In order for end devices in a network to support X.25 calls, they need to be able to negotiate X.25 throughput parameters. This feature enables a router to handle that negotiation on behalf of end devices that cannot do it themselves.
Figure 1. Router Negotiating Throughput Between a Network and an End Device
The router does this by stripping out or inserting values, as appropriate for each case, in the "throughput facility field" of the X.25 calls’ setup and confirmed messages (specifically, in the Call Request, Incoming Call, Call Accepted, and Call Confirmed packets).
In order to insert values appropriately, the router interface connected to the end device must earlier have been configured with the input and output bit rates that are intended to be used by the eventual X.25 call.
The rules according to which the router removes or inserts those bit rates are set by the
x25
subscribe
throughput command, which can have three distinct states:
no,
basic or
never. These forms of the command work as follows when the router receives a call from the network and forwards that call onward to the end device:
- If the router has been configured by the command
no
x25
subscribe
throughput, it will make
no
change to the values it receives in the call’s facility field. The router merely forwards the message, and those values, onward.
- If the router has been configured by the
25
subscribe
throughput
basic form of this command, the router will
insert the bit rate values previously configured on its interface into the call’s facility field. (The only exception is when those values are larger than the call’s values, in which case the router will leave the call’s smaller values in place when it forwards the message.)
In cases when the router has substituted its own configured values for the values it detected in the incoming call, the router also reports those new values in a Call Confirmed packet back out through the network to the source device.
- If the
x25
subscribe
throughput
never form of the command has been entered, the router will
remove the values it receives in the call’s facility field. (And if the values previously configured on the router’s interface are
smaller than those contained in the call, the router will also replace the call’s values with those smaller ones when it forwards the end device’s Call Confirmed packet back out to the network.)
How these behavior rules apply to each possible case is presented in the first table below.
When calls originate not in the network but in the end device, this command’s three states can have somewhat different results, which are detailed in the second table below.
Table 1 Router Treatment of Throughput Facility Field in Incoming Call
Incoming call’s ‘Call Request’ packet
|
Cisco IOS commands applied
|
Results
|
Is interface configured with throughput values?
|
How is Serial Line’s throughput subscription configured?
|
Within ‘Incoming Call’ packet
|
Within ‘Call Confirmed’ packet
|
Contains throughput facility field
|
YES
:
"x25 facility throughput
xxx
yyy
|
no
x25 sub throughput
|
Facility field in message from network is sent to end device unmodified.
|
End device includes no facility field in its Call Accepted packet to the router. And the router includes no facility field in the Call Confirmed packet it sends out to the network.
|
x25 sub throughput
never
|
Router strips out facility field, then forwards message to end device.
|
Router sends values out to network only if the values configured on its interface are
smaller than those received in network call.
|
x25 sub throughput
basic
|
Router compares values in message with those configured on its interface, and sends to end device the lower set.
|
Router sends that lower set out to the network.
|
Has no throughput facility field
|
no
x25 sub throughput
|
No facility field sent to end device.
|
No facility field sent out to network.
|
x25 sub throughput
never
|
No facility field sent to end device.
|
No facility field sent back out to network.
|
x25 sub throughput
basic
|
Router inserts facility field into message, and forwards that to the end device.
|
No facility field sent back out to network.
|
Contains throughput facility field
|
NO
:
"no x25 facility throughput"
|
no
x25 sub throughput
|
Facility field sent to end device.
|
End device includes no facility field in its Call Accepted packet to the router. And the router includes no facility field in the Call Confirmed packet it sends out to the network.
|
x25 sub throughput
never
|
Router strips out facility field, then forwards message to end device.
|
No facility field sent back out to network.
|
x25 sub throughput
basic
|
Facility field sent on to end device.
|
No facility field sent back out to network.
|
Has no throughput facility field
|
no
x25 sub throughput
|
No facility field sent to end device.
|
No facility field sent out to network.
|
x25 sub throughput
never
|
No facility field sent to end device.
|
No facility field sent back out to network.
|
x25 sub throughput
basic
|
No facility field sent to end device.
|
No facility field sent out to network.
|
*Shaded rows (in PDF version) describe calls that contain no throughput facility field before they reach the router.
Table 2 Router Treatment of Throughput Facility Field in Outgoing Call
Outgoing call’s ‘Call Request’ packet
|
Cisco IOS commands applied
|
Results
|
Is interface configured with throughput values?
|
How is Serial Line’s throughput subscription configured?
|
Within outgoing ‘Call Request’ packet
|
Within received ‘Call Confirmed’ packet
|
Contains throughput facility field
|
YES
:
"x25 facility throughput
xxx
yyy
|
no
x25 sub throughput
|
Router forwards facility field it receives in the end device’s Call Request packet out to the network unmodified.
|
Router forwards facility field it receives in the Call Confirmed packet from the network on to the end device unmodified.
|
x25 sub throughput
never
|
Router refuses to forward call on to the network, and cancels it, sending back to the end device a Clear Request packet with the Cause Code field set to 3 (‘3’ stands for "Invalid Facility Request"). Router also sends to the end device a Diagnostic Code field set to 65 (which stands for "Facility Code Not Allowed").
|
x25 sub throughput
basic
|
Router compares values in message with those configured on its interface, and sends to network the lower set.
|
Router sends that lower set to the end device, unless still different values are received in
the
Call
Confirmed
message from the network. In that case, the router forwards that network set to the end device.
|
Has no throughput facility field
|
no
x25 sub throughput
|
No facility field sent to network.
|
No facility field sent to end device.
|
x25 sub throughput
never
|
Router sends values configured on its interface out to the network.
|
No facility field sent to end device.
|
x25 sub throughput
basic
|
Router inserts facility field into message, and forwards that to the network.
|
Router sends the inserted facility field to the end device.
|
Contains throughput facility field
|
NO
:
"no x25 facility throughput"
|
no
x25 sub throughput
|
Router forwards facility field it receives in the end device’s Call Request packet out to the network unmodified.
|
Router forwards facility field it receives in the Call Confirmed packet from the network on to the end device unmodified.
|
x25 sub throughput
never
|
Router refuses to forward call on to the network, and cancels it, sending back to the end device a Clear Request packet with the Cause Code field set to 3 (‘3’ stands for "Invalid Facility Request"). Router also sends to the end device a Diagnostic Code field set to 65 (which stands for "Facility Code Not Allowed").
|
x25 sub throughput
basic
|
Facility field sent on to network.
|
Facility field sent back to end device.
|
Has no throughput facility field
|
no
x25 sub throughput
|
No facility field sent to network.
|
No facility field sent to end device.
|
x25 sub throughput
never
|
No facility field sent to network.
|
No facility field sent to end device.
|
x25 sub throughput
basic
|
No facility field sent to network.
|
No facility field sent to end device.
|
*Shaded rows (in PDF version) describe calls that contain no throughput facility field before they reach the router.