THE TELEPHONE SYSTEM

In the past, signaling throughout the telephone system was analog, (real voice signal). Digital signaling has now become possible, allowing two voltages (-5 volts and +5 volts).

Advantages:

• Signal regeneration is easy over long distances.
• Voice, music, images and data can be intermixed.
• Much faster data transmission speed.
• Maintenance is easier; it is easy to spot mistakes.
• More economical.

Local Ties

They are analog. Computers have to use a modem to convert a digital signal to analog when transmitting on a local loop, and in the telephone company office a codec converts it back to digital to transmit over long-haul trunks, then back to analog. in the local loop of the receiving end and, finally, to digital in another modem to be stored in the destination computer.

Trunks and Multiplexing

One of the goals for long-haul trunks is to collect multiple calls and send them together on a single high-bandwidth line, this is called multiplexing. There are several types of multiplexing:

• Frequency Division Multiplexing (FDM). Filters are used to restrict each phone channel to only 3000 Hz. To ensure good separation 4000 Hz is allocated for each channel. Each voice channel is raised in frequency and then combined; each channel is independent of the others.
• Wavelength Division Multiplexing (WDM Wavelength Division Multiplexing). It's the same idea as MDF, but with light and fiber optics. Since each channel in a fiber cannot be more than a few gigahertz wide (due to the maximum speed of converting between optical and electrical signals), it is a good way to use the roughly 25.000 GHz bandwidth of a fiber. In this case the incoming channels must have different frequencies and are combined with a prism.
• Time Division Multiplexing (TDM). The problem with MDF is that you have to use analog circuitry. By contrast, the MDT can be driven entirely with digital electronics. In MDT each user successively has the full bandwidth of the channel for a moment. MDT can only be used with digital data

SONET (Synchronous Optical Network) Synchronous optical network. It is a synchronous MDT system controlled by a master clock. It consists of switches, multiplexers and repeaters all connected by fiber. The SONET frame is a block of 810 bytes, which is broadcast every 125 microseconds, which implies a speed of 51,84 Mbps. Each frame is a byte rectangle of 90 columns(width) by 9 rows(height).

Switching Offices

Two different switching techniques are used within the telephone system: circuit switching and packet switching.

Circuit Switching

When a user or computer makes a phone call, the telephone system's switching equipment looks for a physical "copper" path (including fiber and radio) from the own phone to the recipient's phone. In the figure below, once a call has been established, a dedicated path exists between both ends and will continue to exist until the call ends.

In circuit switching, a path must be established from one end to the other before any data can be sent. The time it takes to finish dialing and for the ring to start ringing can be as long as 10s, and longer for long distance or international calls. An advantage of the established path is that there is no danger of congestion. An alternate switching strategy is message switching. In this form of switching, a path is not established in advance. Instead, when the sender has a data frame to send, it is stored in the first switching office (router) and then forwarded, one hop at a time. Each frame is received in its entirety, inspected for errors, and then retransmitted. A network that uses this technique is called a store-and-forward network. With message switching, there is no limit to the size of frames, which means that routers (in a modern system) must have disks to buffer long frames. This is also why a single frame can hog a line from router to router for minutes, rendering message switching useless for interactive traffic. In order to solve these problems, packet switching was invented. Packet-switched networks set an upper limit on frame size, allowing packets to be stored in the router's main memory instead of on disk. Packet-switched networks can handle interactive traffic. Computer networks are normally packet-switched, occasionally circuit-switched, and never message-switched.