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ATM versus IP for Voice over IP

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 Sindh Univ. Res. Jour. (Sci. Ser.) Vol. 47 (4) 763-768 (2015)

SINDH UNIVERSITY RESEARCH JOURNAL (SCIENCE SERIES)

ATM versus IP for Voice over IP

H. SHAIKH, M. Y. KOONDHAR++, M. M. RIND, A. B. M. ARAIN*, A. RAZA, A. W. MAHESSAR
Kulliyyah of Information and Communication Technology, International Islamic University Malaysia
Received 21st April 2015 and Revised 3rd August 2015

Abstract: Being packet or cell switching technologies, both IP (Internet Protocol) and ATM (Asynchronous Transfer Mode) have
been competitor of each other for a long time based on business needs, features, functions and capabilities. Suitability of ATM and IP
for carrying voice and the advantages of one over the other has been a contradictory issue. On one hand IP (Internet Protocol) has the
lead of being preferred choice due to lower cost, higher adoption rate and advanced features. On other hand ATM provides many
advantages including higher quality of service, especially for time-sensitive materials because of its bandwidth efficiency. There are
advocates of both of these technologies which claim that each is suited to particular needs and environments and not complementary
of other. QoS (Quality of Service) is the main deciding factor for choosing best of these both technologies that suit the requirement of
end-to-end traffic delivery. This paper shows a comparison of performance between ATM and IP for Voice over IP using OPNET
simulation model. Obtained results are compared and discussed in details.
Keywords: Voice over IP, ATM, QoS, OPNET, Simulation

I. INTRODUCTION low bandwidth traffic will use the cells which contain
Quality of Service (QoS) in context of data networks less bandwidth than high bandwidth traffic for example
is known to be the ability of network to guarantee that video, in which use of cells is not synchronized.
networks is capable to give better results. A. ATM Protocol Stack
The aim behind the development of ATM networks The main concept of the ATM layer is independent
was to integrate services such as voice and video traffic of the physical mechanism which is used for
into a single network. In contrast, IP networks were transmission, it sends data passing towards down by
developed primarily for computer data communication. ATM adaptation layer to its destination. The
Generally, computer data communication is more transmission convergence sub-layer allows an interface
efficiently supported by a connectionless network to the transport medium, where two important functions
service. are cell delineation and header error control generation.
However, ATM is unable to provide functions like
There has been a difference of opinion between
retransmission of corrupted data. ATM Adaptation layer
those who believe that ATM networks are the way
(AAL) indicates end-to-end protocol that gives interface
forward and those who believe that with some
between ATM layer and higher layer protocols with the
modification IP networks can provide the best solution.
applications. The main function of the AAL is to accept
Some researched believe and advocate that the exclusive
messages from higher level protocols and dividing them
use of ATM, even for connecting devices within the end
into smaller entities for the purpose of transport into the
system (Hayter. et al., 2001). Conversely, some claim
cells.
that IP networks can provide any of the services that
ATM can support. Thus the performance of both these B. ATM Services
technologies has been compared from different angles The Main function of the ATM is to support four traffic
and contextual settings. This research paper is focused types that are CBR, VBR, ABR and UBR:
on indenting and comparing the performance of both (1)CBR: Constant Bit Rate is designed for the purpose
ATM and IP technology with the help of some of carrying synchronous time sensitive traffic for
simulations. example as voice.
2. ATM
(2) VBR: Variable Bit Rate contains two sub-classes
Asynchronous Transfer Mode uses only small fixed
which are real time(RT) or non-real time, VBR is for
size packets known as cells. It performs simple functions
that services in which the data stream is not supposed to
in the transit nodes, with a header containing simple
be transported at a constant rate, for example as video.
error detection, error checking and recovery of the cell
(Frossard, et al., 2001)
contents. Its connection oriented information transfer is
allow fast and simple hardware switching in the (3) ABR: Available Bit Rate is designed for those
network, no matter which type of traffic it forwards. The applications where there is no issue of delay, when the
++
Corresponding author: Muhammad Yaqoob Koondhar email: yaqoobkoondhar@gmail.com
*Faculty of Computing, Engineering and Sciences, Staffordshire University, United Kingdom
 H.SHAIKH et al., 764

capacity is more the ABR sources move at a faster rate, Reservation protocol. RSVP is receiver-oriented as it
but when the load of VBR increased then ABR sources request for the resources which can be reserved between
are decreased their rates. (Chao 2000) destination and source. RSVP arrange this by
(4) UBR: Unspecified Bit Rate indicates as a low cost forwarding special IP packets as a PATH message
low QOS service that gives no performance guarantee. which intimates the routers between the source and
(Breivik, et al., 2002). destination to manage the ‘reverse path’ when the
receiver receives the PATH message. The RESV
C. ATM Adaptation Layer (AAL) message includes the QoS specifications for receiver as
It consists of four basic services, each service is to well as for transmitter.
support many applications over an ATM network and
the function of each service is to generate in the form of Once the reservation takes place, the router knows
switching mode, bit rates and delay tolerance. that it has full resources to connect the QoS
requirements, it transfer the RESV message to other
D. ATM Switching
router in the form of tree. This procedure keep going on
The main concept behind ATM is high speed
to the RESV message meet to source. In RSVP, RESV
hardware based switching. ATM switches are normally
message are transferred by the interval of 30 seconds to
multiplexors, which deal with the situation such as,
maintain the connection. If router does not get any
when two cells arrive at same time they are required to
update it deletes the path. (Chen, et al., 2000).
leave from the same port and it can be resolved by
buffering. However both affect QOS by adding delay, 4. VOICE OVER INTERNET PROTOCOL
which can be controlled later by increasing the cell loss VOIP technology provides telephone calls over the
rate. internet using Internet Protocol (IP). It mange to send
analogue voice signals into data packets. VOIP depends
3. INTERNET PROTOCOL (IP)
on H.323 standard to save cost for making long distance
IP is the fundamental pillar of Internet. The Internet
telephone calls. The only disadvantage of VOIP is call
protocol was formed for internetworking as a normal
drop ratio and low quality of voice. (White, et al., 1998).
protocol almost to carry any network. IP allows a ‘best
effort’ service over network layer as a datagram service, The aim of VOIP is promote voice communications
data from the transport layer (TCP or UDP) is changed over packet switched networks. The VOIP packet
into IP datagrams and moved over the network. IP switched networks can be divided into fixed wire Local
network is a network made up of nodes. Internet Area Network (LAN), fixed Wide Area Network
protocol version 4 (IPV4) standard which gives the (WAN) and wireless links. Implementation of VoIP
knowledge of IP addresses as four bytes (i.e,32 bits) in indicates standardised solutions that are the International
length and the advanced Internet protocol version 6 Engineering Task Force (IETF), and International
(IPV6) standard that shows the addresses as sixteen Telecommunications Union (ITU). However, speech
bytes (which is 128 bits) in length. Internet protocol codes are normally used during VoIP calls. These are
network contains a data which is created into the packets often selected from the ITU group of codecs as G.711,
and every IP packet consist of header (which indicates G723.1 and G729 and G729.1. Each of them facilitates
the source, destination and the details of data) as well as VOIP to provide better quality as compared to the
the message data itself. telephone calls which are standard. (Fig .1)
A. QoS or IP
The Transmission Control Protocol/Internet Protocol
(TCP/IP) stack provides a ‘best effort’ service. However
different applications need of several applications that
must be more tightly constraints during the transfer of
information. Many mechanisms established for the
purpose to allow QoS aspect to transmission of IP.
B. Integrated Services
The aim of integrated services is to specify that the
elements of network guarantee to deliver performance
based on quality of service (QoS). The idea behind Fig 1: Using IP topology for VoIP
integrated services suggests that each router in the A. Session Initiation Protocol
network support every application to be given due The SIP is the most famous protocol for voice over
priority. internet protocol. SIP is the text based protocol with
C. Resource Reservation Protocol (RSVP) simple implementation and it consist several syntax-
The essential point of the integrated services and related equally to the Hyper-Text Transfer Protocol
QOS which are related to internet is Resource (HTTP) basically usage in the internet. A SIP call
 ATM versus IP... 765

consist of many different network elements and other as

clients, servers, proxies and registrars.

A client indicates the end point of SIP call and is

known as network element which transfer SIP requests
and receives SIP responses where human user contact
with a client for the purpose to set up and receive
VOIP calls.
Fig.5: Simulation for response of HTTP over IP
A SIP server is the network element that gives the
process of SIP requests and returns back the responses, B. Prioritization
SIP proxy is the network element that includes server as Prioritization is linked with the QoS, the IP QoS
well as client and it will pass messages with in the two protocol is RSVP, which gives the sender for the
SIP clients, and the last is registrar at that will take over purpose of requesting a collection of handling traffic,
registration request from the SIP client and therefore it is nowadays the group of IETF which is the Insert working
instrumented for location of SIP clients by allowing a group establishing a solution, the model which consist of
located service to the servers of SIP. (Fig.1to14) (Dang, Differentiating Services used the Type of Service (ToS)
et al., 2004). octet field of IP header to distribute traffic in between
the service provider and the customer.
C. Voice Compression
Compression plays a main role in VOIP because of
slow speed links for the flow of traffic as the enterprises
which are medium size at the speed of 28.8 kbps are
linked to the virtual private network (VPN).

Fig. 2: Voice source simulations for voice

Fig. 6: Response of HTTP client

D. The Pros and Cons of IP

A major advantage of IP networks is that,
particularly in the case of the Internet, they provide a
system of global interconnectivity that is matched only
by the telephone network. The IP addressing mechanism
provides connectivity between any two or more end
system irrespective of their position within the network
Fig.3: Voice source simulation for IP hierarchy. The connectionless approach of IP has been
shown to be very suitable for supporting computer data
traffic, and over the years reactive congestion control
methods have been developed that are extremely
successful for controlling this type of traffic.
In more recent years, the emergence of distributed
multimedia systems has led to the realisation that future
IP networks will also need to provide some form of
multiservice. A number of resource reservation
protocols have been deployed that are suitable for use
within IP networks. The best known of these is RSVP
(Zang, et al., 1993) which is a receiver oriented protocol
Fig. 4: Simulation for response of HTTP with voice that is optimised for multicast communication. The
H.SHAIKH et al., 766

RSVP model attempts to provide resource reservation

whilst maintaining the connectionless principles of IP.
This compromise generally means that RSVP can only
provide a soft guarantee. Other resource reservation
scheme, for instance, Tenet (Ferrari, et al., 1994)
proposes a connection oriented approach, and in these
cases a firm guarantee can be given.
A resource reservation protocol is just a vehicle for
requesting resources; it does not carry out the actual
resource allocation. To complement RSVP, and other Fig. 9: Response from HTTP server
similar protocols, link sharing schemes (Floyd, et al., 5. VOICE OVER ATM
1995), queuing disciplines (Golestine, et al., 2005) and Being a technology that can perform services in
scheduling algorithms (Ferrari, et al., 1994) have been multiples, the asynchronous transfer mode has the
developed that can provide resource allocation in packet capability of simultaneous transportation of voice, data,
switched networks. A number of these scheduling graphics and video at extremely high speeds.
algorithms can provide a mathematically provable delay
bound, and have an associated admissions test.
In order to provide a delay bound at the packet
switching level these algorithms usually require a bound
on transmission delay across each individual link.
Where a fixed capacity link is used, this does not present
a problem. However, where the link is provided by a
shared medium sub-network an addition access
delay may be encountered. Methods have been
developed to predict link level delay bounds in such Fig.10: Topology using ATM
cases (Ball, et al., 1996). A. ATM Prioritization
By using QoS specifications, we can make good use
of ATM prioritization. The CBR class was supposed to
provide the best quality for voice transmission using the
AAL1, but this could not happen. CBR could provide
Circuit Emulation Service CES, which had the function
of reducing the amount of delay. This brought even
more issues. It could not wait for a little time to send a
Fig. 7: Simulation of voice destination
data frame; instead it is sent empty cells of fixed sizes.
With the inclusion of resource reservation protocols and So actually it was not utilizing 20 bytes of bandwidth
resource allocation mechanisms IP networks should be per ATM cell. On the contrary, there is an extension of
able to add a synchronous service to the existing best the CES: Dynamic Bandwidth Circuit Emulation
effort datagram service. What remains to be shown is Service (DBCES) that can transmit only when a voice
how closely delay can be bounded given a variable sized call is functioning. But as in CES, the cells might remain
packet. The developers of the next generation of IP i.e. partially empty. Hence, the conclusion would be that
Ipv6, have taken into account the need for changes to the AAL1 was responsible for wasting bandwidth and
current Internet. Not only have they included a larger increasing overhead delays for VoATM.
address space, but also the means to identify and control
individual packet flows. It remains to be seen if the
manufactures of future IP routers will take advantage of
these features, or generally ignore them as they did past
with the type of service information provided by IPv4.

Fig. 11: Response from voice source

In order to rectify AAL1’s shortcomings, AAL2 was

introduced. The AAL2 was capable of providing
Fig. 8: Simulation response over IP Variable Bit Rate (VBR-RT) service that could create
ATM versus IP... 767

mini cells for ATM packets. Whichever the emulation, On the other hand, much of the research into ATM
whether structured or not, hence, it improved efficiency has been carried out since the standards were fixed. For
of bandwidth. There were three more positive attributes example, it has never really been shown that 53 octets is
of AAL2: A single ATM connection could an optimal size for the ATM cell.
accommodate multiple channels of voice and could
exclusively support voice compression and silence A major criticism of ATM often cited by its critics is
suppression. that it is connection oriented. Whilst this may be
inconvenient for computer data communication and
when interconnecting legacy LANs, it is much less of a
problem and maybe and advantage, for continuous
media communication. In cases where continuous media
requires a guaranteed level of service the need for
resource allocation implies some degree of connection.
It should also be noted that methods have been
developed that can provided connectionless services
over ATM networks.

Fig. 12: Response from HTTP source

B. The Pros and Cons of ATM

ATM was originally proposed for use as the
transport mechanism of the B-ISDN and was developed
as an ITU standard (De Prycker, et al 2000). Right from
the beginning it was intended that ATM should support
a wide range of traffic types, including voice and video.
Because of this, a major requirement of ATM was that it
should combine the guarantees of circuit switching with
Fig. 14: Simulation result of HTTP server
the flexibility of packet switching.
It has also been demonstrated through the
The choice of a small fixed length cell was largely implementation of ATM switches that provided traffic
motivated by the need to share the bandwidth of the control mechanisms are properly implemented, a fairly
links at a fine granularity. This is clearly an advantage low delay bound can be provided. Therefore, although
when multiplexing video and audio stream but is less ATM networks cannot directly provide an isochronous
convenient if only computer data is being supported. service, in case where the delay bound is small, an
Furthermore, in cases where only computer data is being isochronous service can be emulated without too great
supported the overhead of segmentation and reassemble an increase in latency.
would seem to be an unnecessary overhead.
6. ISSUES AND SOLUTIONS FOR QOS OF VOICE
There are several issues of QoS that are to be looked
at, and obtain a suitable solution for each of them. Echo
is the consequence of getting a signal reflected back to
its source. Echo is caused by delay in return time of the
signal, which if it is more than 50ms causes refectory
characteristics in the signal that generates an echo. The
solution to this issue can be reducing all the delays
between end-systems, such as accumulation delay,
Fig.13: Simulation result of voice destination processing delay, and network delay. Moreover, if a
digital filter is installed on the transmit path into the
The fact that the principles of ATM network were packet network then that could help as well.
developed through the deliberations of a standards
organisation is somewhat of a mixed blessing. On one There is another similar issue which is called jitter, a
hand, inclusion of predictive congestion control measure of the time between two packets that are
mechanisms within the standards emphasises their passing a link. In order for this problem to be solved, the
importance, and makes it difficult for equipment jitter buffer has to be regulated in a way that it is kept to
manufacturers of ATM equipment to exclude them, a minimum or otherwise allowed to adjust at an
irrespective of implementation problems. allowable late packet distribution.
 H.SHAIKH et al., 768

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