Data Communication and Computer NetworksChapter 2Transmission Media and Network Devices
OverviewThe term media is used because not all networks use traditional cable. This term encompasses copper-based (guided) fiber-optic cable (guided) wireless (unguided)Choosing the correct network media is an important consideration because the media forms the foundation for the entire network. This can be based on Interference Speed Max Length Security Installation and Repair ease
Media InterferenceAs a data signal travels through a specific media, it may be subjected to a type of interference known as electromagnetic interference (EMI).Many different things cause EMI; common sources includecomputer monitors fluorescent lighting speakers, basically anything that creates an electromagnetic field.If a network cable is too close to such devices, the signal within the cable can become corrupted.Some network media are more susceptible than others to the effects of EMI. Copper-based media are prone to EMI, whereas fiber-optic cable is completely resistant to it.
CrosstalkData signals may also be subjected to something commonly referred to as crosstalk, which occurs when signals from two cables in close proximity to one another interfere with each other.As a result, the signals on both cables may become corrupted.When you're troubleshooting intermittent network problems, it might be worth your time to confirm that crosstalk or EMI is not at the root of your problems.
Transmission SpeedOne of the important media considerations is the supported data transmission rate or speed. Transmission rates are normally measured by the number of data bits that can traverse the media in a single second. In the early days of data communications this measurement was expressed as bits per second (bps), but today's networks are measured in Mbps (megabits per second) and Gbps (gigabits per second).a network that accommodates huge amounts of data -> transmission rates are a crucial consideration. in small offices where they occasionally share files and maybe a printer -> transmission rate is not a big issue.
Media LengthEach media has a recommended maximum length, and surpassing these recommendations can cause unusual network problems that are often difficult to troubleshoot.In some cases, the network simply will not work.Media have maximum lengths because a signal weakens as it travels farther from its point of origin. The weakening of data signals as they traverse the media is referred to as attenuation.
Copper-based media is susceptible to attenuation. Some copper media, such as Shielded Twisted Pair (STP) use a special shielding inside the cable, which increases the distance the signal travels.Another strategy commonly employed to compensate for attenuation is signal regeneration. The cable itself does not perform the regeneration process; rather, network devices such as switches or repeaters handle signal regeneration. These devices strengthen the signal as it passes, and in doing so, they increase the distance the signal can travel.Fiber-optic cable does not suffer from attenuation. Instead, it suffers from a condition called "chromatic dispersion." Chromatic dispersion refers to the weakening of the light strength as it travels over distance.
Secure TransmissionPhysical media provides a relatively secure transmission medium, because to gain access to the signal on the cable, a person must be able to physically access it, that is, he or she must be able to tap into the cable.Fiber-optic cable is more secure than copper based media because the light transmissions and glass or plastic construction make it particularly hard to tap into. When it comes to security, wireless media is the weakest.
Installation and RepairSome network media are easier to manage and install than others. This might seem like a minor consideration, but in real-world applications, it can be important. For example, fiber-optic cable is far more complex to install and troubleshoot than twisted pair (copper cable). It's so complicated, in fact, that special tools and training are often needed to install a fiber-optic based network.
Network MediaNetwork media can be divided into two distinct categories: cable and wireless, sometimes referred to as guided (bound) and unguided (unbound) media.Cable media come in three common types: twisted-pair, coaxial, and fiber-optic.Wireless media have another range.
Cable MediaThe most widely implemented media There are two types of cable media: copper and glass/plastic. Copper-based cable is widely used to connect LANs and wide area networks (WANs), and optical cable, which uses glass or plastic, is mainly used for large-scale network implementations or over long distances.
Copper Media Copper is relatively inexpensive, easy to work with, and well suited to the needs of the modern network. There are two types of copper cables: coax and twisted-pair cable. Twisted pair is divided into unshielded twisted pair (UTP) and shielded twisted pair (STP). UTP is by far the most common implementation of twisted-pair cable, and it is used for both telephone systems and computer networks.
UTP Insulated copper wires arranged in regular spiral pattern. The oldest, least expensive, and most commonly used media reduce susceptibility to interference than straight pair wires Highly susceptible to electrical noise, interference, and tapping of the signal as compared to the other guided media Arrangement of twisted pairs into group used for high speed (10-100 Mbps) LAN made up of up to four twisted pairs enclosed in a plastic jacket
STPSTP, as its name implies, adds extra shielding within the casing, so it copes with interferenceand attenuation better than regular UTP. Shielded twisted-pair (STP) resembles UTP except that it includes a foil shield that covers the wires and adds another layer of protection against outside magnetic interference. Because of this shielding, cable distances for STP can be greater than for UTP; but, unfortunately, the additional shielding also makes STP considerably more costly than regular UTP.
What's with the Twist?To reduce interference and attenuation, it was discovered that twisting the wires within a cable resulted in greater signal integrity than running the wires parallel to one another. UTP cable is particularly susceptible to crosstalk, and increasing the number of twists per foot in the wire achieves greater resistance against interference. The technique of twisting wires together is not limited to network cable; some internal and external SCSI cables employ a similar strategy.
Categories of twisted-pair cableCategory 1 - telephone cable. susceptible to interference and attenuation and low bandwidth capability, not practical for network applications. Category 2 - capable of transmitting data up to 4Mbps. Still too slow for networks.Category 3 - capable of transmitting data up to 10Mbps. A few years ago, Category 3 was the cable of choice for twisted-pair networks.Category 4 - has potential data throughput of 16Mbps. often implemented in the IBM Token Ring networks.
Category 5 - capable of transmitting data at100Mbps. the cable of choice on twisted-pair networks and is associated with Fast Ethernet technologies.Category 5e - used on networks that run at up to 1000Mbps. can be used up to 350 meters, depending on the implementation.Category 6 - High performance UTP cable capable of transmitting data at over 1000Mbps. is rated up to 550 meters depending on the implementation. marginally more expensive than Category 5e cable.
Coaxial CableCoaxial cable resembles standard TV cable and is constructed using an outside insulation cover, braided metal shielding, and a copper wire at the center. The shielding and insulation help combat attenuation, crosstalk, and EMI. Coaxial cable is rarely used anymore for network backbones or to connect computers, but it is being used today to connect cable modems to the cable providers connection to provide a computer with a broadband Internet connection. Two types of coax are used in networking: thin coax and thick coax. Neither is particularly popular anymore, but you are most likely to encounter thin coax.
Fiber-Optic CableUnlike standard networking cables, which use electric signals to send data transmissions, fiber uses light. As a result, fiber-optic transmissions are not susceptible to EMI or crosstalk, giving fiber cable an obvious advantage over copper-based media. In addition, fiber-optic cable is highly resistant to the signal weakening, referred to as chromatic dispersion. Further advantages of fiber cable include the facts that it's small in diameter, it's lightweight, and it offers significantly faster transmission speeds than other cable media. So, why aren't all networks using fiber cable? The drawback of fiber is that it can be more complex to install than UTP. Creating custom lengths of fiber-optic cable requires trained professionals and specialized tools.
A fiber-optic cableconsists of severalcomponents, includingthe optic core at thecenter, an opticcladding, insulation,and an outer jacket.The optic core isresponsible forcarrying the light signaland is commonlyconstructed of plasticor glass.
Single-mode and Multimode
Two types of optical fiber are commonly available: singlemode and multimode. Multimode fiber (MMF) has a larger core than single-mode. This larger core allows hundreds of light rays to flow through the fiber simultaneously. Single-mode fiber (SMF), on the other hand, has a small core that allows only a single light beam to pass. The light transmissions in single-mode fiber pass through the core in a direct line, like a flashlight beam. The numerous light beams in multimode fiber bounce around inside the core, inching toward their destination. Because light beams bounce within the core, the light beams slow down, reduce in strength, and take some time to travel along the cable. For this reason, single-mode fiber's speed and distance are superior to those of multimode.
Single-mode fiber is used for long runs because it can transmit data 50 times further than multimode fiber and at a faster rate. For example, single-mode fiber might be used on an organizations corporate campus between buildings. Multimode fiber provides high bandwidth at high speeds over medium distances (up to about 3000 feet) but can be inconsistent for very long runs.
Wireless Transmission Wireless methods do not use electrical (cables) or optical (fiber optics) conductors. It uses the earths electromagnetic frequency spectrum.The different type of wireless communications systems includeMicrowave link: directionalRadio link: omnidirectionalInfraredBluetooth
Satellite Point to Point Link
*Lecture slides prepared by Dr Lawrie Brown (UNSW@ADFA) for Data and Computer Communications, 8/e, by William Stallings, Chapter 4 Transmission Media.
*The transmission media that are used to convey information can be classified as guided or unguided. Guided media provide a physical path along which the signals are propagated; these include twisted pair, coaxial cable, and optical fiber. Unguided media employ an antenna for transmitting through air, vacuum, or water.The characteristics and quality of a data transmission are determined both by the characteristics of the medium and the characteristics of the signal. In the case of guided media, the medium itself is more important in determining the limitations of transmission.For unguided media, the bandwidth of the signal produced by the transmitting antenna is more important than the medium in determining transmission characteristics. One key property of signals transmitted by antenna is directionality. In general, signals at lower frequencies are omnidirectional; that is, the signal propagates in all directions from the antenna. At higher frequencies, it is possible to focus the signal into a directional beam. In considering the design of data transmission systems, key concerns are data rate and distance: the greater the data rate and distance the better. *Figure 4.1 depicts the electromagnetic spectrum and indicates the frequencies at which various guided media and unguided transmission techniques operate. In this chapter we examine these guided and unguided alternatives. In all cases, we describe the systems physically, briefly discuss applications, and summarize key transmission characteristics.
*Unguided transmission techniques commonly used for information communications include broadcast radio, terrestrial microwave, and satellite. Infrared transmission is used in some LAN applications. Three general ranges of frequencies are of interest in our discussion of wireless transmission.Frequencies in the range of about 1 to 40 GHz are referred to as microwave frequencies. At these frequencies, highly directional beams are possible, and microwave is quite suitable for point-to-point transmission. Microwave is also used for satellite communications.Frequencies in the range of 30 MHz to 1 GHz are suitable for omnidirectional applications. We refer to this range as the radio range.Another important frequency range is the infrared portion of the spectrum, roughly from 3 1011 to 2 1014 Hz. Infrared is useful to local point-to-point and multipoint applications within confined areas, such as a single room.*Stallings DCC8e Figure 4.6 depicts in a general way two common configurations for satellite communication. In the first, the satellite is being used to provide a point-to-point link between two distant ground-based antennas.