IMPLEMENTING OF EMBEDDED LAN IN TELECOM SYSTEMS - SEE

IMPLEMENTING OF EMBEDDED LAN IN TELECOM SYSTEMS
Ailin Parichehreh Dizaji
Iran Telecom Research Center (ITRC)
Tehran, Iran
email:a_parichehr@itrc.ac.ir
Abstract
Mobile Switching Centers (MSCs) are the heart of a GSM
network. Since MSC is a distributed system, it consists of
different modules. Internal parts of each module can
communicate which each other using HDLC protocol
controller, but its speed isn’t enough for the
communication between main modules, so a high-speed
Embedded Local Area Network (LAN) can be a good
idea for connecting these subsystems.
Design of this Embedded LAN must be according to
MSC’s features as a telecommunication system. So we
need a network that can transfer data and information
with less delay and without any error.
In this paper after designing a suitable network according
to a telecommunication system’s properties and
simulating real conditions, some important parameters
such as throughput, average delay and packet loss rate
have been measured.
Key Words
Embedded LAN Network Performance Throughput
Delay Packet Loss
1. Introduction
One key problem for MSC implementation as a modular
system is creation the ability to transmit data and
information between its subsystems with required
features. In many of previous methods the control of
exchanging messages between subsystems was Message
Buffer’s task which its recent replacement is high speed
LAN. In this paper the details of Embedded LAN
implementation in MSC as a telecom system and its
performance related graphs have been presented.
2. Embedded LAN in MSC
MSC provides the services a person can request by using
a mobile phone and manages the communication between
GSM users and other telecommunication networks. MSC
is a distributed system consisting different modules that
each one has a special part of MSC’s duties. In this way
each module can doing its task while is connected to other
modules and can receive its required information from the
others.
2.1. MSC’s main parts
- Digital Trunk Module (DTM):
DTM’s task is signaling handover between CAS-3bit and
SS7. Each DTM consists 8 DT boards with 4 E1 input
lines for every one. Each DT board has two 4 Mbps lines
in its output that are connected to a duplicated switch. So
this switch has sixteen 4 Mbps lines in its output.
- Echo Canceller (ECM):
Task of this module is omitting undesirable echo related
to connecting mobile and PSTN networks.
- Switch:
With 1024 PCM lines in the network that every 32 of
them are supported with one DTM, we should have 32
DTM units. Total output of this units are 512*4Mbps
lines. So that we need a 64*64 central switch for
switching this lines.
- VLR:
This unit is a database of users in the same MSC region.
- Data Base:
Information related to charging, routing, system
management, etc are registered in this unit.
2.2. Specifications of MSC’s Embedded LAN
We should have two computer networks for total
communication in MSC: HDLC network (with low speed)
for connecting internal parts of each module, and LAN for
connecting main modules. Adjustment of this LAN with
specifications of MSC as a telecommunication system is
so important. Some of these specifications are number of
users, speed, traffic rate, reliability, etc.
MSC’s LAN is selected from 100Base-T type because of
its high speed, low cost and wide domain of contained
protocols. This network has two types of traffic: one is
related to signaling packets transferred for establishment
and release a conversation because mobility of
subscribers, and another one refers to management
packets. It means that most of the traffic is related to
transfer information produced during different stages of
conversation’s establishment and management. So with
selecting client-server model, the only task of server will
be packet handling between sender and receiver, such as a
simple switch. In these conditions peer-to-peer model is
more suitable. Selected protocol is TCP/IP. The main
reason of this selection despite of that IPX/SPX protocol
is faster and more suitable for networks with small
diameter, is using Linux operating system which only
supports TCP/IP. Since MSC is a real-time system any

interrupt in access of its modules to network is not
permitted. So we shall not be heedless to any error such as
cable breaking, NIC or hub/switch fault, etc. One solution
for this can be duplicated LAN. In this case every module
has two NICs with two different IP addresses, so if every
module couldn’t access to the first LAN it will try through
the second one.
-VLR
The table seen below shows the rate of VLR transactions
in different routes:
Route Request Response
2.3. Design of duplicated LAN
Linux can define different routes for access to a special
host in a local network using “route” instruction. Since
these routes can be different in their sources, we can have
routes to LAN two different with two IP address as
different sources for each module using “route”
instruction. Therefore if a module couldn’t send its packet
through LAN over its first IP, then it will resend it over its
second IP. But it is possible that a problem any where of
the route to destination module’s first NIC has been
error’s source. So we have assigned a unique name to
each module. Source module before starting to send
process, calls gethostbyname() function with destination
name and externs both two IP address of destination
module. So if it didn’t receive any acknowledge from
destination it will resend packet to the second IP of
destination module. In this way network modules don’t
need to a table of others’ two IPs which should be
updated periodically that will increase network’s traffic.
2.4. Network performance
The next step is measuring of implemented LAN’s
performance in simulated real environment. Network’s
bandwidth, delay and packet loss rate which are the main
parameters indicating performance of a network will be
worthy if offered load is equal to real traffic.
2.4.1. MSC network traffic
-DTM
In Embedded LAN of MSC there are several types of
users. For example 32 units of them are DTMs which
each one supports 960 channels. According to ITU-T,
every channel on 0.7 hour is busy, and if each
conversation takes 1.2 minutes, so every channel will
have 35 conversations per hour. Therefore we have
960*35 equal to 33600 conversations per hour or 9.33 per
second. Average rate of transmitted packets for every
conversation is 31.5 which half of it is related to one side
of conversation (caller or called), so that with assuming
200 bytes as the length of a packet, the traffic related to a
DTM is equal to 9.33*(35/2)*200*8 = 0.2352 Mbps and
the total traffic of DTMs is 7.5264 Mbps.
-Central Switches
Central switches are another type of users that load two
times of DTMs traffic equal to 15.0528 Mbps to the
network, because they transfer two packets for every sent
or received packet with a DTM.
Mobile to Mobile (caller) 5 7
Mobile to Mobile (called) 6 4
PSTN to Mobile 5 4
Mobile to PSTN 5 4
Average 21/4 19/4
Each DTM has 9.33 conversation/sec, so the rate of
conversations for 32 DTM is about 300 conversation/sec.
Therefore the traffic load of every VLR is:
300(21/4 + 19/4)*200*8 = 4.8 Mbps
-Data Base
Most of Dbase’s load is related to charging records
saving, equal to 300*1*200*8 = 0.48 Mbps.
Since some protocols such as IMSI Attach, IMSI Detach,
Location Updating, etc have been ignored a coefficient
equal to 1.5 is assumed in total traffic accounts, so total
traffic rate of MSC’s Embedded LAN is equal to
1.5(7.5264 + 15.0528 + 4.8 + 0.48) = 41.7888 Mbps
2.4.2. Simulated Embedded LAN
Simulating of Embedded LAN in real conditions and
performance measurement’s conclusions have been
abstracted in following graphs:
Graph (a) shows that for packets with 200 bytes length the
maximum throughput is about 70 Mbps, and for offered
loads less than 55 Mbps throughput graph is liner. So for
41 Mbps offered load throughput of MSC’s Embedded
LAN is near to 41 Mbps.
In graph (b) we can see that the average delay for data
transmitting nearly is 5µs.
From graph (c) we can be sure that the rate of packet loss
for 200 bytes as packet length is near to zero.

3. Conclusion
The most important point in presented graphs is that if
traffic rate of MSC’s Embedded LAN increases up to 65
Mbps, throughput will remain at 100%, delay will be only
10µs, and packet loss rate will be less than 0.1%. So we
can be sure that despite of network traffic rate’s
enhancement up to 1.5 times of current rate, MSC’s
Embedded LAN will be reliable.
Figure (a): Throughput versus offered load
References
[1]: D. LeBlanc, Linux Install and Configuratio
(Coriolis Open Press, 2000).
[2]: M. Mitchell, J. Oldman, Advanced Linux
Programming (New Riders Publishing, 2001).
[3]: G. Keiser, Local Area Network (McGraw – Hill
Higher Education, 2002).
[4]: K. Kummerle, Advanced in Local Area
Networks (IEEE Press 1987 Editorial Board,
1987).
Figure (b): Mean network delay versus throughput
Figure (c): Packet loss rate versus offered load