EFFECTS OF VARIABLE BIT RATE VOCODER ON VOIP QOS

Yael Dux-Rosenberg , Ariel J. Frank

Department of Computer Science, Bar-Ilan University, Ramat-Gan, 52900, Israel

Salomon Serfaty

R&D, Motorola Communications Israel Ltd, Tel-Aviv

Keywords: VoIP, QoS, LAN, WLAN, CRV, CVRV, vocoder

Abstract: Transmission of voice over packet switched networks, such as the Internet (VoIP), has been gradually

evolving due to the advantages it can provide to the different end-users (private user, integrated networks

service providers, business arena, etc). However, in order for VoIP to be commonly used, the Quality of

Service (QoS) offered by VoIP needs to be at least as high as the traditional Plain Old Telephone Service

(POTS).In this research, we aim to improve the QoS parameters of the developing VoIP technology by

substituting the traditional constant bit rate vocoder (CRV) with a new type of vocoder that is based on

continuously variable bit rate (CVRV). Comparative studies of these two vocoders are performed in the

following 3 independent scenarios:

1. LAN, in which the connected terminals transfer/receive voice only.

2. LAN, in which the terminals exchange mixed traffic classes of both voice and data.

3. WLAN, in which the connected terminals transfer/receive voice only.

The results of scenario 3 show a significant improvement in performance with use of CVRV in WLAN

when more than 50 terminals are involved, as exhibited in all the QoS parameters that were tested. The

results of the WLAN are especially interesting and significant as the WLAN is becoming progressively

more common nowadays.

1 INTRODUCTION

Voice has been traditionally transmitted using circuit

switched networks developed specifically for this

purpose while data has been transmitted on packet

data networks. The progress in communication

technology has brought about faster switches,

broader bandwidth and new horizons, such as the

integration of voice and data transmission on the

same digital network.

The mechanism used for transporting voice over an

IP based packet switched network is referred to as

Voice over Internet Protocol (VoIP). In order for

VoIP to be commonly used in the market, the

Quality of Service (QoS) offered by VoIP to the

users, needs to be as good as the traditional Plain

Old Telephony Service (POTS). QoS can be

measured and evaluated by the following individual

parameters: delay, jitter and packet loss.

2 GOALS

In this research, we analyse the performance of two

different vocoders that produce input traffic

(packets) at different rates. The Constant Rate

Vocoder (CRV) produces constant length packets

and the Continuously Variable Rate Vocoder

(CVRV) produces variable length ones. Since the

performance of statistical multiplexing is highly

dependent on input traffic, packets originating from

different modelled sources are expected to exhibit

individual performance results. Based on the above,

it is hypothesized that CVRV could outperform

CRV in terms of VoIP QoS parameters.

The article is organized as follows. Section 3

reviews the model requirements and the offline

simulation model is described in section 4. Section 5

gives the implementation, followed by its PME.

Lastly, conclusions and future directions are given.

Dux-Rosenberg Y., J. Frank A. and Serfaty S. (2004).

EFFECTS OF VARIABLE BIT RATE VOCODER ON VOIP QOS.

In Proceedings of the First International Conference on E-Business and Telecommunication Networks, pages 93-101

DOI: 10.5220/0001393300930101

 SciTePress

3 MODEL REQUIREMENTS

For this research, we require the following

environment characteristics for analysing CRV vs.

CVRV vocoders:

1. Connectivity – The environment needs

to support connectivity between any

number of terminals. The connection can be

between one terminal to another, one

terminal to many terminals or many

terminals to many terminals. The

communication between the connected

terminals can take place as terminals are

receiving data, transmitting or idle.

2. Data Transmission – The environment

needs to support transmission of the

different data types sent by the terminals

and/or the different accessories connected

to them. This input/output data includes

voice, data and multimedia.

3. Event Logging – In order to be able to

analyse the results, all events and processes

should be recordable so that a log file can

be generated.

4 OFFLINE SIMULATION MODEL

This research experiments were carried out on a

simulated network using NS (a Network Simulator)

and additional scripts, so as to achieve the above

requirements. The architecture used, in reference to

the 7-layer OSI model, includes the following layers

(as shown in Figure 1).

4.1 Application Layer

The Application layer supplies the different types of

information: voice and data, which are transferred

by the terminals through the network. The voice

packets are produced by the two different vocoders,

CRV and CVRV, using the same recorded

conversation.

The Continuously Variable Rate Vocoder

(CVRV) is the new vocoder type used in this

research that has the following properties:

‘Continuously’

, i.e., packets are produced at a

constant rate (every 64ms).

‘Variable Rate’

, i.e., the packet length produced

is variable with the subsequent variable rate (with

average length of 82B).

The Constant Rate Vocoder (CRV) is the

traditional vocoder:

‘Constant Rate’

, i.e., packet length is constant

and produced at constant rate. This vocoder

produces packets at a constant length of 82B every

64ms.

The data packets that the terminals transmit are

assumed to originate in an FTP application that is

used as data source for the data terminals.

Figure 1: CRV/CVRV Architecture Reference Model

ICETE 2004 - WIRELESS COMMUNICATION SYSTEMS AND NETWORKS

4.2 Transport Layer

The Transport layer supplies the end-to-end

connectivity by utilizing TCP for terminals

transmitting data and UDP for terminals transmitting

voice.

4.3 Network Layer

The Network layer supplies the connectivity

between hosts. IP is utilized for voice and data

terminals.

4.4 Data link layer and Physical layer

The Data Link and Physical layers are responsible

for moving data to/from the physical link. A LAN or

WLAN are used for this purpose.

4.5 Analysis Module

The purpose of the Analysis module is to produce

the VoIP QoS values, which are delay, jitter and

packet loss, for all test cases derived from the NS

log files. Another parameter that is tested is the

"application packet loss". The application packet

loss parameter measures the percentage of packets

that do not arrive at the destination in their relevant

time frame. These packets become “irrelevant” and

are dropped by the application. Results for this

parameter are calculated repeatedly, for increasing

delays in the play-out buffer.

The delay, jitter, packet loss and application

packet loss, are then used as basic measures for

assessment of the vocoder behaviour.

5 IMPLEMENTATION

The implementation is used for experimentation

with three different scenarios, in which information

(voice or voice+data) is exchanged between

terminals. Each scenario tests the effects of CVRV

on VoIP QoS by checking various network

parameters. The there scenarios include the

following environments: 1) voice traffic over LAN,

2) voice and data traffic over LAN, and 3) voice

traffic over WLAN. Each of these tests is performed

repeatedly with an increasing number of

participating terminals, in order to study the effect of

increasing load on the network.

In the aforesaid experiments, the following

characteristics are studied:

1. Performance of CVRV vs. CRV in the different

scenarios.

2. Influence of the network load on the

performance of each individual test case.

3. Significance of the results as tested by the

repeated runs.

6 PERFORMANCE

MEASUREMENTS

EVALUATION

Here we describe the three scenarios (numbered 1-3)

and their results when comparing CRV and CVRV

vocoders.

6.1 Voice Traffic over LAN

In this Scenario 1, the comparison of CRV and

CVRV is done on a LAN network (802.3 IEEE),

with voice traffic only. Isolating the traffic to voice

packets enables investigation of the behavior of the

Figure 2: Delay Results

EFFECTS OF VARIABLE BIT RATE VOCODER ON VOIP QOS

two vocoders, while modelling the difference

between them.

The scenario is repeatedly tested with an

increasing number of participating terminals,

starting with 100 terminals and going up to 500

terminals (limited by the LAN's steady state, which

is when it's at maximum capacity).

Following we detail the results of the QoS

parameters for this test case.

6.1.1 Delay

The results shown in Figure 2 show that as the

number of terminals increase from one test case to

another, there are more terminals ready to transmit,

and the delay and jitter grow respectively.

6.1.2 Jitter

The jitter increases as the number of packets ready

to be transmitted increase, due to more terminals

participating in the test case. It should be noted that

when only 100 terminals are connected to the LAN,

the jitter is 0. This means that the network transmits

the packets as soon as they are ready to be

transmitted; there is no queue delay.

6.1.3 Packet Loss

None of the test cases in this scenario suffer from

network packet loss. All the transmitted packets

reach their destination.

6.1.4 Application Packet Loss

As seen, no significant difference was found in the

behaviour of CRV and CVRV comparing the delay,

jitter and packet loss results.

However, this isn't true for application packet

loss. Figure 3 shows the results for a test case with

300 terminals. The results of the application packet

loss parameters where at the same level for 100, 200,

400 and 500 terminals as well.

The above results show that in this studied

experiment, as the play-out buffer delay increases,

more packets arrive at the destination in their

relevant time frame, enabling them to be played out

rather than dropped.

It can also be seen in the results that in the range

of 0-50µs, CVRV loses significantly more packets in

comparison to CRV. It is a gap of 37%, 41%, 32%,

35%, 15% at 0µs for 100, 200, 300, 400, 500

terminals, respectively. In the range of 50-250µs, the

packet loss does not differ significantly between the

two vocoders.

In the experiment described in Figure 3, it is

seen that in CVRV vs. CRV test cases, more packets

are also lost when the play-out buffer adds a relative

small delay (0-50us). Furthermore, it should be

noted that the percentage of long packets lost (for

100, 200, 300, 400 and 500 terminals, respectively)

exceeds their prevalence (58%) in the overall packet

population.

6.1.5 Summary and Discussion

In this Scenario 1, it was demonstrated that there is

no substantial difference in the delay and jitter

results of the two vocoders. The behavior of delay

and jitter under these circumstances shows an

exponential trend line that is in agreement with the

literature. The low linear phase in the range of 100-

400 terminals, followed by the exponential growth

between 400 to 500 terminals, corresponds to the

upper limit or saturation of the LAN's load. 600

terminals are out of the testing range.

In this scenario, no network packet loss was

found in the vocoders.

The main difference between the two vocoders

Figure 3: Application Packet Loss Results

ICETE 2004 - WIRELESS COMMUNICATION SYSTEMS AND NETWORKS

in this scenario is in the results of the application

packet loss parameter.

In this scenario, CVRV shows a bigger

application packet loss than CRV in the range of

0-50µs (delay added by the play-out buffer). In this

range, test cases utilizing CVRV lose more packets

than the corresponding test cases using CRV.

It turns out that this phenomenon is due to the

length of the packets. Longer packets travel for a

longer time to the destination in comparison to

shorter ones. This network delay adds to the overall

delay of the packet. For delays longer than 50µs,

there is no difference between the two vocoders.

This is explained by the non-significant contribution

of the network delay when added up to the actual

delay, on top of the play-out buffer.

For demonstration: the length of an average

packet is 82B, the length of the longest packet is

104B, therefore the overhead in travel time for

packets longer than the average would be maximum

17.6µs, as per the following calculation:

When these 17.6µs of delay are added on top of

the 0-50µs delay of the play-out buffer, the addition

of the packet length is significant and influences the

results of the total application packet loss parameter.

The results for the application packet loss

parameter show that for the test case of 500

terminals, the difference in the performance of CRV

and CVRV is less significant compared to the other

test cases (100-400). This is explained by the fact

that the test case with 500 terminals suffered from a

much longer delay and jitter relatively to 100-400

terminals. This causes the effect of the network

delay (caused by packet length) to be less significant

(Queue Delay >> Network Delay), affecting less the

difference in packet loss.

According to the above, when the play-out

buffer for CVRV is designed for the receiver’s end,

the delay added will be according to the maximum’s

packet length, rather than the average packet length,

as in CRV. In order to achieve the same packet loss

rate in CVRV and CRV, the delay added in the play-

out buffer of CVRV will need to be longer,

increasing the total delay time of the packets.

6.2 Voice+Data Traffic over LAN

The scenario of only voice traffic is an isolated case.

The more common situation is a LAN where both

voice and data are transmitted. Scenario 2 that is

investigated here is designed to support both voice

and data traffic. The ratio of voice vs. data terminals

is 1:1. Every data terminal has a ready packet to be

transmitted. The voice terminals perform according

to the pre-designation of CRV and CVRV vocoder,

respectively. The measurements and statistical

studies are applied to the voice packets only. The

scenario is carried out repeatedly with an increasing

number of terminals.

Following we detail the results of the QoS

parameters for this test case.

6.2.1 Delay, Jitter, Packet Loss and

Application Packet Loss

The results for delay, jitter, packet loss and

application packet loss show that when both voice

and data are supplied to CRV and CVRV vocoders,

no significant difference is manifested by the two

vocoder types. These parameters: delay, jitter and

packet loss, increase correspondingly to the number

of added terminals.

6.2.2 Summary and Discussion

In this scenario, of transmission of voice and data,

we have seen no significant difference between the

two vocoders. This is explained below.

The comparison between the vocoders was

designed in such a way that the surrounding

environment and its features are as close as possible.

In this scenario that mixes voice and data, the only

difference between CRV/CVRV test cases is the

length of the voice packets. The data packets are

always ready to be transmitted and a voice packet is

ready every 64ms. Consequently, the packets are

transmitted at the exact same times in both vocoders.

According to the behaviour of the LAN, when

the medium is free, a packet is transmitted.

Collisions occur only when more than one terminal

senses the medium as free, and transmits a packet.

The packets transmitted from multiple terminals

simultaneously collide and will need to be

retransmitted. It takes the transmitting terminal a

constant period of time to notice that the packet it

has sent is corrupted due to collision (twice the

propagation time). According to the test case setup,

the packets in CRV and CVRV scenarios are sent at

the same time, and the collisions occur at the same

times, respectively. The identification, in a constant

time, of a collision eliminates the difference between

the voice packet lengths, controlling the scenario

EFFECTS OF VARIABLE BIT RATE VOCODER ON VOIP QOS

results and maintaining the uniformity in the two

vocoders test cases.

However, a comparison between Scenario 1 and

Scenario 2 shows a remarkable difference in the

delay and jitter value's range of the two scenarios

(0-1ms for Scenario 1 and 19-110ms for Scenario 2),

whereas the performance of the two vocoders in

delay and jitter is similar. This is explained by the

presence of the data packets in Scenario 2, while

Scenario 1 was designed for voice packets only.

There is a substantial difference between the

following two inputs:

1. Data packets are significantly

longer than the voice packets

(500B vs. 82B average).

2. Data packets are always ready

to be sent (rather than every

64ms as in voice).

These two different characteristics result in a

longer queuing of voice packets waiting for the

channel to be idle. As an outcome, the delay and

jitter in Scenario 2 are in the range of tens of ms

rather than µs, as in Scenario 1.

Comparing the application packet loss parameter

results of the two scenarios shows that this

parameter behaves in a different manner than the

delay and jitter ones. The application packet loss is

significantly higher in CVRV than in CRV in

Scenario 1, whereas in Scenario 2, the application

packet loss is similar in both vocoders. The

explanation for this finding is as follows. The values

of the play-out buffer parameter in Scenario 1 are an

outcome of the unfavorable long packets, as it takes

them a longer time to reach the destination. This

phenomenon did not show in Scenario 2, because the

queue delay was much longer than the travel time

and therefore the delay added by the play-out buffer

was in the same magnitude as the queue delay and

not in the range of the network travel time. This

eliminated all the differences between short vs. long

packets.

In summary, the results of the VoIP QoS

parameters of the two vocoders were not

significantly different in spite of the fact that such a

difference was expected based on the statistical

multiplexing analysis. According to the results of

Scenario 2, CVRV did not perform better than CRV.

This is explained by the fact that in both vocoders,

the voice packets were ready to be transmitted every

64ms. Even though CVRV produces packets in

variable lengths, this variety in the length was not

significant enough to achieve the expected improved

behavior of VBR modelled traffic for this vocoder.

6.3 Voice Traffic over WLAN

In Scenario 3, the comparison of CRV and CVRV is

done on a WLAN network (802.11 IEEE), with

voice traffic only. This scenario simulates an ad-hoc

environment, where all terminals can "hear" each

other. Limiting the traffic to voice packets enables

investigating the behavior of the vocoders, and

studying the difference between them.

In the previous scenarios, it was shown that the

results of the tested parameters depend on the

investigated medium. In this scenario, we use the

same test case but on a different medium. We look at

the behavior of the vocoders in order to evaluate the

results by the QoS parameters. The scenario is

carried out repeatedly with an increasing number of

terminals.

Following are the results of the QoS parameters

for this test case.

6.3.1 Delay, Jitter and Packet Loss

The results for 20 and 50 terminals show no

significant difference in the performance of CRV

and CVRV. However, the test case of 80 terminals

shows a significant difference in the delay, jitter and

Figure 4: Delay Results

ICETE 2004 - WIRELESS COMMUNICATION SYSTEMS AND NETWORKS

packet loss between the two vocoders. See Figure 4

for delay results.

6.3.2 Application Packet Loss

The test case of 20 and 50 terminals are not subject

to any application packet loss. In the case of 80

terminals, where application packet loss occurs,

there is no difference in results of the two vocoders.

6.3.3 Results for Collided Packets

The statistical analysis of the WLAN voice only

terminals shows that there is a significant difference

in the performance of CRV vs. CVRV above the

range of 50 terminals. This subsection analyses the

behavior of packets that experienced collision and

retransmission. In this scenario, the prevalence of

collisions, in the incrementing terminal numbers,

increases. In the test case of 20 terminals, there are

no packet collisions, whereas with 50 terminals the

number of packet collisions which is about 1.8% of

the test case packets, and in the test case of 80

terminals, the results is 8.5% of the total packets.

The number of collisions in the test case of 50

terminals is low (and does not affect the results of

delay and jitter). Therefore the analysis is done only

on the test case of 80 terminals where the percentage

of collisions is high and statistically significant.

Figure 5, shows the difference in the

performance of CVRV vs. CRV where the collided

packets are isolated and packet loss is monitored.

As shown, for up to 60ms delay, the

performance of CVRV is better than that of CRV.

CRV looses 7%, 7%, 4%, 4% and 2% more packets

than CVRV for 10, 20, 30, 40 and 50 ms,

respectively.

The experiment described in Figure 6 analyses

the prevalence (in %) of the packets that were

subject to collisions and retransmission.

The packets that have collided 1-7 times are

analyzed exclusively, i.e., the packets that have

collided for 7 times are not included in the column

describing 6, 5, 4, 3, 2 and 1 collisions. The integral

collided packets would be the sum of columns

1+2+…+7 (100%).

Comparison of the number of collisions that

occur in CRV vs. CVRV shows the following:

1. Packets that collided only once are

found in higher prevalence in

CVRV than in CRV – 78.61% vs.

73.86%.

2. Packets that collided more than

once are more prevalent in CRV

than in CVRV (17.22% vs.

14.77% for 2 collisions, 6.03% vs.

4.21% for 3 collisions, 1.86% vs.

1.75% for 4 collisions, 0.70% vs.

0.61% for 5 collisions, and 0.25%

vs. 0.07% for 6 collisions).

3. Packets that collided 7 times are

only present in CRV and not found

in CVRV at all (0.12% vs. 0.00%).

The results show a distinctive superiority of

CVRV over CRV in this scenario. The root cause for

this is explained as follows. Multiple collisions

affect the two vocoders in different magnitudes. In

CVRV, more packets of a single collision are found

than in CRV, indicating that it handled better the

retransmission timing. A packet that has not reached

its destination will be retransmitted until success, up

to 7 times. The fact that there are more packets of

single transmission in CVRV, on account of

multiple retransmissions, shows a better

performance than CRV.

Also, at the other end of the scale – CVRV

performed better than CRV, as manifested by the

fact that the packet transmission is completed by a

Figure 5: Application Packet Loss Results for Collided Packets

EFFECTS OF VARIABLE BIT RATE VOCODER ON VOIP QOS

maximum of 6 retransmissions. CRV reached as

much as 7 retransmissions and was cut off by the

Mac sublayer that is limited to a maximum of 7

retransmissions.

It is suggested that the superior performance of

CVRV is due to the following characteristic in the

WLAN’s network access protocol. In a WLAN

operating with CSMA/CA protocol, the transmitting

terminal concludes that the packet was not received

by the destination, when the ACK packet does not

arrive within its expected time frame. This time

depends on the length of the packet. CVRV packets

are variable length, inserting more randomness into

retransmission compared to what the constant length

packets of CRV enable. This randomness in

retransmission produces the better scheduling of

retransmissions for CVRV.

The outcome of the aforesaid observation for

collided packets (i.e., that each packet in CRV has

been subject to more collisions than the individual

packet in CVRV) is that the delay and jitter of CRV

is bigger than that of CVRV. The difference is

picked up by the QoS parameters of the full scenario

of the transmission on the WLAN, providing a more

favorable service by CVRV.

6.3.4 Summary and Discussion

The results of delay, jitter and packet loss of the

vocoders in Scenario 3 show that the two vocoders

differ significantly in the case of 80 terminals. In all

the parameters, CVRV performs better than CRV,

whereas in the range of up to 50 terminals there is no

difference between the two vocoders.

The difference in the performance of the vocoders

in the higher range of terminals is attributed to the

number of packet collisions when the scenario’s load

is high. The high number of collisions is a “time

consuming” event that influences not only the delay

of the collided packet, but also the jitter, and this is

picked up by the QoS parameters of the entire test

case.

These results are expected to be more prominent

in the WLAN "real world" rather than in a simulator,

since in the simulator the only cause for

retransmissions is when multiple terminals start to

transmit simultaneously, causing the packets to

collide in the "air". In the real world of Wireless and

WLAN in particular, there are additional relevant

attributes, such as: surrounding noises that interrupt

the transmission, the signal is not strong enough, it is

interfered by other devices, etc. These real world

effects can cause the network to have much more

corrupted packets resulting in more packets that need

to be retransmitted. All these are expected to intensify

the advantage of CVRV over CRV in a WLAN.

Generalizing the behavior of CRV/CVRV over

the WLAN shows that the WLAN is not tuned to

work with CBR traffic. The synchronization of the

CBR packets reduces the WLAN effectiveness in

comparison to the VBR traffic. It turns out that the

network access control protocol of WLAN

(CSMA/CA) behaves better with VBR traffic.

7 CONCLUSIONS AND FUTURE

DIRECTIONS

Scenario 1 tested the difference in the performance

of CRV/CVRV of voice traffic only on a LAN. The

results of this scenario show a difference in the

application packet loss parameter. The difference in

the results is in the time magnitude of about 1ms,

explained by the influence of the packet length. But,

as the delay of VoIP measures up to hundreds of ms

Figure 6: Collisions

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100

this difference is noteworthy on theoretical grounds

only. In practice, when testing the VoIP QoS

parameters from the users' point of view, a 1ms

difference has no net effect.

Scenario 2 tested the difference in the

performance of CRV/CVRV of voice and data

traffic on a LAN. The results of this experiment

showed similar behavior in terms of VoIP QoS

parameters of the two tested vocoders, explained by

the statistical multiplexing analysis.

Scenario 3 tested the difference in the

performance of CRV/CVRV of voice traffic only on

a WLAN. The results of this experiment showed

significant difference in the delay and jitter results

for above than 50 terminals participating in the test

case, explained by the WLAN network access

protocol. Generalizing the behavior of CRV/CVRV

over WLAN, it is more effective to utilize traffic

from a VBR source than from a CBR source.

The results of WLAN are especially interesting

and significant as the WLAN is becoming

progressively more common nowadays. We

anticipate that the research results will prove to be

even more prominent in the real world than they

were in the simulated environment. We consider

here two possible future research directions.

First, as most of the vocoders currently used are

constant rate vocoders and the effect of constant

length packets on WLAN causes more collisions,

we suggest to test the WLAN with a different access

method, other than CSMA/CA, to alleviate the

effect of the constant length packet’s collisions due

to their synchronization.

Second, this research shows that the VoIP QoS

depends on the network utilized. LAN terminals

detect that collisions occurred for a transmitted

packet in a constant time whereas WLAN terminals

detect the collision after a period that depends on

the data packet's length. This difference, in the

network access protocol (CSMA/CD for LAN and

CSMA/CA for WLAN), is the root cause of the

difference between the vocoders. Therefore, for

possible further improvement of the VoIP QoS, we

suggest to look into additional networks with

different access methods, such as pure CSMA or

Aloha, which might have an additional positive

influence on the performance of CVRV.

In practice, from the point of view of the end-

user considering the QoS parameters, it was shown

that CVRV is superior to CRV in the WLAN

scenario and performs as well as CRV in the LAN

scenarios. It is thus concluded that this newly

designed vocoder, CVRV, would be the best choice

for the end-user.

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