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For the transmission of bit stream from one machine to another, various physical media can be used. They differ in terms of:
Media can be divided into:
One of the most common ways to transport data from one computer to another is to write them onto magnetic tapes or floppy disks, physically transport the tapes or disks to the destination machine and read them back in again.
Example: Industry standard 8 mm video tape can hold 7 gigabytes. A box of
50 x 50 x 50 cm can hold about 1000 of these tapes for a total
capacity 7000 GB. It can be delivered in 24 hours anywhere in the US. Effective
bandwidth is 56000 gigabits/86400 sec = 648 Mbps (better than high-speed
version of ATM (622 Mbps)). Estimated cost: 10 cents/gigabyte which is
unbeatable. The disadvantage of this kind of the transmission is
definitely big delay.
2.2.2. Twisted pairs
Twisted pair is the oldest and still most common transmission medium. It consists of two insulated copper wires, typically about 1 mm thick. The wires are twisted together to reduce electrical interference from similar pairs close by (two parallel wires constitute a simple antenna, a twisted pair does not).
The most common application of the twisted pair is the telephone system. Twisted pairs can run several km without amplification, but for longer distances repeaters are needed.
Twisted pairs can be used for either analog or digital transmission. The bandwidth depends on the thickness of the wire and the distance traveled (several mbps for a few km can be achieved).
Twisted pair cabling comes in several varieties, two of which are important for computer networks:
Coaxial cable (frequently called "coax") is another common transmission medium. It has better shielding than twisted pairs, so it can span longer distances at higher speeds.
Two kinds of coaxial cables are widely used:
(The distinction is based on historical rather than technical factors.)
A cutaway view of a coaxial cable is shown in Fig. 2-3. The bandwidth depends on the cable length. For 1 km cables, a data rate 1 - 2 Gbps is feasible. Longer cables enable only lower data rates or require periodic amplifiers.
Fig. 2-3. A coaxial cable.
Coaxial cables used to be widely used within the telephone system - now
they are largely replaced by fiber optics on long-haul routes (1000 km of
fiber installed every day in the US).
2.2.4. Broadband Coaxial Cable
75-ohm coaxial cable is used on standard cable television. It is called broadband.
In the telephone world, "broadband" refers to anything wider than 4 kHz. In the computer networking world, "broadband cable" means any cable network using analog transmission (the analog signaling consists of varying voltage with time to represent an information stream).
The cables in broadband networks can be used often up to 450 MHz and can run for nearly 100 km due to the analog signaling which is much less critical than digital signaling. To transmit digital signal on an analog network, outgoing bit stream must be converted to an analog signal and the incoming analog signal to a bit stream. 1 bps may occupy roughly 1 Hz of the bandwidth. At higher frequencies, many bits per Hz are possible using advanced modulation techniques.
Broadband systems are divided up into multiple channels, frequently the 6-MHz channels used for television broadcasting. Each channel can be used for analog TV, CD quality audio (1.4 Mbps) or a digital bit stream at, say, 3 Mbps. Television and data can be mixed on the cable.
Amplifiers in broadcast systems can only transmit signal in one direction. When the cabling is used for connecting computers, so called dual cable systems and single cable systems have been developed (Fig. 2-4).
Fig. 2-4. Broadband networks. (a) Dual cable. (b) Single cable.
In race between computing and communication, communication won (improvement factor 10 vs. 100 per decade during the last two decades) due to using fibre optics in communication.
An optical transmission system has three components:
By attaching a light source to one end of an optical fiber and a detector to the other, we get a unidirectional data transmission system.
The work of this transmission system is based on the refraction of the light ray at the silica/air boundary (Fig. 2-5).
Fig. 2-5. (a) Three examples of a light ray from inside a silica fiber
impinging on the air/silica boundary at different angles.
(b) Light trapped by total internal reflection.
Since any light ray incident on the boundary above the critical angle will be reflected internally, many different rays will be bouncing around at different angles. Each ray is said to have a different mode, so a fiber having this property is called a multimode fiber.
If the fiber's diameter is reduced to a few wavelengths of light, the fibre acts like a wave guide and the light can only propagate in a straight line, without bouncing, yielding a single mode fiber.
Single mode fibers are more expensive but can be used for longer distances
(typically several Gbps for 30 km).
2.2.6. Transmission of Light through Fiber
The glass used for modern optical fibers is so transparent that if the ocean were full of it instead of water, the seabed would be visible from the surface.
The attenuation of light through glass depends on the wavelength (Fig. 2-6). It is expressed in decibels given by the formula:
Attenuation in decibels = 10 log10 transmitted power/received power
Fig. 2-6. Attenuation of light through fiber in the infrared region.
Wavelength 0.85, 1.30, and 1.55 microns (micro meters) are used for communication (Fig. 2-6). 0.85 has higher attenuation but it has a nice property that, at that wavelength, the lasers and electronics can be made from the same material. The bands for all three wavelengths are 25000 - 30000 GHz.
Visible light has slightly shorter wavelength (0.4 - 0.7 microns).
Light pulses sent down a fiber spread out in length as they
propagate. This is called dispersion. The amount of dispersion is wavelength
dependent. It was discovered (so far it works just in lab conditions) that
when pulses have special shape (called solitons) all the dispersion
effects cancel.
2.2.7. Fiber Cables
Fiber optics cables are similar to coax, except without the braid (Fig. 2-7). In multimode fibers, the core is typically 50 microns in diameter, in single mode fibers the core is 8 - 10 microns. The cladding has a lower index of refraction than the core to keep all the light in the core.
Fig. 2-7. (a) Side view of a single fiber. (b) End view of a sheath with three fibers.
Fibers can be connected in three different ways:
Two kinds of light sources can be used to do the signaling:
The properties of the both sources are shown in Fig. 2-8.
Fig. 2-8. A comparison of semiconductor diodes and LEDs as light sources.
The receiving end of an optical fiber consists of a photo diode. The
typical response time of a photodiode is 1 nsec which limits data
rates to about 1 Gbps.
2.2.8. Fiber Optics Networks
Fiber optics can be used for LANs as well as for long-haul transmission. Tapping onto it is more complex than connecting to a copper wire. One way around the problem is shown in Fig. 2-9.
Fig. 2-9. A fiber optic ring with active repeaters.
Another solution is displayed in Fig. 2-10.
Fig. 2-10. A passive star connection in a fiber optics network.
Advantages of fibers:
Disadvantages of fiber:
Nevertheless, the future of all fixed data communication for more than a few meters is clearly with fiber.