Telephony

Access Network

Provides the subscriber’s connection to the Local Exchange (Central Office, CO).

The biggest investment and bottleneck in a telecommunication network.

Access Technologies

• Local Loop (wired)
  Copper wire pairs optimized to carry voice in analog form
• Wireless Local Loops (WLL)
  Replaces the local loop with wireless connection
• Digital Subscriber Loops (DSL)
  Modified to carry data and voice within certain constraints on copper local loops
• Fiber to the x (FTTx)
  Optical fiber local loop (x=curb, business, home)

Switching

Connects subscribers to each other on demand.

• Manual operators: human operators who manually connect calls
• Automatic Switches – Electromechanical switches – Software-controlled switches

Transmission

Transportation of signals between subscribers of networks. Carries thousands of multiplexed calls between exchanges.

High capacity copper cables, microwave (radio) links, optical fibers can be used.

Telephone Numbering System

• Country Code (94)
• Area Code (11)
• Operator Code (2)
• Telephone No. (650301)

Extension codes (3100) are used within an organization or a country for quick identification.

Signalling

Functions:

• Call connection
• Call disconnection
• Billing

Enhanced Services

SMS

Sending and receiving messages of upto 160 characters. Part of Integrated Digital Subscriber Line (ISDN). Available on Mobile and CDMA networks. Messages are stored and forwarded by SMS Centre. Receiver does not have to be on-line when message sent.

Calling Line ID

Aka. CLI. Provides the calling number to the receiver. Part of ISDN. Also available on analog lines. May be enhanced by looking up number in directory. May be disabled to protect caller’s privacy.

Location Based Services

Providing different services based on location of subscriber such as emergence services and delivery services.

For Fixed lines, location is known. For mobiles, location may be identified using:

• cell ID
• GPS
• Triangulation

VoIP

Short for Voice over Internet Protocol. Using packet-switched data networks for interactive voice communications. Analog voice signals are digitized and then converted into packets for transmission over the network. At the receiving end, packets are converted back into analog signals and played through a speaker or headset.

Cheaper compared to traditional telephony. Can reuse already existing infrastructure for internet.

High latency. Packet losses can be there. Low voice quality.