What are the Different Access Techniques?
Access Network, the network between local exchange and subscribers in the Telecom Network accounts for a major portion of resources both in terms of capital and manpower. So far, the subscriber loop has remained in the domain of the copper cable providing cost effective solution in the past. Need for quick deployment of subscriber loop, coverage of inaccessible & remote locations and requirement of more bandwidth for new services coupled with advances in technology have led to the emergence of new Access Technologies. Modern access network technologies are discussed here.
2.0 IMPORTANCE OF ACCESS NETWORK
At present the access network represents approximately 45% to 50% of the total capital investment in the telecom network. It is therefore a very substantial portion of total network and must be given due attention. If this access network is properly maintained most of the problems in the telecom network could be avoided.
However the copper pair cables still dominate the subscriber loop (local network) due to certain reasons primarily based on techno- economic considerations. This copper based local network is considered to be responsible for most of the faults in telecom network. The obvious reasons are congestion of underground facilities, complex network planning and limitation of copper cables to handle digital signals leading to a network inappropriate for extending broadband integrated services digital network (ISDN).
Introduction of Digital Technology coupled with radio transmission and optical fibre cable has revolutionized Telecom Network worldwide. The overall reliability of network has improved vastly. In India too these concepts have been field tried in the access network. The implementation of the above technologies in the access network
can be as follows:
- Using radio in the access network (WILL Technology)
- Using fibre in the access network (FITL Technology)
- c) Exploiting the existing copper network for higher bandwidths. (HDSL,ADSL,VDSL Technologies)
Let us discuss above modern technologies which are largely set to replace copper in subscriber loop.
Radio communication has been employed as a replacement for copper based cables in the long distance media for several years. More recent developments of digital radio and advances in micro-electronic circuits have given rise to wireless in local loop (WILL). It involves using radio to replace the wired link between PSTN switch and the subscriber. WILL is generally used as “the last mile solution” to deliver basic phone services expeditiously where none has existed before. It shall facilitate cordless telephony for residential as well as commercial complexes where people are highly mobile. It is also used in remote areas where it is uneconomical to lay cables and for rapid development of telephone services. The main advantages of this technology are:
- Fast deployment and hence early access to revenue.
- Reduced service interruptions.
- Low maintenance & operational costs.
The radio technology is able to offer the same level of service quality as that provided by wire line technology. The subscribers have no knowledge of their radio connection and may access all the offered PSTN services in exactly the same way as if they were directly connected by wire line. Application of wireless local loop has just started worldwide. The technology employed shall depend upon various radio access techniques like FDMA, TDMA, CDMA.
There is no international standard for this so far. However, a number of national and regional air interface standards for Digital Cellular Mobile Telephone system and cordless telephony are available. These are being adopted for fixed wireless in local loop application. The various technologies available in International market for WILL application are as follows.
TECHNOLOGY OPTIONS FOR WILL
4.1 Basically two types of technology options are available for wireless in local loop. The first one based on cellular mobile Telephone system can be adopted for fixed wireless in local loop application. These systems have Macro cell architecture with cell radius of tens of kilometers ( typically 10-20 Kms), The second type based on Micro cell architecture are extension of cordless telephone systems. These systems have cell radius of few hundred meters (typically 50-200 mts).
4.2 Point to Multi Point (PMP) systems also called Digital MARR systems are becoming available. These systems can cover long range depending upon line of sight conditions (LOS) and repeaters. These systems can be found attractive in hilly areas, isolated islands or largely dispersed habitation where multiple of subscribers are to be served.
4.3 Satellite media can also be used to extend local loop to subscribers who are otherwise located at inaccessible places where laying of cables or line of sight radio media is not economically justified. For example certain villages have been extended gram panchnyat telephones using satellite media for providing rural communication. Generally social factors dominate cost considerations for providing local loops in such cases.
Now a days very small aperture terminals (VSATs) being used for interactive data communication have proved successful in business/ corporate applications. Multichannel per carrier VSATs are also used to extend trunk junctions to remote and inaccessible/ hilly areas.
Through its wide area broadcast capability, a geo stationary earth orbit (GEO) satellite is able to deliver essentially the same throughput signal throughout the country or region at an attractive cost per user.Taking advantage of this factor direct to home (DTH) satellite broadcasting with a smaller antenna at the subscriber roof top is also an extension of local loop over the satellite.
Many personal communication services (PCN) services have been planned using low earth orbit ( LEO) satellites which permit users with portable/ handheld terminals to connect themselves directly to the LEO satellites. In the process the local loop is extended for fixed/mobile application.
- FREQUENCY BAND
The WILL technologies available in different frequency & their important parameter are indicated below: