The Hype of the GSM
Lagos, Feb 20, 2003 (This Day/All Africa Global Media via COMTEX) -- About two years ago when I visited Lagos, there was unprecedentedly upbeat regarding the introduction of GSM to Nigerian market. Most people eloquently talked about GSM as though it would be the end to Nigeria's telecommunications problems.
I remember one of my friends who told me of how people in the Republic of Benin had no telephony problems because of the mass use of GSM in that country. At the time it was understandably unnecessary to tell my friend of the capabilities of other systems because I felt it would be difficult for him to comprehend.
The time is now ripe to delve into the discussion because my friend, and indeed others not in the field of telecom, now understand that GSM is not a panacea for all the telephony ills of the nation. GSM stands for global system for mobile communications. It is one of the second generations (2G) mobile technologies that was adopted by all EU countries. It is a particular type of air interface. Air interface is a standard that defines how a mobile handset interconnects with the mobile office to initiates and receives calls.
There are other 2G air interfaces, namely CDMA and TDMA. CDMA is Code division multiple access and TDMA is a time division multiple access. CDMA is predominantly used in the USA and South Korea. On the other hand TDMA (IS136) is in use in the USA and Japan, etc. GSM uses TDMA technology. TDMA uses time division or chunk of times for the transmission of information and CDMA uses codes for information transfer.
There is no how one can talk about these technologies without being technical, but we shall try to make it as simple as possible, as we have indeed done so far. There is need to discuss the components parts of a mobile network at a high level in order to prepare some of the readers who do not already possess the background skills to understand discussions about mobile telephony.
We shall first discuss a simplified 2G mobile technology. Next we shall discuss a simplified architecture and briefly compare 2G technologies and finally give conclusions. A Simplified 2G Technology
Every 2G mobile technology has four main components as you can see from the diagram in figure 1. The components are the Air interface, the Switching core, the Public Line Mobile Networks (PLMN) and Public Switched Telephone Networks (PSTN). There are three PLMN networks in Nigeria and a fourth one from the second national carrier to follow soon. So, the PLMNs are MTN, ECONET and M-TEL. The Air Interface describes how the mobile handset would interact with the office equipment to facilitate the integrity of and processing of calls. The switching core is responsible for processing and switching through the number the calling subscriber dialed. The mobile calls are switched through a PSTN if it is a call destined or originating from a NITEL or PTO's subscribers. Where the call is bound for a subscriber in another mobile subscriber it can either go through a PSTN or PLMN. Calls can only go through other PLMN directly if the mobile networks have interconnectivity arrangements.
A 2G Architecture
In its simplified form there is the handset or mobile station (MS), a base station transceiver (BTS), a base station controller (BSC) and Mobile switching centre (MSC) and or a Gateway MSC (GMSC). The GSM MS has Subscriber Identification Module (SIM) card. The SIM has inbuilt microprocessor, RAM and ROM. SIM card will have IMSI number. International Mobile Station Identifier (IMSI) in GSM and UMTS is the unique and principal identifying number for subscribers on the network. The number is typically 15 digits long and consists of the Mobile Country Code (MCC) and National Mobile Subscriber Identity (NMSI). The NMSI is further divided into a Mobile Network Code (MNC) and a Mobile Subscriber Identification Number (MSIN). For example the Nigerian number, 234 8033 212511 has 234 as MCC and the rest as NMSI. The NMSI is also divided as follows: 8033 indicates that is an MTN mobile network code (MNC) and the rest of the digits are the mobile subscriber identification number (MSIN).
The Random access memory (RAM) is a programmable memory that houses temporary information such as text message you have just written or received. The Read only memory (ROM) memory holds such information that you can only read, but not change. An example of ROM information is the MENU information.
When you turn on your handset (MS) it sends a control information through the base station transceiver (BTS) to BSC requesting network service and the BSC would require the MS to identify itself, in which case IMSI number would be sent. There are other control information that are constantly sent between the handset and the BTS which would be too technical to discuss, but necessary to keep tracking the movement of the MS from one place to another. In order for an MS to communicate with the network it has to transmit through a predetermined frequency.
Using the GSM technology for example, the original frequency of operation is in the 900 MHz band in most countries. GSM standards have been extended to operate in several other frequency bands as well, where the system is referred to as GSM400, GSM1800 (sometimes called DCS1800), and GSM1900, which operate in the 400, 1800, and 1900 MHz bands.Use of a Subscriber Identity Module (SIM) card permits international roaming even when different frequencies are used in different regions.
The BTS converts the information it receives into electrical signal, which is passed on to the BSC. BSC controls all the information coming from several base stations (BTS) and passes call requests to the Mobile Switching Center for processing and forwarding. The MSC would forward the call to another BSC if the call were a mobile local call within the same MSC. If it is an out going call to a PSTN subscriber such as NITEL subscriber, the call would be passed to another Gateway MSC or directly to PSTN if the MSC has gateway functions as well.
A gateway understands and speaks several languages, which are commonly referred to as protocol. In other words a gateway is needed to convert the voice signal coming out of the mobile network to that which the likes of NITEL fixed network (PSTN) would understand. Traditional voice is represented with 64000 bits per second (64kbps), but the coded voice from the mobile network could be 8000, 16000, 32000 bits per second depending on coding scheme used.
So, the primary function of the gateway is to transform one protocol to another.
As mentioned earlier, if the subscriber to be reached belonged to another mobile network, i.e. from MTN to ECONET and both PLMN have some interconnectivity arrangement then the MSC would forward the call request to the other PLMN. A PLMN has a Home location register (HLR) and Visitors location register (VLR), The HLR is a centralized database that keeps special information about the subscriber, such as services the subscriber have subscribed to and also billing information.
The HLR database would know the where about of a subscriber, whether the subscriber is allowed to roam from one country to another, whether the subscriber should be allowed to send text or have call waiting abilities, etc. On the other hand, VLR is usually embedded in the MSC equipment and the database tracks the information of the subscribers who are visiting from other MSC regions. A Lagos registered subscriber who is visiting Asaba would have its information temporary stored in the VLR for the MSC serving Asaba area. It is important to mention here, for
the benefit of those companies yet to commission their 2G infrastructures, the changes that are currently taken place in the infrastructure design. The current trend is to replace the traditional Mobile switching center (MSC) with two ties equipment that is forward looking. The two-tier approach replaces the MSC with an MSC Server and a media gateway. In other words, the new MSC server does not switch calls, but provides only a controlling function, whereas the media gateway will now have the gateway function and the switching functions. This gets the 2G GSM network ready for evolution into packet technology for voice, which is the future of telephony. So, the second national carrier (SNO) would do well to use this hint.
Comparing Air Interfaces
The above discussions have used GSM as an example. Other than the Air Interface specifics, the discussion is true for other types of Air Interfaces. The three different interfaces: CDMA, GSM and TDMA use different methods to code and transmit information. The Table 1 gives some parameters for comparing the three technologies. I will not discuss them in detail because of the technical jargon involved. The technical minded person could follow the information in the table beyond the explanation, which I will give here. The CDMA uses a system that transmits codes and that means that it is not as susceptible to noise as other methods and that means better quality. CDMA also can produce more capacity than TDMA and GSM. CDMA can support Data rates of 14400 bits per second, whereas the other systems can only support 9600 bits per seconds. Ordinarily, CDMA system used a spectrum method that is more secure than other system, although GSM has an addition encryption function. The CDMA does not drop calls as other systems do because of the way the calls are handed over from one cell to another. If we talk about 3G, there is the attempt to unify the Air Interface and a variation of CDMA is what has been chosen for migrating the various 2G technologies. The GSM would migrate into W-CDMA technology, CDMA into CDMA2000 and TDMA into TD-CDMA. There is no doubt therefore that CDMA is a better technology, despite the fact that there are more GSM users in the world.
Conclusions
In this paper we have discussed briefly the 2G technologies at a level necessary for a non-technical person to follow the current and future discussions regarding the mobile technology. We have also compared the Code division multiple access (CDMA) and Time division multiple access (TDMA) air interfaces. GSM uses a variation of TDMA. While GSM commands the majority of mobile usage in the world, we contended that CDMA is a better technology in terns of quality, capacity, security and data rates. At the time that GSM was planned it was thought by the general public to be a technique that would probably solve all the telephony ills of the nation, but as people are beginning to understand it is fraught with its own problems. However, the GSM is the first 2G technologies that were commercially available. It was a standard adopted by all EU countries and law of large numbers meant that most other nations adopted the technology for compatibility sake. It provides easy roaming among countries in the world.
The current trend is to replace the MSC with an MSC server and a media gateway in order to scale the network ready for evolution to 3G. Any new operators, such as the SNO, must intelligently adopt this two-tier approach, which would cut future capital investment costs and provide a better and a more modern qualitative network. That has been the trend.
Odinma is GM, Network Technology & Solutions for Europe, Middle East and Africa, Lucent Technologies
by Augustine OdinmaCopyright This Day. Distributed by All Africa Global Media(AllAfrica.com)
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