COURSE NAME: Fundamentals of Information Technology
TOPIC: Telecommunications and Networks
LECTURE: 13
Transmission Media
Communication involves the transfer of information in the form of text, graphic images, sound or moving pictures from one place to another. The medium through which this transfer takes place—the transmission media—determines the effectiveness of such communication. It is important to understand the types of media available, their method of construction and their advantages and disadvantages in different situations.
Transmission media refers to the physical means by which information is transferred. It does not include books, compact disks, tapes, floppy disks, photographs and slides which are themselves moved from one site to another. Transmission media, then, includes electrical wires, coaxial cables, optical fibers and electromagnetic waves. Transmission media can therefore be said to be wired or wireless.
When choosing the transmission media, the following factors have to be considered:
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- Transmission Rate
- Distances
- Cost and Ease of Installation
- Resistance to Environmental
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- Conditions
Wired Transmission Media
What is Network Cabling?
Cable is the medium through which information usually moves from one network device to another. There are several types of cables, which are commonly used with LANs. In some cases, a network will utilize only one type of cable, other networks will use a variety of cable types. The type of cable chosen for a network is related to the network's topology, protocol, and size. Understanding the characteristics of different types of cable and how they relate to other aspects of a network is necessary for the development of a successful network.
The following types of wired media might be used to set up a network:
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Twisted Pairs
- Coaxial Cable
- Fiber Optic Cable
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Twisted Pairs
Twisted pair cabling comes in two varieties: shielded and unshielded. Unshielded twisted pair (UTP) is the most popular and is generally the best option for school networks (See fig. 1).
Fig.1. Unshielded twisted pair
The quality of UTP may vary from telephone-grade wire to extremely high-speed cable. The cable has four pairs of wires inside the jacket. Each pair is twisted with a different number of twists per inch to help eliminate interference from adjacent pairs and other electrical devices. The tighter the twisting, the higher the supported transmission rate and the greater the cost per foot.
Unshielded Twisted Pair Connector
The standard connector for unshielded twisted pair cabling is an RJ-45 connector. This is a plastic connector that looks like a large telephone-style connector (See fig. 2). A slot allows the RJ-45 to be inserted only one way. RJ stands for Registered Jack, implying that the connector follows a standard borrowed from the telephone industry. This standard designates which wire goes with each pin inside the connector.
Fig. 2. RJ-45 connector
Shielded Twisted Pair (STP) Cable
A disadvantage of UTP is that it may be susceptible to radio and electrical frequency interference. Shielded twisted pair (STP) is suitable for environments with electrical interference; however, the extra shielding can make the cables quite bulky. Shielded twisted pair is often used on networks using Token Ring topology.
Coaxial Cable
Coaxial cabling has a single copper conductor at its center. A plastic layer provides insulation between the center conductor and a braided metal shield (See fig. 3). The metal shield helps to block any outside interference from fluorescent lights, motors, and other computers.
Fig. 3. Coaxial cable
Although coaxial cabling is difficult to install, it is highly resistant to signal interference. In addition, it can support greater cable lengths between network devices than twisted pair cable. The two types of coaxial cabling are thick coaxial and thin coaxial.
Fiber Optic Cable
Fiber optic cabling consists of a center glass core surrounded by several layers of protective materials (See fig. 5). It transmits light rather than electronic signals eliminating the problem of electrical interference. This makes it ideal for certain environments that contain a large amount of electrical interference. It has also made it thestandard for connecting networks between buildings, due to its immunity to the effects of moisture and lighting.
Fiber optic cable has the ability to transmit signals over much longer distances than coaxial and twisted pair. It also has the capability to carry information at vastly greater speeds. This capacity broadens communication possibilities to include services such as video conferencing and interactive services. The cost of fiber optic cabling is comparable to copper cabling; however, it is more difficult to install and modify.
Fig.5. Fiber optic cable
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Transmission Media Comparison Chart
------
| MEDIA | ADVANTAGES | DISADVANTAGES |
|------|------|------|
| Twisted | Inexpensive | Sensitive to noise |
| Pair | Well understood | Short distances |
| Cable | Easy to add nodes | Limited bandwidth |
| | | Security - easily tapped |
|------|------|------|
| Coaxial | High bandwidth | |
| Cable | Long distances | Security - easily tapped |
| | Noise immunity | |
|------|------|------|
| Optical | Very high bandwidth | Connections |
| Fiber | Noise immunity | T splitters |
| Cable | Long distances | |
| | High security | |
| | Small size | |
------|
Wireless Transmission Media
Wireless LANs
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Not all networks are connected with cabling; some networks are wireless. Wireless LANs use high frequency radio signals, infrared light beams, or lasers to communicate between the workstations and the file server or hubs. Each workstation and file server on a wireless network has some sort of transceiver/antenna to send and receive the data. Information is relayed between transceivers as if they were physically connected. For longer distance, wireless communications can also take place through cellular telephone technology, microwave transmission,or by satellite.
Wireless networks are great for allowing laptop computers or remote computers to connect to the LAN. Wireless networks are also beneficial in older buildings where it may be difficult or impossible to install cables.
Infrared
The two most common types of infrared communications used in schools are line-of-sight and scattered broadcast. Line-of-sight communication means that there must be an unblocked direct line between the workstation and the transceiver. If a person walks within the line-of-sight while there is a transmission, the information would need to be sent again. This kind of obstruction can slow down the wireless network.
Scattered infrared communication is a broadcast of infrared transmissions sent out in multiple directions that bounces off walls and ceilings until it eventually hits the receiver. Networking communications with laser are virtually the same as line-of-sight infrared networks.
Wireless LANs have several disadvantages. They provide poor security, and are susceptible to interference from lights and electronic devices. They are also slower than LANs using cabling.
Microwaves
An extremely high frequency (1 GHz or higher) "line of sight" radio link carries a point-to-point signal, which is accurately focused using dish antennas. Since it is a line of sight transmission, repeater towers need to be located about every 35 km. In a microwave radio system, telecommunications traffic is transmitted in the form of directed beams of microwaves. Microwaves are a kind of electromagnetic radiation like light or like the radio waves used in ordinary broadcasting, but of a frequency intermediate between these.
Microwave transmission refers to the technique of transmitting information over a Microwave link. Since microwaves are highly susceptible to attenuation by the atmosphere (especially during wet weather), the use of microwave transmission is limited to a few contexts.
A microwave link is a transmissiondevice which allows video / audio / data to be sent using radio waves between two locations from just a few feet to several miles apart. It is commonly used by television broadcasters to transmit a programme across the country. For instance, from an Outside Broadcast back to a studio. Mobile units can be camera mounted and are used to allow cameras the freedom to move around without trailing cables. Often seen on the touchline of field sports games on a Steadicam system.
Microwave systems have sufficient bandwidth capacity to support a large number of voice channels and one or two television channels and can achieve high bit rates over moderately long distances. They have been used extensively in inner city data systems where cabling would be very expensive.
Uses of Microwave
- In communication between the Satellites and base stations.
- As a backbone carrier for Cellular Systems.
- In short range indoor communications
Each base station uses two forms of technology to function. GSM radio signals connect mobile handsets to a tower, while microwave transmission links one tower to another in a linear network.
This worker is taking a compass reading to make sure a tower's antenna is pointing in the correct direction.
Satellites
Satellites are a special application of a microwave system. Here, ground stations with large dishes direct information to a communications satellite in geostationary orbit above the Equator. Originally these satellites were passive objects but the modern satellite captures the signal, amplifies it using a transponder, and relays it back to Earth or on to another satellite. Each channel is managed by a transponder, which can support thousands of speech channels and about four television channels simultaneously. Despite their high cost of construction and launch, satellites have become part of our way of life for live television, international telephone and data links
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BlueTooth
This is a specification for the use of low-power radio communications to wirelessly link phones, computers and other network devices over short distances. Bluetooth technology was designed primarily to support simple wireless networking of personal consumer devices and peripherals, including cell phones, PDAs, and wireless headsets. Wireless signals transmitted with Bluetooth cover short distances, typically up to 30 feet (10 meters). Bluetooth devices generally communicate at less than 1 Mbps.
Wi-Fi
Wi-Fi (short for "wireless fidelity"),itis the industry name for wireless LAN (WLAN) communication technology related to the IEEE 802.11 family of wireless networking standards.
Although the Blue tooth standard utilizes the same 2.4 Ghz range as, Blue tooth technology is not a suitable Wi-Fi replacement. Compared to Wi-Fi, Bluetooth networking is much slower, a bit more limited in range, and supports many fewer devices.
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