Call performance studies on the ATM forum UNI signalling implementations

Abstract

Asynchronous Transfer Mode (ATM) is widely accepted as the transfer mode for Broadband Integrated Services Digital Networks (B-ISDN). As ATM networks provide connection-oriented services, signalling is an essential component of these networks. The performance of the signalling system is crucial in determining the scalability of the network. Call performance measurements on two different ATM switches were carried out using a broadband network analyser. Calls are generated according to a deterministic/Poisson distribution and various components of the call set up delay are analysed. The probability distribution of the delays is obtained and the correlation between the set up delay and its different constituents is established. It is observed that the delays at the calling and the called user entities of the UNI form a major component of the call set up delay. The average call set up delay is found to be 21 and 27 ms, respectively, when a single switch is involved in call set up and 50 ms when two switches are involved in call set up. With simultaneous call arrivals, though with higher delays, both the switches are found to establish calls at the rate of 100 calls/s.

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CALL PERFORMANCE STUDIES ON THE ATM FORUM UNI SIGNALLING
IMPLEMENTATIONS
Su Kwe Long, R. Radhakrishna Pillai1, Jit Biswas1, and Tham Chen Khong
Department of Electrical Engineering
National University of Singapore
10 Kent Ridge Crescent, Singapore 119260.
e-mail: gisskl@sgh.gov.sg; {pillai,jit}@iss.nus.sg; cktham@cesun1.ee.nus.sg

1 Institute of Systems Science, National University of Singapore, Heng Mui Keng Terrace, Kent Ridge, Singapore 119597.
2 According to ITU-T draft Recommendation Q.2931, the Signalling virtual channel uses VPI=0, VCI=5.
KEYWORDS
Computer systems, Telecommunications, Emulation, Performance
analysis, Communications
ABSTRACT
Asynchronous Transfer Mode (ATM) is widely accepted as the transfer
mode for Broadband Integrated Services Digital Networks. Since ATM
networks provide connection-oriented services, signalling is an essential
component of these networks. The performance of the signalling system is
crucial in determining the scalability of the network. Call performance
measurements on two different ATM switches were carried out using a
broadband network analyser. The performance measures considered are
call setup and release delays and the call throughput of the switches. Calls
are generated according to a deterministic/Poisson distribution and
various components of the call setup delay are analysed. The probability
distribution of the delays are obtained and the correlation between the
setup delay and its different constituents are established. It is observed
that the delays at the calling and the called user entities of the UNI form a
major component of the call setup delay. The average call setup time with
the switches are found to be 21 and 27 msecs respectively. The call setup
delay does not seem to increase linearly with the number of ATM switches
involved whereas the call release delay increases linearly with the number
of ATM switches involved. With simultaneous call arrivals, though with
higher delays, both the switches are found to establish calls at the rate of
100 calls/sec.
INTRODUCTION
Asynchronous Transfer Mode (ATM) is widely accepted as transfer mode
for Broadband Integrated Services Digital Networks (B-ISDN). Since ATM
networks provide connection-oriented service, signalling is an essential
component of such networks. The performance of the signalling system is
crucial in deciding the scalability of a communication network. Today
several switch vendors have implemented the ATM Forum UNI
specifications for signalling. This paper reports the call performance
studies on the ATM Forum UNI 3.0 signalling implementations.
The measurement was carried out on Fore ASX-100 and ASX-200 ATM
switches using a Hewlett-Packard Broadband Series Test System (BSTS).
The BSTS was connected to the ATM switch through OC-3 links operating
at 155 Mbps and call requests were generated using its UNI emulation
function (Figure 1). The call requests followed a Deterministic/Poisson
distribution. The signalling response from the switch was analysed to
determine the delays involved in call setup and release. The call throughput
of the switch is determined by stressing the switch with simultaneous call
requests.
Figure 1 Hardware setup for the measurement
ATM FORUM UNI SIGNALLING
The procedure involved in setting up a switched virtual circuit (SVC)
according to UNI 3.0 specification [ATM Forum 1995] is shown in Figure
2.
Call Request
The calling party (client) initiates call establishment by transferring a
SETUP message on the signalling virtual channel2 across the interface and
starting timer T303. This SETUP message contains several information
elements (IE) to identify the message, specify various ATM Adaptation
Layer (AAL) parameters, calling and called party addresses, Quality of
Service (QOS) requirements, and a number of other elements if needed.
Following the transmission of the SETUP message, it is considered to be in
the Call Initiated state. If no response to the SETUP message is received by
the user before the first expiry of timer T303, the SETUP message may be
retransmitted and timer T303 is restarted. If the user does not receive any
response to the SETUP message after the second expiry of timer T303, the
user shall internally clear the call.
Tx
Rx
R
Rx
HP Broadband Series Test System
ATM Switch
Attenuator
Tx

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Call Proceeding
If the network and called party (server) can determine that access to the
requested service is authorised and available, the network and called
party may send a CALL PROCEEDING message to the calling user to
acknowledge the SETUP message and indicate that the call is being
processed, and enter the Incoming Call Proceeding state. In this state, no
more call establishment information is needed, nor will any call request
be accepted. When the calling party receives the CALL PROCEEDING
message, the user shall stop the timer T303, start timer T310, and enter the
Outgoing Call Proceeding state.
Figure 2 UNI Signalling for connection setup
Call Confirmation
Upon receiving an indication that the call has been accepted, the
called party shall send a CONNECT message across the interface to
the calling party and enter the Active state. This CONNECT message
contains some parameters such as call reference, message type,
accepted AAL parameters and other identifiers created as a result of
the IEs in the SETUP message. On receipt of the CONNECT
message, the calling party shall stop timer T310, send a CONNECT
ACKNOWLEDGE and enter the Active state. If the calling party has
received CALL PROCEEDING message, but does not receive a
CONNECT or RELEASE message prior to the expiration of timer
T310, then the calling party shall initiate clearing procedures towards
the network.
Call Release
ATM Client ATM ServerATM Switch
SETUP
SETUP
CALL PROCEEDING
CALL PROCEEDING
CONNECT
CONNECT
CONNECT ACK
CONNECT ACK
Pcell
SW
PD
PRser
PRcl
PRsw
Call
Setup
Delay
T
Call
Processing
Time
R
Time
PD - Propagation Delay
Pcell - Cell Transmission Time
SW - ATM Switch switching time
PRser - Delay between receiving
SETUP message and sending
CONNECT message at server
PRcl - Delay between receiving
CONNECT message and
sending CONNECT ACK
message at client
PRsw - Delay between receiving and
sending CONNECT message
at switch

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If the calling party wants to release the connection, the party shall
send a RELEASE message to the called party, start timer T308,
disconnect the virtual circuit and enter Release Indication state. This
message only contains the basic information to identify the message
across the network. Once the virtual channel used for the call has been
disconnected, the called party shall send a RELEASE COMPLETE
message to the network and enter the Idle state.
On receipt of the RELEASE COMPLETE message, the calling party
shall stop timer T308 and enter the Idle state. The called party can
initiate the call release too, and all the responses will occur in the
reverse direction.
Figure 3 Call setup delay distribution
Call Setup Delay
The call setup delay is defined as the delay between the connection
request (sending SETUP message by the client) and connection
confirmation (sending CONNECTION ACKNOWLEDGE by the
client). It is denoted by T in Figure 2.
In order to measure the delay, the following assumptions are made:
•
The propagation delay PD is negligible as the distance from the
user to switch is less than 100 m.
The Segmentation and Reassembly (SAR) delay at an ATM
user is negligible as most of the UNI messages can be
segmented into one ATM cell.
The cell transmission time Pcell is small and ignored because of
the small cell size (53 bytes).
The time stamp is made at ATM layer.
•
•
•
Let the delay between receiving CONNECT and sending CONNECT
ACKNOWLEDGE at client be PRcl, the delay between receiving
SETUP and sending CONNECT at server be PRser and the delay
between receiving CONNECT and sending CONNECT
ACKNOWLEDGE at switch be PRsw. When the switch receives
SETUP message, it takes SW to route the SETUP message to the
destination. Thus the call setup delay T can be expressed as:
T = SW + PRser + PRsw + PRcl (1)
where the sum of PRser and PRsw is termed call processing time (R) at
server.
Equation (1) is valid only if the ATM switch can handle UNI
Signalling messages simultaneously. However, due to the scheduling
of CPU in ATM switch, there is a small delay when the switch sends
SETUP message to the server and CALL PROCEEDING message to
the client. Similarly, there is a small delay when the switch sends
CONNECT to the client and CONNECT ACKNOWLEDGE to the
server. In order to make the measurement more accurate, these two
delays must be deducted from the Equation (1).
Throughput
The throughput is defined as the ratio of the number of successful
connection requests over the number of connection requests made
per unit time. In another words, the throughput is the ratio of the
number of successful call setup over total number of call setup
requests generated.
RESULTS AND DISCUSSION
Call Setup Delay Measurement
In this measurement, the effect of the following factors on call
performance are examined:
•
•
•
The characteristic of ATM switches
The effect of call holding time
The effect of number of hops on ATM connection setup delay
The Fore ATM switch was connected to two Optical Line Interfaces
of HP BSTS, where each of them was emulated as either ATM client
or server (Figure 1). The call setup requests were generated at a
constant rate of 1 call per second and later the calls were released at
the same rate. The results obtained from a sample size of 1000 for
the ATM switches Fore ASX-100 and ASX-200 are listed in Table 1
and Table 2.
DelayMean
(ms)
5.533769
4.869945
5.268169
10.403714
21.669853
Table 1 Call setup delays at Fore ASX-100
Standard
Deviation
4.456737
2.141427
2.752163
4.946343
6.530267
Minimum
(ms)
3.748700
3.679700
3.727500
7.701000
16.943200
Maximum
(ms)
53.789600
22.458000
38.099900
58.126800
67.478100
Prser
PRsw
PRcl
R
T
DelayMean
(ms)
6.111260
8.225683
4.944853
14.336943
27.060092
Table 2 Call setup delays at Fore ASX-200
Standard
Deviation
4.303982
8.039236
1.077882
9.614887
12.536468
Minimum
(ms)
3.851700
4.911600
3.783800
10.050900
20.072900
Maximum
(ms)
57.277400
137.730400
31.277600
143.682200
157.275300
Prser
PRsw
PRcl
R
T
The call setup delay at ASX-100 is shorter than that of ASX-200.
Comparison of these two sets of results shows that PRsw makes the
difference. This delay is the only switch-related delay in the
measurement, other delays are user-related. The results show that
ASX-200 switch needs 3.36 ms more to assign VPI and VCI values
for the calling (client) and called party (server) compared to ASX-
100 switch.
To calculate the value of SW, Equation (1) is used. The following
calculation is used for ASX-100:
Fore ASX-100
Fore ASX-200
152025303540
0
0.02
0.04
0.06
0.08
0.1
Call Setup Delay (ms)
Probability

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Figure 4: The effect of call holding on call setup delay
SW = T - (PRser + PRsw + PRcl)
= 21.67 - (5.53 + 4.87 + 5.27) ms
= 6.00 ms
Similarly, the value of SW at ASX-200 is calculated as 10.43 ms.
However, due to the scheduling delay mentioned earlier, the actual
value of SW should be smaller. After synchronising the ATM client
and server, the scheduling delay was found to be 1.44 ms at ASX-
200. Thus,
SW = 10.43 - 1.44 = 8.99 ms
As the ASX-100 switch does not respond with CALL PROCEEDING
message back to client when it receives SETUP message, this
scheduling delay does not involve in SW measurement at ASX-100
switch.
From the calculation, it shows that the value of SW is 2.99 ms
shorter at ASX-100 switch than that at the ASX-200 switch. Again,
the value of SW is switch-related.
In order to find the relationship between call setup delay and various
delay parameters in Equation (1), the correlation coefficients3
between call setup delay and the various delay components are
calculated and listed in Table 3.
Delay Correlation Coefficient
ASX-100
0.632991
0.415092
0.398215
ASX-200
0.364740
0.116607
0.746436
PRser
PRsw
PRcl
Table 3 Correlation coefficients between call setup delay and various
delay components (single switch)
The results show that call setup delay has a stronger linear
relationship with PRser at ASX-100 switch, while the call setup delay

3 If Correlation is rxy , µx and µy are mean of x and y, then Correlation
xy r
σσ
Coefficient ρxy =
yx
yx
µµ
−
, where σx and σy are standard
deviation of x and y.
at ASX-200 has a stronger relationship with PRcl. The call setup delay
distribution for a single switch is plotted in Figure 3.
To measure the effect of call holding on call setup delay, the ATM
connections were setup one after another and not released. The call
setup delay for every call was measured. Figure 4 shows the mean
call setup delay (each data point is the mean of 5 measurements)
under different number of unreleased calls in the system. There was
no difference in call setup delay with increase in the the number of
unreleased calls. This shows that the call holding has no effect on the
call setup delay.
Next, in order to measure the effect of number of hops on the call
setup delay, two switches are used. The emulated server was
connected to ASX-100 and the emulated client was connected to
ASX-200. The two switches were connected to each other. The
measurement results are shown in Tables 4 and 5.
DelayMean
(ms)
4.287765
5.200153
4.568313
9.485909
50.403307
Standard
Deviation
2.794265
2.818096
3.464478
3.908586
25.809557
Minimum
(ms)
3.690900
3.681600
3.278200
7.553600
27.227400
Maximum
(ms)
42.961300
22.621000
47.376900
47.053800
196.865900
Prser
PRsw
PRcl
R
T
Table 4 Call setup delays with two switches
Figure 5 Call setup delay distribution with two switches
Figure 6 Call setup delay under different number of simultaneous
calls
010 20304050
0
5
10
15
20
25
30
35
40
Number of Unreleased Calls
Call Setup Delay (ms)
Mean = 20.402986 ms
Std. Dev. = 2.396822
Min. = 17.231200 ms
Max. = 26.790575 ms
2040 6080100120140
0
0.005
0.01
0.015
0.02
0.025
Probability
0.03
0.035
0.04
Call Setup Delay (ms)
Fore ASX-100
Fore ASX-200
20406080100
0
500
1000
1500
2000
Number of Simultaneous calls
Mean Call Setup Delay (ms)

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Figure 7 Call processing time under different number of
simultaneous calls
Delay
PRser
PRsw
PRcl
Correlation Coefficient
0.053468
0.046853
0.029577
Table 5 Correlation coefficients between call setup delay and various
delay components (two switches)
The results show that the call setup delay with two switches is higher
than the sum of the setup delays measured previously at two
individual switches. By comparing the results with single and two
switches, it is noticed that the delay parameters in Equation (1) do
not vary much. From the correlation coefficients, it shows that the call
setup delay has little relationship with the delay components. The
main contribution to the call setup delay with two switches is the
delay involved between two ATM switches. The delay related to
ATM network (ATM switches) can be calculated as:
T - PRser - PRcl
= 50.40 - 4.29 - 4.57 ms
= 41.54 ms
This delay is much larger than that of a single switch; 10.87 ms for
ASX-100 and 16.01 ms for ASX-200 .
Since the communication and control between two ATM switches are
handled by Network-Network Interface (NNI), the details of NNI
Figure 8 Throughput under simultaneous call generation
delays are not discussed in this paper. However, the results show that
NNI introduces a significant delay in call setup involving multiple
switches. The call setup delay distribution with two switches is
plotted in Figure 5.
Throughput Measurement
In this measurement, simultaneous4 call requests were
generated by the analyser to measure the throughput for each switch.
Number of call requests and number of successful calls were
recorded, as well as the call setup delay of the successful calls.
Because of the software and hardware limitation, a maximum of 100
simultaneous calls were generated. The focus was on the increase in
call setup delay when the number of simultaneous calls increased. The
measurement results are listed in Figure 6, Figure 7 and Figure 8.
Each data point is the mean value from 5 experiments.
Notice that the two ATM switches performed differently in this
measurement. The call setup delay at ASX-100 increased
exponentially when the number of simultaneous calls increased, but
that of ASX-200 increased logarithmically when the number of calls
was beyond 30. The increase in delay was caused by the queuing
delay at the switch as the switch can handle only 100 calls per
second[Fore Systems]. The server call processing time also increased
in the same manner as the call setup delay because the incoming call
request rate was faster than the server response rate. Though the
delays are different, both the Fore ATM switches yielded 100%
throughput.
To find out which delay parameter causes the increase of the call
setup delay, the correlation coefficients between call setup delay and
various delay components calculated as shown in Tables 6 and 7.

4 Because of hardware limitation, the ÒsimultaneousÓ calls are in fact
generated one after another with interval of 5 ms.
Fore ASX-100
Fore ASX-200
20406080100
0
500
1000
1500
Mean Call Processing Time (ms)
Number of Simultaneous calls
2040 6080100
10
20
30
40
50
60
70
80
90
Number of Call Requests
Number of Successful Calls