4.6.3 Active Optical Network(AON)

In this type of implementation the passive splitting point is replaced with OLD(Optical Line Distribution unit) which is a powered unit making it possible to have higher bit rate on individual routes over longer distances than PON

Features of Active Optical Network

• cost effective fibre network
• higher number of optical components compared to PON
• long link lengths possible
• additional active element(OLD)
• high flexibility and capacity
• Higher power supply problem due to OLD

• POTs
• ISDN BA
• ISDN PRA
• Leased Lines

Advantages of PON/AON

• Sharing of fibre and equipment as in case of PONs
• Increase in range at the cost of an additional active component(splitter)
• high flexibility in structures
• higher capacity
• Management
  • Configuration management:provision, modification and cessation of capabilities
  • Performance management:routine testing, error monitoring
  • Fault management:fault location, degradation discovering
  • Security management:prevention of unauthorized access, security of transmission, detection of access

4.6.4 SDH in the access network

The current network based on Plesiochronous Digital Hierarchy(PDH) is unable to meet the demand of flexibly and reliably providing bandwidth on demand for video conferencing, remote data-base access, BB-multimedia, interactive video and other services that are round the corner. We had seen that this is primarily due to the inability of identifying and directly extracting 2 Mbps streams from higher order signals. The "mux mountain" problem makes the system inflexible and expensive.

Synchronous Digital Hierarchy addresses these problems. ITU-T standards G.707, G.708, G.709 define the hierarchy with the basic rates of 155 Mb/s or STM- 1(Synchronous Transport Module-1), 622 Mb/s or STM-4 and 2.4 Gb/s or STM-16, 10 Gbps or STM-64 and further levels are proposed. As can be seen from fig.10, with a single add- drop multiplexer a 2 Mb/s stream can be inserted or dropped with ease. A great advantage of SDH is that the present PDH systems can also be integrated with it.

The deployment of SDH until now has been focussed on the core transport network. With hastening demand for more flexible and more advanced services from business and residential customer groups alike, the time is now ripe to re-assess the rolling out of SDH all the way up to the edge of the network, closer to the customer.

Why use SDH?

• Provides network simplification: SDH requires less equipment for the same functionality.
• Transports mixed voice and data services over a single pipe.
• Single stage multiplexer: management of tributaries by a single STM-n multiplexer. The planner is not burdened by the rigidity of the MUX hierarchy in the PDH. Planners have at their disposal bandwidth flexibility essential for introduction of new services.
• Reduced capital investment and operating cost
• PDH has limited facility for management SDH has good management capabilities.
• Survivability: Better methods of protection and restoration. Self healing rings
• Offers automatic reconfiguration

• Greater bandwidth: 155 Mbps or more to major sites. Subscriber can get any service in the network at short notice

• Standardisation: enables multi vendor networks, Inter-operability of equipment from different vendors
• Improved network resilience: A synchronous network will be more reliable due to increased reliability of individual elements and more resilient structures of the network will allow development of network topologies that will be able to achieve network protection i.e. survive failures by reconfiguring and maintaining service by alternate ways. This can be achieved by cross-connects or self-healing rings.
• Future proof networks

SDH planning

The SDH add-drop multiplexer(ADM) discussed above can be used in a number of configurations of which point-to-point and ring topologies are of interest.

Point-to-point configuration

Used as PDH replacement in the National network at STM-4, STM-16 and in other high utilization connections--like 2 point links

Ring configuration

For high survivability, ‘self healing’ fibre rings are planned. These rings are able to reconfigure without intervention of the external network management should equipment or cable failure occur. The ring structures are used in national, junction and access networks

National: STM-4, STM-16 and higher

Junction: STM-4

Access: STM-1

There could be a number of variations of the basic ring structure. Most suitable ring type dependent on traffic flow in the network. The planner must examine traffic on your network.

Strategic decisions to be made by planners

• Decide protection method 1+1 or shared protection
• Decide configuration: All traffic on a ring: easier to manage
• Have more than one rings and split traffic between rings: better survivability
• Partitioning: deciding number of rings and their interconnections
• Determine collection points between tiers and between rings in a tier
• Should high volume connections be point to point
• Handling traffic growth
• Single or dual, manual or electronic, switching at VC12/VC4

It is seen that optimization is sometime impractical or at least difficult to achieve. As the stations in the network grow, the number of possible rings grows exponentially.

Hill climbing technique for planning is very often followed: start with an intuitive plan. Decide on the number of rings and ADMs on each ring. Add or drop ADMs on ring as the scenario changes.

Rings provide better protection than meshes. Meshes may provide better restoration. Overall better solution is determined by current equipment cost and other engineering considerations.

5.0 V5 interfaces

The V5 interface is the interface between an access network and the local exchange which has been standardized by ITU( G.964/965) and ETSI. It has two variations V5.1 and V5.2. The V5 interface is not limited to any specific access technology though much of the work has been done with optical access network in mind. These can be used with radio access networks as well. Interconnection between telecommunications networks of different operators is also a potential use of V5

5.1 V5.1 interface

The V5.1 interface only allows one single link between the access network and the local exchange.

The services supported by V5.1 are PSTN analog, ISDN BRA, semi-permanent leased lines, permanent leased lines. A V5.1 can only support basic rate ISDN because it does not have sufficient capacity for standard primary rate ISDN. A single V5.1 interface can support upto 30 PSTN ports or 15 ISDN ports. Both PSTN and ISDN ports can be supported on the same 2 Mb link.

5.2 V5.2 interface

The V5.2 interface can support upto 16 2 Mbps links. In addition to the functions of the V5.1 interface, the V5.2 supports concentration of traffic. The physical configuration can be depicted as follows:

A V5.2 interface can support both basic rate and primary rate ISDN. It also supports both semi-permanent and permanent leased lines. A single V5.1 interface supports 30 PSTN ports while a single V5.2 can support several thousand ports utilizing concentration and dynamic allocation.

6.0 Conclusion

The bottom line is that in future the subscriber has to be provided a mixture distribution media and services at the same access point within the network. Today there are different networks i.e. telephone, telex, data, video, with separate access facilities. This restricts flexible service assignment to the subscriber the intention then is to migrate to a network architecture capable of cost effectively offering all services over only one access point. Such a network structure will fulfill the following requirements.

1. Flexible and cost effective extension of services
2. Save cost by reusing the existing infrastructure
3. Provide common management for the whole network independent of the distribution media.
4. Rapid provisioning and high reliability.

What does all this mean? The telecommunications environment in the access area is changing in terms of services, distribution media and network structures. We require a solution which allows migration form the network of today to that of future protecting the investment. It should be easy to add new services as also provide existing services quickly to the subscribers. Future will therefore see more of fiber and more of intelligence being pushed into the local loop.

1. Introduction to Telecommunications Networks
2. Structure of the Access Network
3. Subscriber Demand and Traffic Forecasting
4. Planning Access Networks
5. New Technologies in the Access Network
home