Line access subsystem of a dss

Analog subscriber lines use Z interface to access analog customer units. It is possible to connect the following terminals using Z interface.

1. phones and fax machines;

2. local and toll payphones;

3. modems;

4. modem pools of IPOPs;

5. hardware of different voice services

The U interface enables ISDN services with the help of the basic rate access. Such a line requires NT at customer premises and LT on the side of a TE. The NT provides the S and T interfaces for subscriber terminals

There is a V₁ interface between LTs (up to 16) and a switching network. Some digital switching systems allow to connect 2, 4, 8 or 16 remote analog subscribers via the U interface. Such subscribers use two B channels (usually multiplexed 32kbit/s channels per each B channel). In this case multiplexing equipment must be installed instead of LT and NT.

A remote subscriber module (RSubM) uses the V₂ interface to access host equipment (using the E1 trunks). The RSubM provides the Z and U interfaces for subscriber lines.

The V₃ interface enables ISDN services with the help of the primary rate access. Such a line can support a PBX, local computer nets, or high bit rate terminals.

The V₄ interface is necessary for a subscriber multiplexer connection. Such a multiplexer provides the Z and U interfaces for subscriber lines.

ASL

E1 trunk

DSN

TE

TE

TE

TE

TE

TE

TE

TE

TE

TE

TE

BAU

D

A

D

A

LAN

PBX

WLL

equip

DSL

E1 trunk

E1 trunk

E1 trunk

E1/2 trunk

E1/2 trunk

E1/2 trunk

Physical trunk

TS

V_B5.2

Optical trunk

Figure 11 – Typical subscriber and network interfaces

There are two modifications of the V₅ interface available. They are V_5.2 and V_5.1. The V_5.1 protocol allows to connect either 30 analogue subscriber lines (one E1 trunk) or 30 B channels. However, it does not support primary rate access. The V_5.2 protocol supports 16 E1 trunks, concentrates subscriber lines, and supports primary rate access. Moreover, it supports lines from broadband access unit and wireless local loop. It provides separate signaling channels for every trunk.

A broadband access unit can provide the V_B5 broadband interfaces for optical or coaxial lines of B-ISDN (155 Mbit/s or 622 Mbit/s). The broadband access unit provides V_5.2 interfaces for xDSL lines and an interface (STM-1) for ATM network.

The A interface is needed for an E1 trunk (32 time channels, 2048 kbit/s). The A₁ interface is necessary for an E1/2 trunk (16 time channels, 1024 kbit/s).

The C₁ interface is applied between D/A converters and 4-wire physical trunks while the C₁₁ interface is designed for TDM transmission systems.

The C₂ interface is applied between D/A converters and 2 or 3-wire physical trunks.

Up to date digital switching system supports subscriber lines of many types. Subscriber access can be different (analogue, narrowband digital, broadband digital, optical, and wireless).

Analogue subscriber access

Narrowband subscriber access subsystem allows subscriber modules to connect analogue subscriber lines via Z interface. An analogue customer unit performs BORSCHT functions. Figure 2 shows an analogue subscriber line connected to a subscriber module.

Figure 2 – Analogue subscriber line connected to a subscriber module

An analogue phone accesses an analogue customer unit via 2-wire analogue subscriber line. ACU converts analogue informational signal to digital and vice versa. The customer unit has 4-wire output.

Analogue customer unit performs the following functions.

1. It senses “off-hook” state.

2. It receives the dialed digits.

3. It provides power supply for a phone.

4. It performs A/D and D/A conversions.

5. It sends ringing.

6. It senses “on-hook” state.