The Local Loop: Modems, ADSL, and Fiber

The Local Loop: Modems, ADSL, and Fiber

THE LOCAL LOOP: MODEMS, ADSL, AND FIBER – It is time to start our detailed study of the operation of the telephone system. Let’s start with the part that most people are familiar with: a local two-wire loop that comes from the phone desk and offices. The local loop is often called the “last mile”, although it can be several kilometers long. It has been carrying analog information for over 100 years and will likely continue to do so for many years due to the high cost of digital conversion.

Much effort has gone into removing existing local copper loops data networks. Telephone modems transmit digital data between computers over a narrow channel provided by the telephone network for voice calls. They were once widely used, but have been largely supplanted by broadband technologies such as ADSL. reuse the local loop to send digital data from the customer to the final office, where it is transmitted to the Internet.

Modems and ADSL must take into account the limitations of older local circuits: relatively narrow bandwidth, attenuation, and distortion of the signal, and susceptibility to electrical interference, such as interference.

The following local loops are often considered important:

  • Telephone modems
  • Digital subscriber lines
  • Fiber to the home

Telephone Modems

Telephone modems are used to transfer bits between two computers on a voice phone line, rather than for a conversation, which usually occupies the line. The main difficulty is that the voice telephone line is limited to 3,100 Hz, which is enough for a conversation. This bandwidth is more than four orders of magnitude less than the bandwidth used for Ethernet or 802.11 (WiFi). As expected, data rates for telephone modems are also four orders of magnitude lower than those for Ethernet and 802.11. A device that converts a stream of digital bits and an analog signal that represents bits is called a modem, which is an abbreviation for “modulator-demodulator“. Modems are of different types: telephone modems, DSL modems, cable modems, wireless modems, and so on.

The modem can be integrated with a computer (now common to telephone modems) or be a separate unit (common to both DSL and cable modems). Logically, a modem is inserted between the computer (digital) and the telephone system (analog), as shown in the following figure.

the local loop

Figure: The use of both analog and digital transmission for a

computer-to-computer call. Conversion is done by the modems and codecs.

Digital Subscriber Lines

Initially, there were several high-speed incrustations under the general name xDSL (digital subscriber line) for several x’s. Services with more bandwidth than standard telephone services are sometimes called broadband, although this term is a marketing concept rather than a specific technical concept. Unfortunately, the capacity of the local loop decreases quite rapidly with the distance from the end office, as the signal deteriorates along the cable. It also depends on the thickness and overall quality of the twisted pair. The graph of potential bandwidth versus distance is shown in the following figure. In this figure, it is assumed that all other factors are optimal (new cables, modest packages, etc.).

Digital Subscriber Lines

Figure: Bandwidth versus distance over Category 3 UTP for DSL

The international ADSL standard, known as G.dmt, was approved in 1999. It provides speeds of up to 8 Mbps downlink and 1 Mbps uplink. It was replaced by the second generation in 2002, called ADSL2. Some enhancements have been made to achieve speeds of up to 12 Mbps downlink and 1 Mbps uplink. We now have ADSL2 +, which doubles the downstream speed to 24 Mbps, doubling the bandwidth to use 2.2 MHz on a twisted pair.

Fiber To The Home

In general, fiber houses are combined so that only one fiber reaches the final office per group of 100 houses. In the downward direction, the optical dividers divide the final desktop signal so that it reaches all the homes. Encryption is necessary for security if only one house can decode a signal. Upward, the optical adders combine the house signals into a single signal, which is received in the last office. This architecture is called PON (passive optical network) and is shown in the following figure. In general, for all downlink channels, the total wavelength is used for transmission in the downstream direction and another wavelength is used for transmission in the upstream direction.

Fiber To The Home

Figure: Passive optical network for Fiber to The Home.

Read More Structure Of The Telephone System


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