What are the Different Access Techniques?
Access Network, the network between the 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 a cost-effective solutions 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 the 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 wireline 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 wireline. The 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 applications. The various technologies available in the 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 applications. These systems have Macrocell architecture with cell radius of tens of kilometers ( typically 10-20 Kms), The second type based on Micro cell architecture is an 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 panchayat telephones using satellite media for providing rural communication. Generally, social factors dominate cost considerations for providing local loops in such cases.
Nowadays 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 geostationary 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 rooftop 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:
|SYSTEM TECHNOLOGY||MACRO CELLULAR||MICRO CELLULAR|
890-915 935-960 Mhz
824-849 869-894 Mhz
824-849 869-894 Mhz
|RF CHL SPACING||200 Khz||30 Khz||1250 Khz||100 Khz||1728 Khz||300 Khz|
|VOICE CHL / CARRIER
|MODULATION||GMSK||PIE/4- QPSK||CDMA||FSK||GFSK||PIC/4 QPSK|
GSM Global System for Mobile Communication
DAMPS Digital Advance Mobile Phone Service
CDMA Code Division Multiple Access
CT-2 Cordless Telephony-2
DECT Digital Enhanced Cordless Technology
PHS Personal Handiphone Service
5.0 FIBRE IN LOCAL LOOP (FITL)
In order to overcome the limitations of copper cable and to be able to support value added broadband service like data, cable Video, HDTV and increased use of computer which require bandwidth on demand, optical fibre is introduced in the local loop. In light of its infinite bandwidth and high reliability, optical fibre cable is the automatic choice for the local loop.
(i) Impact of environmental factors is almost negligible on optical fibre cable.
(ii) Optical fibre cables are not susceptible to electromagnetic interference and hence there is no possibility of intercepting information at any point
(iii) The limitation on loop resistance is eliminated by introduction of active elements providing appropriate amplification.
(iv) Because of fibre’s unlimited bandwidth capacity up-gradation is very simple as it can be affected by simply changing the end terminal equipment. Repeated digging & cable laying is not required.
(v) Small size of fibre cable avoids congestion in ducts & crowding at MDF.
(vi) Due to their inherent wide band width capability optical fibre cable can support narrow band and broadband ISDN services. They can also support video transmission, thus bringing the telephone services and cable TV operations together.
The advantages offered by FITL and limitations of copper access network can be tabulated as:
|Copper Access Network Challenges||FITL Advantages|
5.2 FITL NETWORKING
The long term objective of FITL is to take the fibre right upto the subscriber premises or else to extend the fibre as close .to subscriber as possible. The various approaches towards the end goal depending upon its penetration in the access network can be listed below
(I) Fibre to the Building (FTTB)
(ii) Fibre to the Curb (FTTC)
(iii) Fibre to the Home (FTTH)
While the use of fibre optics in the access network is extremely
advantageous and promising but it is still in the initial stages. Currently fibre costs are higher as compared to copper but there is a trend towards decreasing costs of opto-electoronics and optical fibre cables. Some of the implementation issues and limitations of FITL specially in Indian contest are as follows:
(I) The reliable power supply at the remote end other than the exchange end is a must to exploit the reliability and other advantages promised by FITL concept.
(II) There is no major identified demand for broad band ISDN service. Thus unless the cost economics of FITL is justified its application may be some what slow.
While there is no denying the fact that the fibre will eventually take over the last mile (access portion) of the network ,it is felt & generally accepted that it would not be economically justified immediately and, would take some more time before it penetrates in the access network. The Telecom Administrations world over have already invested a lot in, terms of their copper based network and will continue to do so until the fibre becomes more techno-economically feasible. Till such time there is no alternative but to exploit the already buried (but not dead) copper to carry more and more bandwidth.
HDSL is one such1 technology employing a transmission technique which derives substantial capacity advantage in transporting digital signals in local network over existing copper pairs by providing clear 64 kb/s channels supporting voice, FAX and data services with a improved transmission performance. In other words HDSL is able to convert the copper pairs into high speed digital line carriers what were essentially individual voice telephone line carriers. HDSL technology promises following advantages
a) The subscriber connectivity over a. distance of 4.5 Kms on 0.5 mm
copper pairs carrying 2.048 Mb/s data.
b) The adaptive digital signal processing used in HDSL allows near
c) Remote power fading over copper wires from exchange to subscribers.
d) There is no need for any cable conditioning or selection of pairs.
e) It is possible to extend the range of operation to 9 Kms over
HDSL Technology can prove very useful in improving inter exchange junction working and subscriber access network utilizing The already buried copper pair cables.
Recently some new technologies Asymmetric Digital Subscriber Line (ADSI.) and very high Speed Digital Subscriber Line (VDSL) have come up which promise to implement digital TV broadcast , video on demand interactive distance learning and home shopping on the same simple existing copper lines. ADSL can carry 6 Mb/s signal over 3.5 Kms (approx)and VDSL can carry 26 Mb/s to 52Mb/s over a short distance of 600-1000 meters approximately. Later on it is expected to carry even 620 Mb/s over 100 meters. VDSL may find its use in business applications within a building. Many Telecom Administrations are beginning to evaluate and install ADSL services.
Subscriber loops form a very important part of telecom network The increasing appreciation of draw backs of present copper based network and introduction of high band width services have necessitated many alternate modern access technologies. Flexibility and expediency are becoming the key driving factors behind the deployment of WILL. This is a step towards mobile communications leading towards personal communication services.(PCS)
The deployment of optical fiber in the access network promises many advantages as compared to traditional copper cable network. Fibre extension closer to subscriber premises will provide an economical, flexible and easily up gradable transport media for carrying existing and emerging range of services including telephony, distributive video services, high bit rate data and Broadband ISDN services. HDSL, ADSL and VDSL technologies allow a techno-economically feasible migration from copper based network to a broad band fibre based network of future.
These modern Access Technologies shall avoid further large scale deployment of copper cable and shall pave the way for setting up a strong Access Network infrastructure required to step in the future Telecom Network of 21st Century.
PLANNING GUIDELINES FOR OFC OVERLAY ACCESS NETWORK
In January 2001 BSNL Board in its meeting has decided to provide direct fiber to the Commercially Important Customers. Minutes of the meeting have been issued vide letter number 214-117/2000 TPL ( CX ) dated 31/01/01. Initially the project was to be taken up in 8 cities, viz Ahmadabad, Bangalore, Chennai, Coimbatore, Ernakulam, Hyderabad, Kolkata and Pune, where the No of DELs have exceeded 3 Lakhs. The business cases of Chennai, Hyderabad and Ernakulam Access Network is planned for 37 cities during this year, where more than 1 Lakh DELs are existing. These guidelines are issued in order to achieve the following objectives.
- Provision of fiber to customers from end to end at a later date.
- Provide strong fiber infrastructure in an organized manner
- To achieve convergence
- Minimise the investment on the outdoor plant.
- Avoid repeated trenching in the city limits in the same areas.
- Integrate the needs of all the services/ wings of BSNL requiring the fiber media.
At present our fiber network is in the Fiber To The Network (FTTN) state. The Access Network is proposed to reach upto the Curb so that the Fiber To The Curb (FTTC) can be achieved. Fiber To The Building ( FTTB ) and Fiber To The Home can be achieved in the later phases of expansion. Thus Access Network is essential in every city where data/ leased line customers are in a good number. The last mile to the customer is the most unreliable link compared to the reliable communication system we have from one exchange to other. It is more appropriate to call this last mile as First mile since the customer is always comes first in any direction to the exchange. Access Network shall strengthen the First mile and also allows us to deliver the giga bit band width to the customers. It shall also enable us to enter in to Service Level Agreements with customers to the extent of 99.9%.
The Access Network is proposed to be used as the platform for offering future services such as Metro Local Area Network ( Metro LAN ), Wide Area Network (WAN), Ethernet over SDH, Ethernet over MPLS ( EoMPLS) and Storage Area Networks ( SAN).
The planning aspects can be mainly classified under following points :Network Planning
- Type of Duct
- Duct Planning
- Duct Utilization
- Detailed Survey
- Fiber Planning
- Project Estimate
- Duct Laying
- Cable Laying
- Preventive Maintenance
1. NETWORK PLANNING
The Network ultimately should provide end to end fiber connectivity for high bandwidth users. The Network should provide additional fiber connectivity between BSNL exchanges and Transmission centers. It should also be capable of building up of leased lines traffic from exchanges to other exchanges or transmission centers as required. The Network should provide alternate media between exchanges, RLUs, RSUs and DLCs. Broadband data traffic should have access and aggregate points in the network. The duct planning should be done in order to cover the potential customers as listed below. Universities, colleges, schools, hospitals, hostels, multistoried buildings, cinema theatres, software technology parks, industrial pockets, exhibition grounds, Air ports, posts, railway station, travel agencies internet service providers, cable operators, call centers, training institution, research centers, banks, cellular service providers etc.
The following information is to be collected before planning the Network :-
- Existing duct plan
- Existing OFC Network (N/W)
- DLCs working & planned
- Pillar locations
- Location of existing exchanges, RLUs, RSUs
- Location of TAX, & transmission centers
- Route for cable laying where permission is already granted
- Routes for which permission applied for
- Road map of the city
- Location of important places such as Hospitals, Colleges, Business establishments etc.
All the above data is to be drawn on the road map of the city. This shall be the starting point for the planning of the network.
- TYPE OF DUCT :
In order to provide fibers for CICs, DLCs, junction traffic, long distance traffic and other value added services such as cellular, WILL High Count fibers are to be used. Provision of fibers for these needs may be required to be done in many phases. This indicates that multiple pipes are essentially to be laid in city limits. 40 mm Permanently lubricated HDPE pipes (PLB HDPE pipes) are to be used for laying the OF cables. Coloured pipes should be used to identify the multiple cables to be laid.
Quality of the pipe is plays very critical role in the Access network. The PLB pipe being laid is proposed to serve as the permanent reusable duct. The cables are expected to be deblown and blown in these ducts, when high count fibers are to be laid in place of the existing cables.
TEC specification is available for 40mm & 32 mm (outer diameter) PLB HDPE pipes. OF cables with 16 mm Outer diameter, can be easily blown in 40mm Pipe. The 96 F/ 48F OF cable supplied by M/s Sterlite and M/s Icomm have an outer diameter of 18 mm. The 96F cable has been successfully blown in 40mm PLB pipe in Chennai. 96/48F can be also pulled in the PLB pipe subject to the condition that the pulling force does not exceed 2.5x Weight of cable for 1 KM.
Cable blowing is to be planned in the PLB pipe used in the Access Network. In general the cable blowing shall be done by the pipe supplier within the agreement time. Access Work may require more PLB couplers and end plugs. Since pipes are being laid by other private operators, it becomes essential to identify our pipes. Accordingly manufacturer may be asked to put BSNL logo, name on the pipe.
Protection arrangements for these multiple PLB ducts should be reconsidered. When multiple pipes are laid, the trench width shall increase. For protecting these multiple pipes, RCC slabs or rocky slabs ( eg: cuddapah stones) of width atleast 40 cms may have to be used. ( 40 cm x 50 cm x 4cm ). Use of higher diameter GI and RCC round pipes are required at the bridges and culverts. A minimum of 7 PLB pipes can be accommodated in 150 mm GI / RCC pipes.
3. DUCT PLANNING
Trenching and reinstatement in cities is becoming very difficult, time consuming and highly expensive. Lot of coordination with corporation/municipality and other infrastructure agencies is required. Apart from this, the inconvenience to public and other infrastructure agencies imply that the trenching for laying the PLB ducts should be a one time job. Hence instead of laying one or two pipes, looking at the current need, it is essential that multiple pipes are to be laid.
In general, the number of ducts may be planned as 8/7/6/5/4. In the core areas more than 8 number of ducts may be planned. More number of ducts may be planned within 200 Mtrs of the exchange surroundings.
While doing the duct planning, existing concrete duct is to be considered. Possibility of removing existing unused copper cables in ducts may have to be explored. The new duct being planned should cover all the existing pillar locations, DLC locations. In general polygonal architecture is proposed, with the main telephone exchanges, RLUs, RSU, Transmission centers as the polygon vertices.
Many places as the geography does not permit such Polygons with the distribution arms meeting at a place, the polygons may be planned as shown below.
The advantages of planning the ducts in the polygon fashion is as given below.
- The exchange areas are demarcated automatically
- OF Cable ring structures are formed inherently.
- This structure helps in growing towards mesh network.
- Facilitates direct connectivity among different exchange areas.
- Transmission Network integration is achieved.
- The network is scalable as the Polygons can be added externally as well as internally.
Triangle and rectangular(parallelogram) type polygons may be planned in the core areas and Pentagons, Hexagons and above may be planned in the areas surrounding core areas and Peripheral areas.