This section describes four of the most significant challenges that BT addressed in its LLU implementation:
the design of a plan to control crosstalk;
implementation of physical collocation;
modifications to the BT operational processes;
the design of computer interfaces between BT and the other operators.
Incumbent operators in other countries have faced similar problems but the detailed solutions may be different.
In sub-loop unbundling, the LLU operator connects to the loop at a sub-loop access point located between the MDF and the end-user premises). Sub-loop access is suitable for use in conjunction with a high-speed DSL technology such as VDSL [6]. Sub-loop LLU lines may be shared or dedicated. Under shared sub-loop access the incumbent will provide an analogue telephone service to the end-user while the OLO provides a data service.
Collocation is not relevant to sub-loop unbundling as access points are located away from local exchange buildings. To use sub-loop unbundling an LLU operator must provide a street cabinet close to the sub-loop access point and connect it using a tie cable. This is analogous to distant location for classic LLU.
If ADSL technology [4,5] is used it is possible for an unbundled loop to be shared between incumbent and the LLU operator. In shared access an LLU operator provides an ADSL-based data service to an end-user while BT provides analogue telephony using the same access line. Line sharing requires extra tie cables and wiring to be provided.
The basic form of local loop unbundling, sometimes called full or classic LLU, involves leasing the whole capacity of the access pair to another operator. The LLU operator can use the access pair to provide a range of services. The services offered are limited only by a few rules designed to protect the integrity of the network. The network owner remains responsible for the maintenance of the unbundled local loop from the network terminating point in the end-user premises to the connection point to the LLU operator equipment at the HDF or at a joint in a tie cable.
The two major variants of LLU are full unbundling and sub-loop unbundling. Each of these variants may be offered exclusively or with shared use. Full unbundling involves leasing the use of a complete access pair from a local exchange site to the end-user premises. The LLU operator may choose to make exclusive use of the line or may choose to share it with the incumbent operator. In shared access, the incumbent uses the lower frequency part of the line to provide conventional telephony service, while the LLU operator uses ADSL technology to provide a data service.
In sub-loop unbundling the LLU operator is given access to the line at a sub-loop access point which will usually be located at a flexibility point between an exchange building and the end user premises. This may be attractive to operators who wish to use very-high-speed DSL or VDSL technology. If the sub-loop access is shared, the incumbent will use the lower-frequency part of the loop to provide a conventional telephone or basic rate ISDN service.
LLU operators will usually require certain ancillary services to allow them to make use of unbundled loops. A key service is space for the operator to place its network equipment. In the case of full unbundling, the LLU operator may be able to lease some space within the exchange building; this is called collocation. Alternatively, the operator may be able to place its equipment in a nearby building or street cabinet. This is called distant location.
The LLU operator does not need physical access to the main distribution frame. Tie cables are used to connect a handover distribution frame (HDF) in the LLU operator accommodation to the MDF. If distant location is used, the tie cable may be provided by either incumbent or by the LLU operator in conjunction with the incumbent.
Close co-operation between the LLU operators and network owner is particularly necessary during the ordering and maintenance stages. Much of this interaction is being automated using operational support systems that can be connected to the LLU operator's systems. The approach to systems is described later in the chapter.
LLU operators also need core network transmission links to connect the unbundled loops to the rest of their network. These links may be leased from any suitable operator or may be built by the LLU operator themselves. The LLU product variants are described in more detail below.
All developed countries have near ubiquitous metallic access networks and they represent one of many Telcos' largest and most valuable assets. Once limited to providing voice access and voice band modems, the metallic access network is now capable of carrying multi-megabit data services thanks to the introduction of digital subscriber line (DSL) technology made possible by recent developments in micro electronics and in digital signal processing. Each customer access line consists of a pair of wires running from the customer premises to the nearest local exchange. The line consists of a number of elements that provide connection, jointing and cross-connection functionality. The key architectural features of BT's metallic access network in the UK are shown. In the UK, access network cables terminate on a main distribution frame (MDF) within the local exchange building and this represents the cross-connection point between the external network and transmission equipment. There are flexibility points in the external network at primary cross-connection points (PCPs) usually located in cabinets seen in the street and at distribution points (DPs) located close to end-user premises, typically on telephone poles. The wire between the DP and end-user is known as a customer feed or customer drop. Access network cable sizes range from 10 pairs to 4800 pairs. In general, cables near a local exchange building have larger pair counts and smaller conductor sizes. Conversely, nearer to customers, the network cables have smaller pair count and larger conductor diameter.
A technical task force was set up under the auspices of the Network Interoperability Consultative Committee (NICC) to develop technical specifications for the new products:
a plan to control crosstalk interference in the network;
technical specifications for the unbundled loops;
a code of practice for maintenance of unbundled loops;
a code of practice to deal with alleged cases of crosstalk interference.
Definition of the processes and support systems was delegated to the Oftel sponsored Operator Policy Forum (OPF). This group had to produce a number of complex agreements including:
exchange area data to support LLU operator planning processes;
processes to supply LLU service to specific customers;
operations and maintenance processes;
testing processes supporting both provisioning and maintenance;
billing processes.
An essential part of this work was to document and agree the business policies to be adopted by all parties to support LLU services.
