Telecommunications - X.25 Parameters - Counters and Timers

Counters and Timers


Timer T1

Timer T2

Timer T3

Timer T4

N1 - Maximum number of bits in an I frame

N2 - Maximum number of transmissions

k - Maximum number of outstanding I frames

see also X.25 throughput issues


Timer T1

The value of the DTE timer 1 system parameter may be different than the value of the DCE timer T1, the value of the DCE T1 timer is agreed at subscription time, Public Network values are normally as follows:

packet length 128 256 512 1024
2400 bps 2.4 seconds 3.0 seconds 6.0 seconds 11.0 seconds
9600/19200 bps 1.0 seconds 1.0 seconds 2.0 seconds 3.0 seconds
48K / 64Kbps 0.5 seconds 0.5 seconds 0.5 seconds 1.0 seconds

The period of timer T1, at the end of which retransmission of a frame may be initiated according to the procedures described under error correction, will take into account whether T1 is started at the beginning or the end of the transmission of a frame (I dont know which Public Network uses).

The proper operation of the procedure requires that the transmitter's timer T1 be greater than the maximum time between transmission of frames and the reception of the corresponding frame returned as an answer to this frame. Therefore the receiver should not delay the response or acknowledging frame returned to the above frames by more than a value T2.

As far as I can work out the rules for Public Network are as follows:

In the case of Public Network if the timer expires, Public Network will send a command RR and restart T1, if this happens N2 times without a response then polling with SABMs starts, after another N2 times Public Network then polls with DISCs.


Timer T2

T2 is the amount of time available at the DTE or DCE before the acknowledging frame must be initiated in order to ensure its receipt by the other end, prior to timer T1 running out. ie (T2 < T1)


Timer T3

The period of Timer T3 will provide an adequate interval of time to justify considering the data link to be in a disconnected (out of service) state. (T3 >>> T4)

default: t1 * N2


Timer T4

Timer T4 is a DTE system parameter that may be used to identify an excessive period of no-activity on the data link, suggesting a possible faulty data or physical link condition.

The period of timer T4, which is a system parameter, represents the maximum time a DTE will allow without frames being exchanged

on the data link. The value of T4 should be greater than T1 and may be very large for applications which are not concerned with early detection of faulty data link or physical link conditions. T4>>T1.


N1 - Maximum number of bits in an I frame

The value of the DTE N1 system parameter will indicate the maximum number of bits in an I frame (excluding flags and 0 bits inserted for transparency) that the DTE or DCE is willing to accept from the remote end. In order to support universal DCE operation, a DTE will support a value of DTE N1 which is not less than 1080 bits (135 octets).


N2 - Maximum number of transmissions

The value of N2 will indicate the maximum number of attempts that will be made by the sender to complete the successful transmission of a frame.

Public Network default : 20


k - Maximum number of outstanding I frames

The frame window size - always 7 for Public Network

The value of the DTE k system parameter will be the same as the value of the DCE k system parameter.


 


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