Core 3: Communication Systems

Transmission media is the medium by which data is transferred. It can be bounded (cabled) or unbounded (wireless). Data is sent as ‘signals’ along transmission media.

Wired Wired Transmission media restricts the signal so that it is contained within a cable and therefore follows the path of the cable. It can be shielded to protect from electromagnetic interference and has recommended distances between nodes.

Twisted Pair Twisted pair is composed of copper wire twisted together within plastic insulation, and an outer sheath. Twisted pair can be UTP or STP (shielded or unshielded). The most common is UTP, specifically CAT-5 or CAT-6.

Twisted Pair Contd… UTP is classified into categories where higher category cable supports higher frequency and hence higher data speeds. Eg. 16MHz for Cat 3 up to 250 MHz for Cat 6.

Coaxial Cable Coaxial cable was originally for transmitting analogue TV broadcasts. Coaxial contains a solid copper core, a nylon insulator, enclosed within an aluminium foil wrap. This is wrapped inside braided copper and black plastic sheath.

Coaxial Cable Contd… Coaxial does not suffer from the electromagnetic interference that twisted pair can suffer from. Coaxial cable is expensive and larger than UTP and can transmit on frequencies tanging from 30MHz to 3GHz.

Optical Fibres Optic fibre is the fastest physical transmission media. Optic fibre is composed of optical fibres containing light waves. Light reflects off the inside of the cladding that surrounds the cable. Both the core and cladding are made of pure glass.

Optical Fibres Contd… Light waves are extremely high frequency electromagnetic waves. Fibre optic operates on frequencies of around 200,000Ghz – 350,000GHz.

Wireless Wireless or unbounded transmission uses the atmosphere to carry electromagnetic waves between nodes. Wireless has advantages over wired in that it can traverse rugged ground and allows nodes to move freely. BUT… this makes it susceptible to interference and unsuitable for high speed critical connections.

Point to Point Terrestrial Microwave This is used to relay wireless signals across large distances. A direct and uninterrupted line of is required between towers. Sequences of transmitters and receivers, known as transponders, are arranged into chains..

Point to Point Terrestrial Microwave These towers are being slowly replaced by satellite connections for radio, TV and data transfer. They were most common in the 1980’s.

Satellite Satellites use microwaves to carry DIGITAL signals from and to both ground based stations and also between satellites themselves. Satellites contain transponders that amplify and then re-transmit microwaves on a specific frequency.

Wireless LANS (WLAN’s) WLANS currently use the g series, with frequencies in the vicinity of 2.4GHz. Common applications include, cordless phones, Bluetooth devices and remote control toys. WLAN transmits and receives at a maximum of 54Mbps.

Wireless LANs (WLAN’s)

Bluetooth Bluetooth is a communication system for short range transmission. It was designed to replace cables connecting portable devices.

Bluetooth Bluetooth devices automatically recognise each other, forming an adhoc or piconet network. Low power Bluetooth connections should be 1m apart whilst high powered can reach up to 100m.

Infrared Infrared waves travel in straight lines hence a direct line of sight is required between source and destination. Infrared is only used over short distances. Common applications include remote control or portable devices.

Mobile Phones Mobile phone networks are split into areas called cells, hence why mobile phones are called cell phones. Each cell contains its own central base station that sends and receives data to and from phones.

Mobile Phones As users roam between cells the base stations pass the calls onto the next base station.

Here we will look at the devices used to connect nodes to form a LAN and also to transfer data within a network. Each node requires a NIC (Network Interface Card) that complies with the Transmission level protocols on the network.

In most LANs a star topology is used so we need a central node in the form of a Hub, Switch or Wireless Access Point. Gateways connect networks that use different protocols whilst Bridges connect networks using the same protocols. Modems allow LANs to communicate with WANs and Routers operate at the communication control and accessing level to direct data along the best path. For a small LAN the functions of many of the above devices are combined into a single piece of hardware – a Router.

NIC (Network Interface Card) NIC’s convert data between the computer into a form suitable for transmission.

Repeater A repeater receives a signal amplifies it and sends the signal on its way.

Hub A Hub repeats all messages to all nodes on a single LAN segment. Hubs have been phased out by more intelligent switches.

Bridge A bridge separates networks into different segments using the same protocol. Switches have replaced this function more recently.

Switch A switch is an intelligent hub or a multi- port bridge. Determines the MAC address of sender & receiver for each message.

Gateway A Gateway connects between networks. This means a gateway can connect between protocols.

Wireless Access Point (WAP) A connection point on a wireless network. The issue here is security. Any user can access the network. Therefore we use WEP (Wired Equivalent Privacy) & WPA (Wireless Protected Access).

Modem A modem is short for modulation and demodulation which are the functions a modem serves. Modems are used to connect a computer to an ISP. Modems can be ADSL (broadband over existing copper wire network) or cable (net connection via coaxial cable).

Router Routers direct traffic over the most efficient path to a destination. Routing decisions are based on IP addresses. Your wireless router at home operates as a modem and a gateway to the wider internet as well as including a variety of security measures.

Router

A server provides a specific processing services to other nodes (clients). The following servers are the most common servers founds within a LAN or network.

ServerDescription File Server A file server manages storage and retrieval of files and also application software in response to client requests. This demands large amounts of secondary storage. These use RAID hard-drives. Print Server A print server controls access to one or more printers for many clients. This uses a print queue. Some print servers can take priority requests. Database Server Database servers run database management system software. These execute SQL statements and retrieve queries. Mail Server Governs networks such as Microsoft Outlook and uses application and presentation layers of protocol. Web Server Web servers provide services to web browsers – they retrieve web pages and transmit them back to the requesting client web browser. Proxy ServerProxy servers are intermediary servers that exists between clients and main servers. They attempt to fulfil requests instead of the main server so that wait times are reduced and the main server gets less traffic.