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The term access network refers to the network between the local exchange and the subscriber. In many countries this network is still predominantly made up of the copper cable based point to point connections. This has kept the network in large proportions passive, inflexible and relatively unreliable. This traditional network has long been tailored to the services generally provided i.e. voice, leased lines, and low rate data sometimes each by separate equipment and networks. The technology has not changed much during the last many decades even though considerable changes have been introduced in the field of switching and transport. With the advent of digital technology, the process of installation, maintenance have become less cumbersome and quality of services has improved. It is therefore felt that the any cause for dissatisfaction, among customers about present services, is predominantly due to the frequent failures in the access network and the time taken for restoring them.
Fig 1. The Access Network
One of the most fundamental and remarkable of the driving technologies of today is the optical fibre. Without it the current telecommunications revolution would have been a non-event. Supporting the high growth telecommunications demand with copper, radio and satellite alone would have stalled the information revolution. These would not have made bandwidth and connectivity for all possible. Increasingly, therefore twisted pair copper cable is being replaced by optical fiber cable with new transmission technologies. The term access network has, infact, gained popularity after the advent of new technologies in the local loop. Another change which is now becoming evident is change of character of the access network from passive to active. These changes hold the promise of removing the limitations of the copper cable network.
In the coming years the telephone companies will increasingly have to offer services like video on demand, broad band data, interactive video. This demand for higher bandwidth with the requirement of rapid provisioning of wide variety of services the above drawbacks of the traditional network become more glaring.
How can telephone companies keep pace with the changing expectations? This requires them to take the following steps:
In short the operator of telecom services will have to change the service profile, transmission media and the network structure.
Considering the above limitations and also the requirements of bandwidth for new services alternative access technologies need to be inducted in the subscriber network. The two main access technologies which are set to largely replace copper in subs loop are 1) fiber 2) radio. In the interim period technologies that enhance bandwidth of copper would prove to be useful.
There are more than 800 million lines worldwide built up with copper pairs. This amounts to a large investment on the part of the existing operators and administrations. It would make good economic sense to be able to make use of this existing network to deliver enough bandwidth to support the services required. Fibre to every home is of course the ultimate dream, but when are we going to accomplish this? Not in the near future! It is therefore imperative for administrations to give a serious look to the digital technologies that promise to deliver amazingly higher bandwidth on copper.
A family of technologies that have begun to transform the narrow band copper access network into broadband network is the xDSL family of technologies. The term DSL, or digital subscriber line, refers to the modem which when connected at either ends of a normal twisted wire pair line, converts it into a digital line capable of handling data rates well into broad band. By using higher frequencies, DSL technologies enable much higher speeds over the twisted pair lines. Speeds up to 2 Mbps are readily achievable -- 35 times faster than today's fastest analog modems. Much higher speeds, up to about 52 Mbps, are today possible. This means that customers can have applications that need these higher speeds even if their towns or villages do not have optical fibre in the local loop.
The letter "x" indicates that there are many variants of DSL technology. Some of these are HDSL, SDSL, ADSL and VDSL. Although the transmission varies greatly among these technologies, there are some distinct similarities. They are all bi-directional, digital signals that run on non-loaded, copper loop. Notwithstanding these similarities each type of DSL technology has its own characteristics. A brief discussion is given below
3.1 High data rate digital subscriber line (HDSL) is a technology that delivers 2 Mbps to the customers. It is in effect a better way of transmitting E1 over a twisted pair of wires. HDSL requires 2 non- loaded copper pairs for bi-directional transmission. The loop length limitation is 4 to 5 kms on 0.5mm cable. Typical applications include PBX network connections, cellular base stations, Internet servers, WAN/LAN access. In some applications it is likely to give way to ADSL and SDSL in near future.3.2 Single line digital subscriber line (SDSL) is the single line version of HDSL, transmitting E1/T1 signals over a single twisted pair and in most cases operating over POTS so a single line can support POTS and E1/T1 simultaneously. Most subscriber premises are equipped with a single pair and SDSL has an edge under such circumstances. Since SDSL works on a single pair, it commonly provides high- speed connections for work-at-home and Internet access applications over residential local loops. It should however be noted that SDSL has a limitation of about 3 to 4 km, a distance over which ADSL gives much higher rates.
3.3 Asymmetric digital subscriber line (ADSL) transmits an asymmetric data stream, much more going downstream to the subscriber and much less coming back. This is not a serious limitation because the most of the target applications for digital subscriber services are asymmetric. Fast internet access, video on demand, home shopping, remote LAN access, multimedia access, specialized PC services all feature high data rate demands downstream but relatively low data rate demands upstream. ADSL requires only one pair and delivers upto 2-8 Mbps. For 2Mbps the distance is about 5.5 km and for 8 Mbps it is about 3 km. ADSL enables POTS to be delivered over the same copper pair.
3.4 Very high Data rate Digital Subscriber Line (VDSL) is a new technology not expected to be in use in public networks for some years. VDSL transmit asymmetric streams at data rates higher than ADSL but over shorter lines. It is expected to provide speeds as high as 52 Mbps downstream and between 1.5 and 2.3 Mbps upstream. The distance at the upper end at 52 Mbps would be about 0.3kms and more at lower speeds (1.3 km at 13 Mbps). These could work in conjunction with fibre to the curb or building to deliver high bandwidth to the homes. VDSL can take care of all ADSL applications and additionally high definition TV.
Fig 2. General Model of DSL systems
Comparison of various DSL technologies is given below:
| Name | Meaning | Data Rate | Connection Type | Distance to exchange | Applications |
| DSL | Digital subscriber Line | 160kb/s | Symmetrical | ~5 km | ISDN series, voice and data comm. |
| HDSL | High Data Rate Digital Subscriber Line | 2Mb/s | Symmetrical | 4-5km | No POTS, E1 LAN/WAN, Service access feeder plant. |
| SDSL | Single Line Digital Subscriber Line | 2Mb/s | Symmetrical | 3-4km | Same as HDSL + POTS |
| ADSL | Asymmetric Digital Subscriber Line | 1.5 to 8Mb/s Down 128kb-768kb Up | Asymmetrical | 3-6km | Interest access, video on demand, simplex video, remote LAN access, interactive multimedia. |
| VDSL | Very High Data Rate Digital Subscriber Line | 13-52Mbp/s Down 1.5-2.3Mbp/s Up | Asymmetrical | 0.3-1.5km | Same as ADSL plus HDTV |
Optical fibers, clearly the chosen technology for transmission media, are beginning to find their place in the subscriber's loop. Currently fiber costs are high as compared to copper but there is a trend towards decreasing costs of optical fiber cables and photonics employed. In addition the tremendous advantages in terms of information capacity of fiber, its small weight and size over copper cable are making it a very attractive technology to replace copper in subs loop when advanced broadband services need to be offered to the customer. To carry the same information as one fiber cable we would need hundreds of reels of twisted wire Cu cables. Further, fiber is 23 times lighter than Cu cable and 36 times less in cross- sectional area. These features of light weight and small size make it easier to handle fiber cable. In crowded city networks they can easily be accommodated in existing ducted systems. Fiber in loop (FITL) can be developed in several configurations.
4.1 Objectives of fibre in the loop
4.2 Fibre in the loop driving forces
4.3 Advantages of FITL network
4.4 General planning issues
4.5 Planning deployment strategies
4.5.1 Methods of deployment of fibre
The three methods that are normally used take their name from the location of the remote terminal equipment. Accordingly we have
Fibre to the Curb(FTTC) in which the terminal equipment is located on the curb from where it would be convenient to serve a suitable service area. Since the distribution would still be copper, suitable location for the terminal would be one which optimizes the cost, reduces back-feeding, reduces distribution cost and takes safety factors into consideration. Wayleave and power availability need to be confirmed before finalising the location.
Fibre to the building(FTTB) in which the terminal equipment is located inside a multistoreyed building. This brings higher bandwidth closer to the subscriber. The distribution part is still copper. For new buildings, the planners may negotiate for suitable location well in time.
Fibre to the home/Office(FTTH/FTTO) in this method the fibre goes upto the subscriber premises
4.5.2 Fibre in the loop architectures
A planner has a choice of the following architectures to choose from: point to point Fibre Optic cabinet based configuration(with or without V5.2 interface), fibre sharing with passive splitting, fibre sharing with active splitting and ring configurations. The location of the cabinet or equivalent units may be decided based on the distribution of subscribers and point to which bandwidth is to be delivered.
a) In the point to point based implementation the planner plans remote terminals with corresponding central office terminals. The actual location will depend on the method of deployment chosen and the demand profile.
Fig 3. Point-to-Point Architecture
In configurations with V5.2 interface between the switch and the access network, the FO cabinets can be connected to the switch with 2 Mbps links and the central office terminals would not be required.
b) In point to multipoint/star architecture a number of remoter terminals can be parented to one central office terminal forming star structure
Fig. 4 Star Architecture
c) Tree structures with passive or active splitting gives advantages of fibre and equipment sharing.
Fig. 5 Tree Architecture
d) Ring structures have been made possible with the advent of SDH technology
Fig 6. Ring Architecture
We will discuss more about these architectures and technologies in the later sections.
4.6 Technological options for planners
We have been mentioning names like fibre optic cabinets, PONs, ONUs, OLT, SDH, ADMs etc without much of explanation of these. In this section we would put these in right perspective.
Much of the equipment that is currently in the access network conforms to PDH(plesiochronous digital hierarchy) that was described above. Examples of these are the fibre optic cabinets and PON(passive optical network). In case of PDH technology cabinets having analog interface with switch, the installation requires a central office terminal for for opto-electrical conversion and multiplexing/demultiplexing signals to/from the remoter terminals. Connection between the DLC and the switch is through the channel banks on VF basis.
Synchronous Digital Hierarchy(SDH) is the new standard which promises higher data rates, more reliable, flexible and manageable access networks. V5.2 interface is used between the access network and the switch the central office terminal is not required. Signal is transported from the fibre optic cabinet(or DLC/RT) to the switch through SDH access multiplexers(AM) house in the cabinet and near the switch. The switch end AM is connected to the switch with 2 Mbps streams. SDH also allows formation of rings and a number of cabinets can be connected to a ring. The V5.2 interface allows concentration so that the number of subscribers per cabinet can be more than the number of available channels. For example, if the traffic conditions permit, a 34 Mbps(480 chl) cabinet can even serve 2000 subscribers.
4.6.1 Fibre optic cabinets
The optical fibre cabinet consists of fibre optic transmission equipment and customer access equipment. It consists of three internal chambers. A battery chamber that houses upto 2 batteries, an MDF chamber housing MDF, alarms and fibre splice box, an equipment chamber housing transmission and access equipment.
Exchange side of cabinets connect to exchange on 2Mbps or channel level(with VF pairs) and subscriber side of cabinets connect to subscribers via copper lines. These can be installed as outdoor or indoor cabinets. Outdoor cabinets are environmentally fitted and could be installed on curbs or in remote areas. Usual capacities of fibre optic cabinets have capacities 120, 240 and 480 channels. Each cabinet requires two fibres for operation and one dark fibre-pair is usually kept as spare. One of the relatively new type of FO cabinet, the Fujitsu FSX2000, meets the size requirements of 30, 120,480 or 1920 lines. In case of 4:1 concentration it can provide upto 1920 lines with 16 E1s from the exchange using V5.2 interface. Indoor cabinets of sizes 30, 120, 480 lines and outdoor cabinet for 480 lines can be configured. It gives existing and new services and is migratable to SDH transport thereby giving the possibility of establishing high bandwidth ring structures.
The FSX2000 remote terminal(RT) can act as a hub for upto 16 satellite RTs each equipped with 120 POTs giving 1920 lines. It can be used in non-V5 mode as DLC(Digital Loop Carrier) in which case we have a central office terminal(COT) and an RT and can have upto 480 subscriber lines. The switched services supported in the DLC mode are POTS lines, Payphone lines, PABX lines and centrex lines. Interfaces are available for these. Digital 64 kbps and 2 Mbps services are suppported on G.703 interface. Also supports PRA and BRA. It also supports fractional E1 services nx64 kbps. Each line card connects 15 lines and for 480 line configurations a total of 32 line cards are required.
In integrated access mode FSX2000 eliminates the need for the central office terminal provided there is an available means of transporting RT trunks back to the switching system. FLX ring network (Using STM-1) is one example. These systems can be implemented as fibre to the curb(FTTC), Fibre to the building(FTTB) or fibre to the home(FTTH) depending on where the cabinet is located.
It is possible to upgrade the existing COT/RT DLC network to integrated network when the terminal exchange supports ETSI V5 interface. To do this, at the central office end remove the channel banks and the VF physical links and add 2 Mb interface cards and E1 circuits connected to the switch.
The outdoor cabinet has three internal chambers
Equipment chamber is surrounded by an air cavity which shields from direct solar radiation, also it is sealed to prevent ingress of dust and moisture from external air. Fans are provided for cooling. It can be installed rapidly on the prepared concrete plinth.
MDF accomodates pairs of VF cables using line isolation type termination with optional gas lightening arrestors. Fibre splice tray is capable of holding four spliced fibres and two unspliced fibres.
Battery chamber has two 4x12 V, 50 or 100 AH batteries. AC power cables enter through the floor of the battery chamber. Plant cables and fibre optic cables enter the cabinet through the floor of the MDF chamber. Earth bar is in the MDF chamber.
The fibre optic cabinets offer point to point connections and can take care of POTS, ISDN(BA and PRI), DID, Payphones, 64Kbps leased lines
Alcatel's Lightspan 1540 is a multipurpose platform that can be customised for many applications. It allows normal telephony services, IDSN, Frame Relay, VoIP and xDSL . It has broadband multimedia services, MPEG on ATM and ATM 25 Mbps. It can be used as DLC with PDH and SDH transport.
The management system is window-based plug and play and permits using software downloads for upgradation(say from V5.1 to V5.2)
4.6.2 Passive optical network(PON)
PON systems implement a variation of point to multipoint in the form of a tree or passive double star architecture.
Simple PON configuration
Basic PON system components
Optical Line Terminal(OLT)
Splitter
Optical Network Unit (ONU)
Typical implementation of PONs
PON Services
PON benefits
PON systems offer a number of benefits to the operator and the end users. Fiber is less costly to maintain than copper based systems so operators can reduce costs, increase profits or lower costs to the end-users. The technology conserves fibre and optical interfaces. All this leads to cost effective service delivery. Optical fibre future proofs the network and increases reliability. Both business and residential customers can be served on the same platform. Powerful network management makes network low cost to run. Customers get better quality of service. Network can be upgraded to support future services
PON planning options
Planning considerations regarding some of the units are given below:
1. Optical Line Terminals(OLT)
This unit interfaces with the switch and provides transport control, operation, administration and management functions. The ones that are currently used have a capacity of upto 1920 channels utilizing 140 Mbps system. An OLT can typically support upto four PONs each having a capacity of 480 POTS lines. The equipment typically operates in the 1300 nm window.
It supports a variety of signalling schemes including V5. Allows remote downloading of software to the ONUs thereby simplifying installation and upgradation of the outdoor equipment.
2. Optical splitters
Split the beam into a number of directions. A two way splitter may have 2 input and two outputs. Signal may be fed to only one of the inputs with the other one being ‘standby’. Upto 1:32 splitters are available.
Two types of optical splitters are commonly used:
3. Optical Network Units(ONUs)
Provides interface between the customer's equipment and the PON. Each ONU provides a multi-service delivery platform for POTS, ISDN, leased lines and 2 Mbps services. Several types of ONUs are available – street, wall and rack mounted. Loop back and line test capabilities are available.
Typical ONU sizes are 4B to 120B. The smaller ones being used for FTTH/FTTB and the larger ones for FTTB/FTTC implementations ONUs require power supply which can be supplied from the central office or another distant point on copper cables or it can be made available locally. Location depends on the supply area and ONU size.
Future of PON systems
Technical advances and changing profile of services has drawn increasing interest to optical distribution networks with Asynchronous Transfer Mode(ATM) over Passive Optical Network(PON). This is referred to as ATM-PON or APON. It is seen by many operators as the most promising approach to achieving large scale full service access network deployment that could meet evolving service needs of network users. It has been seen that APON could support a wide range of "FTTx" access network architectures.
The network components supporting APON are OLT, ONT/ONU and passive splitter. One fiber is passively split upto 64 times. This allows users to share bandwidth and reduces cost. Costs are further reduced by a decrease in the number of opto-electronic devices needed at the OLT. The APON uses a double star architecture. The first star is at the OLT and the second at the splitter.
In addition to the fibre and interface sharing benefits of the PON systems, the APON allows operators to serve more customers as compared to other technologies. At the same time operators can give QoS guarantees. It is estimated that APON technology can achieve savings of 20 to 40% over circuit based access systems. Because these systems are ATM based, they can adopt to virtually any service desired. Operators can deliver all of their legacy services as well as new services.
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