Telecommunications - Throughput of X25 links

X.25 link level

• Counters and timers

X.25 Packet level

• Extended formats
• Packet Size
• Window size

also see configuration information

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Maximum Achievable Throughput (including overhead octets at levels 2 & 3

Assuming an average Network loop delay of 0.7s

Maximum data Packet size octets	Line Signalling Rate (kbit/s)
	2.4	4.8	9.6	48
128	1.51(2) 2.4(4)	2.27(2) 4.8(5)	3.04(2) 9.6(7)	4.16(2) 14.56(7) 48(24)
256	1.77(2) 2.4(3)	2.88(2) 4.8(4)	4.16(2) 9.6(5)	6.48(2) 22.7 (7) 48(15)
512	1.95(2) 2.4(3)	3.33(2) 4.8(3)	5.16(2) 9.6(4)	9.17(2) 32.09(7) 48(11)
1024	2.06(2) 2.4(3)	3.64(2) 4.8(3)	5.88(2) 6(4)	11.64(2)40.74(7) 48(9)

the values expressed represent - throughput, kbit/s (window size) respectively

Assuming an average Network loop delay of 2s

Maximum data Packet size octets	Line Signalling Rate (kbit/s)
	2.4	4.8	9.6	48
128	0.9 (2) 2.4(6)	1.11(2) 3.89(7) 4.8(9)	1.27(2) 4.44(7) 9.6(16)	1.43(2) 5.01 (7) 48(67)
256	1.23(2) 2.4(4)	1.67(2) 4.8(6)	1.97(2) 6.91(7) 9.6(10)	2.39(2) 8.37 (7) 48(48)
512	1.517(2) 2.4(4)	2.23(2) 4.8(5)	2.93(2) 9.6(7)	3.89(2) 13.61(7) 48(25)
1024	1.72(2) 2.4(3)	2.69(2) 4.84)	3.77(2) 9.6(6)	5.52(2) 19.32(7) 48(18)

the values expressed represent - throughput, kbit/s (window size) respectively

Public Network

..............................

x25 dte . . x25 dte

-- t1 -----?. -----------t3-------------? . ---- t1 ------?

. .

?- t2 ----- . ?----------t4------------- . ?--- t2 ------

. .

................................

2t1+t3 = time taken to transfer a full data packet between two

X25 DTEs

2t2+t4 = time taken to transfer an RR packet between two X25 DTEs

t3+t4 = network loop delay

assumptions

1. Both the sender and receiver are X25 DTEs
2. The time for the receiver to accept, process and acknowledge a data packet is negligible.
3. There is no queuing at the DTE/DCE interfaces (neither in the DTE nor in the network)
4. The time for the sender to process a P(R) and prepare a new data packet is negligible.
5. The data signalling rate, D, is the same at both X.25 interfaces.
6. The maximum attainable throughput indicates the overhead octets introduced at x25 levels 2 and 3.
7. Successive information frames transmitted on a customer access links and inter-exchange links are separated by a single flag.
8. That P(R)s are transported by RR packets.

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Calculation of maximum attainable throughput, D.

values of D were calculated using the formula :-

D= w * fm

--------------------

2(t1+t2) + ( t3+t4)

where w = window size

fm= length of I-frame carrying a full data packet

Calculations of the network loop delay (t3+t4) in table 1 were based on an estimated network transfer time t3 of 270 ms for an 128 octet data packet in what is expected to be a maximum length call in the UK packet-switched network.

For inter-network calls (table 2) the network loop delay was taken as :-

2 * (corresponding value in table 1) + 500 ms.

Values of T3+T4 are listed below:-

Max packet Network Loop Delay (t3+t4) ms

size (octets)	table 1	table 2
128	475	1450
256	560.5	1621
512	731	1964
1024	1072	2644

Note: we cannot guarantee or assure throughput outside the Public Network network

 

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