Though located in the exchange building, MDF is as much a part of the external network as it is of internal plant. It is the meeting point of internal and external plant. It provides terminating space for the primary cable and the cables from the exchange line terminating units. MDF provides the flexibility of connecting any of the exchange side circuit to any of the external pairs by jumper wires. The MDF has traditionally consisted of iron framwork of verticals and horizontals. The number verticals will depend on the size of the exchange. The verticals are 'numbered' A, B, C...and are also known as bars. There are ten horizontals resulting in ten cross points per vertical. On the the line side side where the primary cables are terminated, each cross-point will have a terminating block with a terminating capacity of 100 pairs.
The exchange side(or the equipment side) of the MDF is not only reserved for line circuit termination but is also used as a termination point for transmission circuits and other miscellaneous systems. The number these terminations per vertical will vary depending on the type of exchange and the block used.
A code like C132 would refer to vertical 3 block number 1 and pair 32.
The MDF also gives a convenient place to put devices for overvoltage and overcurrent protection. It also provides an isolation point for testing the line-side and exchange side separately.
There many variants to the above basic theme. In one type of MDF manufactured by Krone each vertical has 8 modules of 100 pairs each for the line side and 13 horizontals each with connection unit of 256 pairs(for 6 verticals) for the exchange side.
The main distribution frame for FO systems is also called Optical Distribution Frame(ODF). It is used as the interface between the optical fibre cable system and the transmission equipment. It is where the line technology and the fibre optic transmission technology meets. Splice cassettes can be used to accommodate splice connections; these cassettes are then mounted in cassette housings. An ODF may have a number of distributors. All exchange lines can be terminated on one distributor from where it is jumpered via patch cords to the user side which is connected to other distributors.
There are broadly two types of cables in the PSTN access network; copper cables having copper conductors for transmission and optical fibre cables having glass fibres for carrying signals.
As far as the construction and layout is concerned the cables can be PEUT(poly ethylene unit twin) and PET(Poly ethylene twin). Both have polyethylene(PE) sheathing. In the latter the twins(twisted pairs) are put with staggered cross-over points to reduce cross-talk. The PEUT cables are bigger in size so the pairs are bundled into units of 25 or 50 for easy handling and identification. The core is formed of a number of conductors arranged in units of 25 or 50 pairs arranged in layers. Over these is wrapped two coatings of paper, polyethylene coated aluminium and finally the Polyethylene sheath. The cable needs to be filled up with some dielectric to avoid discharge between conductors should high voltage be induced by nearby electric cables or lightning. Commonly used dielectrics are air and jelly. Air filled cables will usually be pressurized to prevent ingress of water and for ease of fault localization. The primary cables are normally air-spaced as they have to be pressurized. The distribution cables are jelly filled.
The common conductor diameters are 0.4 mm, 0.5 mm and 0.63 mm and 0.9 mm. The commonly used sizes are 5, 10, 20, 50, 100, 200, 600, 800, 1000, 1200, 1600 and 2000 pairs. The cables can also be armourd or unarmoured depending on whether they are to be buried directly or laid in duct.
Optical fibre cables normally consist of 4 or more glass fibres each of which is constructed according to on of the following structures:
The above basic fiber constructions would normally be used multiply to form cables. Cables of 4, 6, 8, 12, 16, 24, 48, 72 fibers have been commonly used. Now cables are available with hundreds of fibres. Cable construction can be loose tube, tight buffer, open channel or ribbon type. In the loose tube cable the fibres are loosely and helically placed in a PVC tube which in turn is encapsulated in aluminium moisture barrier(in all externally laid cables) and outer sheath. In tight buffer construction SCOF fibres are laid in tighter construction with the empty spaces being filled with nylon yarn. In open channel construction fibres are located in grooves formed in the central strength member. Ribbon fibre cable has fibres in ribbonised form in the grooves in the central strength member. Ribbons are flat structures of 4, 6 or 8 fibres.
The fibre does not have same loss characteristics at all the wavelengths. The loss profile shows low losses around some wavelengths and high to very high losses at others. The three wavelengths around which the losses are low are 850 nm, 1310nm and 1550 nm. Theoretical minimum losses are around 0.15 db/km at the current state of technology. The main reasons for the losses are:
In addition to OTDR simpler methods are available to prove system functioning. A light source can be used to light the fibre and a power meter to measure the power reaching the other end.
Cabinets provide flexibility points in the network. These are numbered sequentially in an exchange area. We shall see two types of cabinets: conventional copper cable cabinets and OF cabinets
A cabinet has an arrayed arrangement of termination block. Cabinets are available with varying termination capacity. An example could be a cabinet with a total termination capacity of 1600 pairs including 800 for primary cable and 800 for distribution cable. A cabinet is provided in the network to provide flexibility, separate primary side from distribution side, provide test point for maintenance, cross connect primary pairs to distribution pairs.
This cabinet consists of fibre optic transmission equipment and customer access equipment. It consists of three internal chambers. A power supply and 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. The equipment chamber has circuits for optical to electrical conversion and vice versa, multiplexing equipment and line terminating equipment. We will see more about these in another chapter.
Ducts are provided to facilitate cabling and provide protection to the cable network. Telecommunication ducts can be of different sizes to suit different purposes. On the routes for laying primary, distribution and junction cable the duct pipe that is usually used has around 100mm internal diameter and is usually 6meters in length. A number of such pipes will be combined to make one "way" from one point to another. These pipes would be laid in rectangular formations ie 4X4 for 16 way duct, 2X4 for 8 way duct etc. The ducts can be laid in sand or in concrete. For optical fibre cables smaller ducts are preferred.
Jointing Chamber is defined as any underground structure which can be used to make cable joints or placing accessories in a ducted network. The are two types of jointing chambers
(a) Joint boxes (b) Manholes
Joint boxes are surface opening jointing chambers fitted with a cover or covers the removal of which exposes substantially the whole of the interior.
To give an idea of the size, a joint box can have the following dimensions
length=1120mm width=635mm depth=920mm
Manholes are roofed jointing chambers, usually bigger in dimension as compared to joint boxes. They are always reinforced type. Access can be gained through an opening in the roof. Cable personnel go into the manhole to perform jointing and other operations. An example of the manhole dimensions is given here:
length=3950mm width=1600mm depth=2125mm
The distribution point provides termination for the D side cable and a place for interconnection of customers' lines from their premises. Commonly used sizes are 10 pairs and 20 pairs. Four(4) types of D.P.s are used in the network
The following types of sheath closures are standard:
In the Distribution Network where pair distribution i.e. breakdown is required then is sleeves can be used. Where "through" (straight) joints are planned, taped sleeve and collars can be effectively utilized.
One type of sleeve, Sleeve 31A, consists of a cylindrical base through which the cables enter and a dome shaped cap to close the joint. The two parts are clamped together by hinged collar secured by a hinged bolt and thumb nut. The base incorporates an insert with partitions. These partitions divide the base into one large and 5 small compartments. The cables are fitted through holes pierced in the base of the sleeve and the appropriate compartment. Subsequently the compartments used are filled with quick setting resin to provide waterproof seals between the cables and the case. The central larger compartment will accept 2 cables 100/0.63 or 1 cable 100/0.63 and 2 cables 50/0.63 or 1 cable 50/0/63 and several smaller cables. For any other combination that meets the criterion of 3mm space between the cables and the wall of the compartment and cable.
In the Primary Network Polyethylene injection moulding (PIM) also known as injection welding has been used for the air-spaced cables. Other prevalent types of joints are thermoshrink and mechanical joint closure system.
Splicing of fibres: Splicing is a term used for joining of two fibres. Two types of splices are prevalent:
Typical loss figures are:
Connecters 0.5 dB
Splices 0.2 dB
Multimode fibre -3 dB/km at 850 nm , -1 dB/km at 1300 nm
Single Mode fiber -0.4 dB/km at 1300 nm, -0.3 dB/km at 1550 nm
You may be interested in one of these...
1. Introduction to telecommunication networks
3. Subscriber Demand and Traffic Forecasting
4. Planning access networks
5. New technologies in the access network
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