Wired geographic networks: distribution network

The commercial telephone service was born in 1877 and, from the outset, adopted a structure based on a multitude of switch board seach of which collected users from a restricted area and was in some way connected to all the others.

In a telephone network we can distinguish:

• Distribution network: the part of the network between the switchboard and the user. A fundamental characteristic of the distribution network connections is that they are each assigned to a single user (even if the physical carrier can be shared).
• Junction network: that part of the network that allows the connection between the different control panels. Its networks are typically shared, that is, assigned from time to time to different users, thanks to the activity of the switching center (only in a small percentage of cases there are circuits reserved for a specific user on this network).

In the structure now almost universally adopted for the distribution network, the geographical area pertaining to a local power plant, called the power plant area, is divided into various cable areas, along different routes, each served by a single high-capacity cable, traditionally in copper.

In urban areas, multiple cables can go out along the same route, serving different areas. Each cable feeds along its path several distribution cabinets, each destined to distribute the outgoing pairs towards the user in a specific cabinet area. In correspondence with the cabinets, which constitute points of flexibility in the network, it is possible to install numeric user collection systems (user multiplexers). From the cabinet to the
users there is an additional point of elasticity constituted by the distribution box or distributor.

So in a copper distribution network we distinguish several elements:

• Primary network: connects the control unit to the cabinets (elastic network) with underground cables or in tubes of 400 to 2400 pairs.
• Cabinet: allows the concentration of the pairs of the secondary network (600 couples) towards the primary network (400 couples).
• Secondary network: connects the cabinet to the distributors with underground cables from 30 to 400 pairs or overhead cables from 10 to 200 pairs.
• Distributor: the termination of the cable network located inside or near buildings with cables of 10 to 50 pairs.
• Fittings/connections: Couples of cables that connect the single real estate units to the distributors.

In the hundred years since the birth of telephony, the distribution network has undergone a relatively slow evolution. The current network, although renewed in materials, is not so very different from that of a few dozen years ago.

The basic aspect that must be kept in mind when creating a distribution network is to make available to each user the necessary carrier for the service requested with the minimum cost, but with resources that can cope with demand development over time, and in space, not known a priori.

Understanding the architecture of the copper distribution network is important because it is precisely on this that, in the last twenty years, the provision of Digital Subscriber Line (DSL) services to users has been based.

Bibliography::

• La rete di accesso per telecomunicazioni, Angelo Luvison, Federico Tosco
• Broadband Telecommunications Handbook, Regis Bates
• Building Broadband Networks, Marlyn Kemper Littman