TELEPHONE NETWORK

Telephone networks use circuit switching. The telephone network had its beginnings in the late 1800s. The entire network, which is referred to as the plain old telephone system (POTS), was originally an analog system using analog signals to transmit voice.

Topics discussed in this section:

• Major Components
• LATAs
• Signaling
• Services Provided by Telephone Networks

Major Component

Three major components:

1. Local loops,
2. Trunks, and
3. Switching offices.

Local Loops

The local loop is a twisted-pair cable that connects the subscriber telephone to the

nearest end office or local central office.

The local loop, when used for voice, has a bandwidth of 4000 Hz (4 kHz).

The first three digits of a local telephone number define the office, and the next four digits define the local loop number.

Trunks

Trunks are transmission media that handle then communication between offices.

A trunk normally handles hundreds or thousands of connections through multiplexing.

Transmission is usually through optical fibers or satellite links.

Switching Offices

To avoid having a permanent physical link between any two subscribers, the telephone company has switches located in a switching office.

A switch connects several local loops or trunks and allows a connection between different subscribers.

Circuit Switching

• Uses a dedicated path between two stations
• Has three phases
• Establish
• Transfer
• Disconnect
• Inefficient
• Channel capacity dedicated for duration of connection
• If no data, capacity wasted
• Set up (connection) takes time
• Once connected, transfer is transparent

A telephone system

Switched Network

LATA's

After the divestiture of 1984 , the United States was divided into more than 200 local-access transport areas (LATAs). The number of LATAs has increased since then.

A LATA can be a small or large metropolitan area.

A small state may have one single LATA.a large state may have several LATAs.

A LATA boundary may overlap the boundary of a state; part of a LATA can be in one state, part in another state.

Intra-LATA Services

The services offered by the common carriers (telephone companies) inside a LATA are called intra-

LATA services. The carrier that handles these services is called a local exchange carrier (LEC).

Inter-LATA Services

The services between LATAs are handled by inter exchange carriers (IXCs). These carriers, sometimes called long-distance companies, provide communication services between two customers in different LATAs.

Switching offices in a LATA (local-access transport area)

Note

Intra-LATA (local access transport area)

services are provided by local exchange

carriers (LECs).

Since 1996, there are two

types of LECs: incumbent local

exchange carriers and competitive

local exchange carriers.

IXC (Interexchange carrier, long distance company)

Points of Presence

A point of presence (POP) is a switching office.

Each IXC that wants to provide inter LATA services in a LATA must have a POP in that LATA.

The LECs that provide services inside the LATA must provide connections so that every subscriber can have access to all POPs.

Point of presences (POPs)

Making a Connection

Subscriber who needs to make a connection with another subscriber is connected first to an end switch.
Then, either directly or through a tandem switch, to a POP.
The call now goes from the POP of an IXC (the one the subscriber has chosen) in the source LATA to the POP of the same IXC in the destination LATA.
The call is passed through the toll office of the IXC and is carried through the network provided by the IXC.

Signaling

• The signaling system was required to perform tasks such as
• It uses the digital signals(each digit in a multidigit telephone number) to create a connection between the caller and the called parties.
• Providing dial tone, ring tone, and busy tone
• Transferring telephone numbers between offices
• Maintaining and monitoring the call
• Keeping billing information
• Maintaining and monitoring the status of the telephone network equipment
• Providing other functions such as caller ID, voice mail, and so on.

Note

The tasks of data transfer and signaling

are separated in modern telephone

networks: data transfer is done by one

network, signaling by another.

Note

Although the two networks are separate,

this does not mean that there are

separate physical links everywhere; the

two networks may use separate

channels of the same link in parts of the

system.

Data transfer and signaling networks

Terms Related to Signaling

• The user telephone or computer is connected to the signal points (SPs). The link between the telephone set and SP is common for the two networks.
• The signaling network uses nodes called signal transport ports (STPs) that receive and forward signaling messages.
• The signaling network also includes a service control point (SCP) that controls the whole operation of the network.
• Other systems such as a database center may be included to provide stored information about the entire signaling network.

Signaling System Seven (SS7)

Layers in SS7 (signaling system seven)

Physical Layer:

It uses several physical layer specifications such as T-l (1.544 Mbps) and DCa (64 kbps).

Data Link Layer:

It provides typical data link layer services such as packetizing, using source and destination address in the packet header, and CRC for error checking.

Network Layer:

It provides end-to-end connectivity by using the datagram approach to switching. Routers and switches route the signal packets from the source to the destination.

Transport Layer:

SCCP The signaling connection control point (SCCP) is used for special services such as VoIP-call processing.

Upper Layers:

TUP, TCAP, and ISUP There are three protocols at the upper layers.

Telephone user port (TUP) is responsible for setting up voice calls. It receives the dialed digits and routes the calls.

Transaction capabilities application port (TCAP) provides remote calls that let an application program on a computer invoke a procedure on another computer. ISDN user port (ISUP) can replace TUP to provide services similar to those of an ISDN network.