuits.
The relation between packets and frames in various X.25 layers is shown in the 
following diagram.
Figure 60   X.25 packet and LAPB frame
X.25 link layer specifies the frame switching process between DTE and DCE. In 
terms of hierarchy, the link layer seems to bridge the packet layer interface of DTE 
and that of DCE. Through this bridge, the packets can be transmitted continuously 
between the packet layer of DTE and that of DCE. The link layer has such main 
functions as follows:
■Transmit the data effectively between DTE and DCE
■Ensure the synchronization of information between the receiver and 
transmitter
■Detect and correct the error in the transmission
■Identify and report the procedure error to the higher layer protocol
■Inform the packet layer of the link layer state
As specified in international standards, X.25 link layer protocol LAPB adopts the 
frame structure of high-level data link control (HDLC) and the frame structure is a 
subset of LAPB. The bi-directional link will be established when either site sends an 
SABM (Set Asynchronous Balanced mode) command and the other replies with 
UA. 
Defined as X.25 layer-2 protocol, LAPB is actually a separate link layer protocol, 
which can transmit the data with LAPB bearing non-X.25 upper layer protocol. 
3Com Router
 series can configure the link protocol of serial interface to LAPB and 
perform simple local data transmission. Meanwhile, X.25 of 3Com Router series 
has switching function, that is to say, the router can be used as a small X.25 
packet switch. The following diagram shows the relations among LAPB, X.25 and 
X.25 switching.
Packet
headerUser data
DataFrame
headerFCSFrame
delimiter
Bit stream
X.25 layer 3 Packet
X.25 layer 2 Frame
X.25 layer 1
Frame
delimiter 
						

							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 Parameter
aConfigure LAPB operating mode (also called modulo)
There are two LAPB modulus: Modulo 8 and Modulo128. Each data frame (I 
frame) is numbered by sequence, the number can be any from 0 to modulo 
minus 1, and the sequence number is selected periodically within the range of 
the modulo.
In the interface view, configure as follows:
Ta b l e 230   Configure LAPB frame numbering mode
By default, the LAPB modulus is Modulo 8.
bConfigure LAPB parameter K
The parameter K in the LAPB window represents the maximum number of I 
frames numbered in sequence that is to be identified by the DTE or DCE in any 
specified time.
In the interface view, configure as follows:
Ta b l e 231   Configure LAPB window parameter K
By default, k is 7.
c Configure LAPB N1, N2
Inter-network protocol
X.25
switching
X.25
LAPB
OperationCommand
Configure the link layer protocol of the interface to 
LAPBlink-protocol lapb [ dte | 
dce ] [ ip ]
OperationCommand
Configure LAPB frame numbering mode (also called 
modulo)lapb modulo { 128 | 8 }
OperationCommand
Configure LAPB window parameter Klapb window-size k-value
Restore the default value of LAPB window parameter 
Kundo lapb window-size 
						

							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 larger than the maximum interval between sending a frame and 
receiving its response frame. Retransmission timer is started after sending the 
frame but if response is not received and timer is expired then frame will be 
retransmitted.
LAPB T2, the receiving timer (T2) determines when to send a confirmation 
frame to the opposite DCE (or DTE) before T1 expires (T2T1). If T3 is 0, it indicates that the timer is not set.
Ta b l e 233   Configure LAPB system timer T1, T2, T3
By default, T1 is 2000ms; T2 is 1000ms and T3 is 0ms.
Configure X.25 X.25 configuration includes:
■Configure X.25 interface
■Configure X.25 interface supplementary parameter
■Configure X.25 datagram transmission
■Configure the supplementary parameters of X.25 datagram transmission
■Configure X.25 sub interface
■Configure X.25 switching
■Configure X.25 load balancing
Besides configuring X.25, appropriate modification to some LAPB parameters in 
certain cases can also optimize the performance of X.25.
OperationCommand
Configure LAPB parameter N1lapb max-frame n1-value
Restore the default value of LAPB parameter N1undo lapb max-frame
Configure LAPB parameter N2lapb retry n2-value
Restore the default value of LAPB parameter N2undo lapb retry 
OperationCommand
Configure LAPB system timer T1, T2, T3lapb timer { t1 t1-value | 
t2 t1-value | t3 t3-value }
Restore the default value of LAPB system timer T1, 
T2, T3undo lapb timer{ t1 | t2 | 
t3 } 
						

							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, depending on the specific 
condition of X.25 network that is accessed.
1Configure X.121 address
If the 3Com Router series does not originate or terminate calls but only 
participates in X.25 switching, there is no need to configure an X.121 address. 
However, if the 3Com Router series is attached to a X.25 Network, you should 
configure an X.121 address for the interface, and usually, the address is provided 
by the ISP.
To set/cancel the X.121 address, perform the following task in interface view.
Ta b l e 234   Set/Cancel the X.121 address of the interface
2Configure X.25 working mode
To configure X.25 working mode, perform the following task in the interface view.
Ta b l e 235   Set X.25 working mode
Layer 3 of X.25 supported by 3Com Router series can work in both DTE mode and 
DCE mode. It can also specify the datagram format among the two optional 
formats: IETF and Nonstandard.
Note that generally speaking, public X.25 packet switching network requires the 
router to access at DTE side and requires the IETF format. Therefore, the working 
mode of X.25 should be DTE and the format should be IETF. If a pair of serial 
interfaces of two routers is directly connected for data transmission, make sure the 
two transmission ends are DTE and DCE and the formats are the same.
For X.25 supported by 3Com Router series, default working mode is DTE and 
default format is IETF.
3Configure X.25 virtual circuit range
OperationCommand
Set the X.121 address of the interfacex25 x121-address 
x.121-address
Cancel the set X.121 address of the interfaceundo x25 x121-address  
OperationCommand
Set the working mode and packet format of X.25 
interfacelink-protocol x25 [ dte | 
dce ] [ nonstandard | ietf ] 
						

							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. 
However, they are not much different at the user side.
X.25 protocol divides the logical channel into 4 areas. (listed here in numerically 
increasing order): Permanent virtual circuits (PVCs), Incoming-only circuits, 
Two-way circuits, Outgoing-only circuits.
According to the X.25 recommendation proposed by ITU-T, DCE selects an 
available logical channel with a smaller number from the “one-way incoming call 
channel range” and “two-way channel range” to initiate a call, while DCE selects 
an available logical channel with a larger number from the “one-way incoming call 
channel range” and “two-way channel range” to initiate a call. Thus, we can 
avoid the case that one side of the communication occupies all the channels, and 
minimize the possibility of call collision.
In X.25 protocol, six parameters are employed to delimit the four sections, as 
shown in the diagram below.
Figure 62   X.25 channel delimitation
For the meanings of these six parameters, please refer to the following table.
Ta b l e 236   X.25 channel delimitation parameters
Perform the following task in the interface view:
ParameterMeaning
LICLowest Incoming-only Channel
HICHighest Incoming-only Channel
LTCLowest Two-way Channel
HTCHighest Two-way Channel
LOCLowest Outgoing-only Channel
HOCHighest Outgoing-only Channel
1
LT C
LIC
HTC
HIC
LOC
HOC
4095
Permanent virtual circuit
Incoming-only channel
Unused
Two-way channel
Unused
Outgoing-only channel
Unused 
						

							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. Therefore, this task may be optional without special 
requirements of service provider.
After setting window size and maximum packet size, the SVCs that can be 
established only with call process will use these values if related parameters are 
not negotiated in the call process. The PVCs that can be established without call 
process will also use these values if no window size or packet size option is 
assigned when specifying PVC.
X.25 transmitting end will fragment the too long data packet of upper layer 
according to the maximum packet size and mark in the last fragment packet (M 
bit is not set). When the packet reaches the receiving end, X.25 reassembles all 
these fragment packets, and judges whether a complete packet is received 
according to M bit marker. Therefore, too small value of the maximum packet size 
will consume too much router resources on packet fragmenting and assembling, 
thus lowering efficiency.
Finally, the following two points should be noted:
■Maximum packet size < MTU*8 < LAPB N1.
■New configuration will take effect only after executing shutdown and undo 
shutdown
 commands
To set/cancel the default flow control parameter, perform the following tasks.
Ta b l e 239   Set the default flow control parameter
Configure X.25 Interface 
Supplementary 
Parameter■The Configuration of X.25 interface supplementary parameter includes:
■Configure the time delay of X.25 layer 3 timer
■Configure the attributes related to X.25 address, including the following 
configuration items:
■Configure the alias of interface address
■Configure to skip the calling or called address
■Configure whether to check the address code block in call accepting packet.
■Configure whether to carry the address code block in call accept packet
■Configure default upper layer protocol
OperationCommand
Set the receiving window and sending window size 
of virtual circuitx25 window-size in-packets 
out-packets
Cancel the set receiving and sending window size 
of virtual circuitundo x25 window-size
Set the receiving and sending maximum packet 
lengthx25 packet-size in-packets 
out-packets
Cancel the set receiving and sending maximum 
packet lengthundo x25 packet-size 
						

							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 and corresponding procedures of these timers are 
shown in the following table.
Ta b l e 240   X.25 layer 3 timer
In the table, T28 is the timer of “sending register request”, and is only defined at 
the DTE side. It is used to dynamically apply to stop the selective services in the 
network. Its reference value is 300 seconds, and cannot be modified. Perform the 
following tasks in the interface view.
Ta b l e 241   Set X.25 layer 3 timer delay
2Configure the attribute related to X.25 address
■To establish a SVC with a call, X.25 address is needed, which adopts the 
address format specified in ITU-T Recommendation X.121. X.121 address is a 
character string consists of the Arabic numerals from 0 to 9, and it is of 0 to 15 
characters. 
■Configure an alias for the interface
When an X.25 call is forwarded across the network, different networks will be 
likely to make some modifications on the called address according to their own 
needs, such as adding or deleting the prefix. In such cases, the destination address 
Procedure name
Timer name
DTE sideDCE side
RestartT20T10
CallT21T11
RestoreT22T12
ClearT23T13
RegisterT28
OperationCommand
Set the timer delay value of restart procedure
Default value (second): DTE: 180     DCE: 60 
x25 timer tx0 seconds
Cancel the set timer delay value of restart procedure undo x25 timer tx0 
Set the timer delay value of call procedure
Default value (second): DTE: 200     DCE: 180
x25 timer tx1 seconds
Cancel the set timer delay value of call procedureundo x25 timer tx1 
Set the timer delay value of restore procedure
Default value (second): DTE: 180     DCE: 60
x25 timer tx2 seconds
Cancel the set timer delay value of restore procedureundo x25 timer tx2 
Set the timer delay value of clearing procedure
Default value (second): DTE: 180    DCE: 60
x25 timer tx3 seconds
Cancel the set timer delay value of clearing procedureundo x25 timer tx3  
						

							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 modes and the 
formats of corresponding alias strings are defined for X.25 in 3Com Router series, 
as shown in the following table.
Ta b l e 243   Alias match modes and meanings
3Configure the attributes related to the address code block in the call packet or call 
accept packet
As specified in X.25 protocol, the call packet must carry the information set of 
both the calling DTE address (source address) and the called DTE address 
(destination address). This address information set is called the address code block. 
While in call accept packet, some networks require that both (the calling DTE 
address and the called DTE address) be carried, some networks require that only 
one of the two be carried, while some others require that neither should be 
carried. X.25 in 3Com Router series enables users to make choices according to 
the requirement of specific network. Perform the following task in interface view.
TaskCommand
Specify an alias for the interfacex25 alias-policy match-type  
alias-string
Cancel the specification of an alias for the 
interfaceundo x25 alias-policy 
match-type  alias-string
Matching modeMeaningExample
FreeFree matching, the alias string is in 
the form of 12341234 will match with 561234, 
1234567 and 956123478, but will 
not match with 12354.
free-extExtended free matching, the alias 
string is in the form of …1234..…1234.. will match with 
678123459, but will not match 
with 68123459, 67812345 and 
6781234591.
LeftLeft-justified matching mode, the 
alias string is in the form of $1234$1234 will match with 1234567 
and 12346790, but will not match 
with 3123478 and 123784.
left-extExtended left-justified matching 
mode, the alias string is in the 
form of $1234…$1234… will match with 1234679 
and 1234872, but will not match 
with 123468 and 12346890.
RightRight-justified match mode, the 
alias string is in the form of 1234$1234$ will match with 791234 and 
6901234, but will not match with 
7912345 and 6212534.
right-extExtended right-justified matching 
mode, the alias string is in the 
form of ….1234$….1234$ will match with 
79001234 and 86901234, but will 
not match with 7912345 and 
506212534.
StrictStrict matching mode, the alias 
string is in the form of $1234$$1234$ can only match with 1234
WholeWhole matching mode, the alias 
string is in the form of ................ will match with all the valid 
X.121 addresses with the length of 
8
whole-extExtended whole matching mode, 
the alias string can only be ** will match with all the valid X.121 
addresses 
						

							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 layer protocol can be 
specified as the default protocol borne on the X.25 of the 3Com Router series. 
When the X.25 of the 3Com Router series receives a call with an unrecognizable 
CUD, it will treat it as the default upper layer protocol specified by user.
In the interface view, perform the following task to set/cancel the default upper 
layer protocol borne on X.25.
Ta b l e 245   Set/Cancel the default upper layer protocol borne on X.25
By default, the upper protocol carried by X.25 is IP protocol.
OperationCommand
Not carrying the called DTE address information when a 
call is originated
Default: carry
x25 ignore 
called-address (by 
default)
Cancel not carrying the called DTE address information 
when a call is originatedundo x25 ignore 
called-address
Not carrying the calling DTE address information when a 
call is originated
Default: carry
x25 ignore 
calling-address (by 
default)
Cancel not carrying of the calling DTE address 
information in a callundo x25 ignore 
calling-address
Not carrying the called DTE address information when 
the originated call is accepted
Default: not carry
x25 response 
called-address
Cancel not carrying of the called DTE address 
information when the originated call is acceptedundo x25 response 
called-address (by 
default)
Not carrying the calling DTE address information when 
the originated call is accepted
Default: not carry
x25 response 
calling-address
Cancel not carrying the calling DTE address information 
when the originated call is acceptedundo x25 response 
calling-address (by 
default)
Check the address code block after the response of the 
call is received
Default: check
x25 
check-response-address 
(by default)
Cancel check the address code block after the response 
of the call is receivedundo x25  
check-response-address
OperationCommand
Specify the default upper layer protocol borne on 
X.25
Default: IP
x25 default-protocol [ ip 
| ipx ]
Cancel the specifying of the default upper layer 
protocol borne on X.25undo x25 default-protocol 
[ ip | ipx ]