Multiplexing

Multiplexing is a technique used to combine and send the multiple data streams over a single medium. The process of combining the data streams is known as multiplexing and hardware used for multiplexing is known as a multiplexer.

Multiplexing is achieved by using a device called Multiplexer (MUX) that combines n input lines to generate a single output line. Multiplexing follows many-to-one, i.e., n input lines and one output line.

Demultiplexing is achieved by using a device called Demultiplexer (DEMUX) available at the receiving end. DEMUX separates a signal into its component signals (one input and n outputs).

The transmission medium is used to send the signal from sender to receiver. The medium can only have one signal at a time. If there are multiple signals to share one medium, then the medium must be divided in such a way that each signal is given some portion of the available bandwidth.

For example: If there are 10 signals and bandwidth of medium is 100 units, then the 10 unit is shared by each signal.

When multiple signals share the common medium, there is a possibility of collision. Multiplexing concept is used to avoid such collision.

Types of Multiplexing

Frequency Division Multiplexing (FDM) –

Frequency spectrum is divided among the logical channels and each user has exclusive access to his channel. It sends signals in several distinct frequency ranges and carries multiple video channels on a single cable. Each signal is modulated onto a different carrier frequency and carrier frequencies are separated by guard bands.

 

Bandwidth of the transmission medium exceeds required bandwidth of all the signals. Usually for frequency division multiplexing analog signalling is used in order to transmit the signals, i.e. more susceptible to noise. 

A multiplexer accepts inputs and assigns frequencies to each device. The multiplexer is attached to the high speed communication line. A corresponding multiplexer or de-multiplexer is on the end of the high speed line and separates the multiplexed signals. The frequency spectrum is divided up among the logical channels where each user hangs onto a particular frequency. The radio spectrum are examples of the media and the mechanism for extracting information from the medium.

Disadvantage of FDM:

One problem with FDM is that it cannot utilize the full capacity of the cable. It is important that the frequency bands do not overlap. Indeed, there must be a considerable gap between the frequency bands in order to ensure that signals from one band do not effect signals in another band.

Time Division Multiplexing (TDM) 

Each user periodically gets the entire bandwidth for a small burst of time, i.e. entire channel is dedicated to one user but only for a short period of time. It is very extensively used in computer communication and tele-communication. Sharing of the channel is accomplished by dividing available transmission time on a medium among users. It exclusively uses the Digital Signaling instead of dividing the cable into frequency bands. 

TDM splits cable usage into time slots. Data rate of transmission media exceeds dats rate of signals. Uses a frame and one slot for each slice of time and the time slots are transmitted whether source has data or not.

There are two types of TDMs which are as follows:

1. Synchronous Time Division Multiplexing:

It is synchronous because the multiplexer and the de-multiplexer has to agree about the time slots. The original time division multiplexing. The multiplexer accepts input from attached devices in a round robin fashion and transit the data in a never ending pattern. Each input connection has an allotment even if it is not sending data.

Some common examples of this are T-1 and ISDN telephone lines. 

One problem with TDM is to handle different data rates. Three techniques or combination are used to handle 

a. Multilevel Multiplexing

b. Multiple-slot Multiplexing

c. Pulse stuffing.

Multilevel Multiplexing:

In this technique different data rates are handled by combining two or more input links into one link as shown in below figure.

Multiple-slot Multiplexing:

In this technique one data rate signal is split into two or more links as shown in below figure.

Pulse stuffing:

In this technique if a link has less data rate then pulse stuffing is done to make the data rate equal to other links as shown in the figure below.

2. Statistical Time Division Multiplexing:

Time-division but on demand rather than fixed, a slot is given in the output frame only if three input line has a slot's worth of data to send. It allows connection of more nodes to the circuit than the capacity of the circuit. Works on the premise that not all the nodes will transmit at full capacity at all times. 

    It must transmit a terminal identification i.e destination identification number, and may require storage. A statistical multiplexer transmits only the data from active workstations. If a workstation is not active, no space is wasted on the multiplexed stream. It accepts the incoming data streams and creates a frame containing only the data to be transmitted.

In statistical division multiplexing a slot needs to carry data as well as the address of destination.

Time Slot comparison of Syn TDM and Stat TDM

TRANSMISSION MEDIA

A transmission medium can be broadly defined as anything that can carry information from a source to a destination.

Guided Media

Guided media, which are those that provide a conduit from one device to another, include twisted-pair cable, coaxial cable, and fiber-optic cable. A signal traveling along any of these media is directed and contained by the physical limits of the medium. Twisted-pair and coaxial cable use metallic (copper) conductors that accept and transport signals in the form of electric current. Optical fiber is a cable that accepts and transports signals in the form of light.

1. Twisted-Pair Cable

A twisted pair consists of two conductors (normally copper), each with its own plastic insulation, twisted together.

One of the wires is used to carry signals to the receiver, and the other is used only as a ground reference. The receiver uses the difference between the two. In addition to the signal sent by the sender on one of the wires, interference (noise) and crosstalk may affect both wires and create unwanted signals. If the two wires are parallel, the effect of these unwanted signals is not the same in both wires because they are at different locations relative to the noise or crosstalk sources (e,g., one is closer and the other is farther). This results in a difference at the receiver. By twisting the pairs, a balance is maintained.

Applications

Twisted-pair cables are used in telephone lines to provide voice and data channels. The local loop-the line that connects subscribers to the central telephone office-commonly consists of unshielded twisted-pair cables. The DSL lines that are used by the telephone companies to provide high-data-rate connections also use the high-bandwidth capability of unshielded twisted- pair cables. Local-area networks, such as lOBase-T and lOOBase-T, also use twisted-pair cables.

2. Coaxial Cable

Coaxial cable (or coax) carries signals of higher frequency ranges than those in twisted pair cable, in part because the two media are constructed quite differently. Instead of having two wires, coax has a central core conductor of solid or stranded wire (usually copper) enclosed in an insulating sheath, which is, in turn, encased in an outer conductor of metal foil, braid, or a combination of the two. The outer metallic wrapping serves both as a shield against noise and as the second conductor, which completes the circuit. This outer conductor is also enclosed in an insulating sheath, and the whole cable is protected by a plastic cover

Applications

Coaxial cable was widely used in analog telephone networks where a single coaxial network could carry 10,000 voice signals. Later it was used in digital telephone networks where a single coaxial cable could carry digital data up to 600 Mbps.

Cable TV uses RG-59 coaxial cable. Another common application of coaxial cable is in traditional Ethernet LANs. Because of its high bandwidth, and consequently high data rate, coaxial cable was chosen for digital transmission in early Ethernet LANs.

3. Fiber Optic Cable: 

A fiber-optic cable is made of glass or plastic and transmits signals in the form of light. Optical fibers use reflection to guide light through a channel. A glass or plastic core is surrounded by a cladding of less dense glass or plastic. The difference in density of the two materials must be such that a beam of light moving through the core is reflected off the cladding instead of being refracted into it.

The outer jacket is made of either PVC or Teflon. Inside the jacket are Kevlar strands to strengthen the cable. Kevlar is a strong material used in the fabrication of bulletproof vests. Below the Kevlar is another plastic coating to cushion the fiber. The fiber is at the center of the cable, and it consists of cladding and core.

Applications

Fiber-optic cable is often found in backbone networks because its wide bandwidth is cost- effective. Today, with wavelength-division multiplexing (WDM), we can transfer data at a rate of 1600 Gbps. The SONET network provides such a backbone. Some cable TV companies use a combination of optical fiber and coaxial cable, thus creating a hybrid network.

Local-area networks such as 100Base-FX network (Fast Ethernet) and 1000Base-X also use fiber-optic cable.

Advantages and Disadvantages of Optical Fiber

Advantages

Fiber-optic cable has several advantages over metallic cable (twisted pair or coaxial).

a. Higher bandwidth. Fiber-optic cable can support dramatically higher bandwidths (and hence data rates) than either twisted-pair or coaxial cable. Currently, data rates and bandwidth utilization over fiber-optic cable are limited not by the medium but by the signal generation and reception technology available.

b. Less signal attenuation. Fiber-optic transmission distance is significantly greater than that of other guided media. A signal can run for 50 km without requiring regeneration. We need repeaters every 5 km for coaxial or twisted-pair cable.

c. Immunity to electromagnetic interference. Electromagnetic noise cannot affect fiber-optic cables.

d. Resistance to corrosive materials. Glass is more resistant to corrosive materials than copper.

e. Light weight. Fiber-optic cables are much lighter than copper cables.

f. Greater immunity to tapping. Fiber-optic cables are more immune to tapping than copper cables. Copper cables create antenna effects that can easily be tapped.

Disadvantages

There are some disadvantages in the use of optical fiber.

a. Installation and maintenance. Fiber-optic cable is a relatively new technology. Its installation and maintenance require expertise that is not yet available everywhere.

b. Unidirectional light propagation. Propagation of light is unidirectional. If we need bidirectional communication, two fibers are needed.

c. Cost. The cable and the interfaces are relatively more expensive than those of other guided media. If the demand for bandwidth is not high, often the use of optical fiber cannot be justified.

UNGUIDED MEDIA: WIRELESS

Unguided media transport electromagnetic waves without using a physical conductor. This type of communication is often referred to as wireless communication. Signals are normally broadcast through free space and thus are available to anyone who has a device capable of receiving them. Unguided signals can travel from the source to destination in several ways: ground propagation, sky propagation, and line-of-sight propagation.

In ground propagation, radio waves travel through the lowest portion of the atmosphere, hugging the earth. These low- frequency signals emanate in all directions from the transmitting antenna and follow the curvature of the planet. Distance depends on the amount of power in the signal: The greater the power, the greater the distance. In sky propagation, higher-frequency radio waves radiate upward into the ionosphere where they are reflected back to earth. This type of transmission allows for greater distances with lower output power. In line-or-sight propagation, very high-frequency signals are transmitted in straight lines directly from antenna to antenna. Antennas must be directional, facing each other, and either tall enough or close enough together not to be affected by the curvature of the earth. Line-of-sight propagation is tricky because radio transmissions cannot be completely focused.