SUMMARY – Communication SystemsNathan Rees – 2017

Characteristics of Communication Systems

  • Communication systems enable people and systems to share and exchange data and information electronically.
  • The communication occurs between transmitting and receiving hardware/software over a network, where each device on a network is called a node.

  • The framework in which communication systems function is demonstrated in this model:

Protocol Levels

The OSI model (Open Systems Interconnection) is a seven-layer model that is a universal standard for creating all types of transmission hardware and software. The IPT syllabus, however, only has three levels (shown right).

IPT Application Level

  • The actual data to be transmitted is created by a software application in a format that is understood by the receiving application.
  • Protocols operating at this level include HTTP; DNS; FTP; SMTP; POP; IMAP; SSL.

IPT Communication Control and Addressing Level

  • Communication with the network is established.
  • It determines when a session starts and finishes.
  • It manages the correct transmission of each data packet.
  • Packets failing to reach the destination are resent until all packets are transmitted correctly.
  • Protocols operating at this level include TCP/IP.

IPT Transmission Level

  • This layer manages the physical transmission of data, converting bits into signals that are sent through a medium.
  • The final size of the packets, the speed of the transfer and various other characteristics are determined.
  • Switches and the Ethernet protocol are used to direct messages based on their destination MAC address.
  • Physical cables for transfer, such as twisted pair; copper telephone cable; optical fibre; coaxial cable; wireless transmission.
  • Protocols operating at this level include Token Ring; SPNET; FDDI.

Handshaking

Handshaking is an agreement that determines which protocols will be used and the specific detail of each protocol that must occur before any data can be transmitted or received.

Protocols

Application Level Protocols

  • HTTP (Hypertext Transfer Protocol): used in web browsers to communicate and receive web pages to and from a web server.
  • SMTP (Simple Mail Transfer Protocol): used for sending and receiving emails to and from the client to the mail server.
  • SSL (Secure Sockets Layer): uses two keys to encrypt data so that private documents can be securely transmitted via the internet (eg. HTTPS – shows active by ‘S’).

Communication Control and Addressing Level Protocols

  • IP (Internet Protocol): moves data packets from sender to receiver. It has the ability to reroute messages over the most efficient path to their destination.IP does NOT require TCP to operate.
  • TCP (Transmission Control Protocol): ensures that messages are delivered correctly by using error checking methods. TCP packets contain 576 bytes. TCP requires IP to operate.

Transmission Level Protocols

  • Ethernet:the most commonly used LAN (Local Area Network) technology.

-Exists in the TCP/IP stack.

-Allows simultaneous data flow in both directions.

-Collisions will occur, therefore CSMA/CD (Carrier Sense Multiple Access and Collisions Detection) was developed to counteract this.

-Modern Ethernet networks use switches to prevent collisions all together.

-Can transfer through Cat 5 cable at speeds of 1000Mbps.

  • Token ring: a token (special data packet) circulates through nodes on a LAN.

-Nodes wishing to send data wait for the token to come around and then attach data packets onto the token.

-Once the recipient node has received the message, a confirmation is attached to the token ring and is sent back to the original node.

-No collisions occur, however Ethernet has largely replaced the token ring protocol.

Passing Messages Between Source and Destination

Message Creation

The message is compiled using a software application from the data source in preparation for sending. Examples include:

  • A user writing an email on Outlook
  • Speaking during a VOIP (Voice Over Internet Protocol) phone conversation
  • Pressing the delete key to remove a file stored on a file server

Organisation of Packets at the Interface Between Source and Transmitter

As a message descends through protocols, each packet is wrapped with multiple headers and trailers. At the control level the protocols are managed by the operating system. These protocols can ensure the packets reach their destination correctly by using error checks.

Signal Generation by the Transmitter

The transmitter is the physical hardware that generates or encodes the data onto the medium creating a signal. The transmitter represents individual bit as a wave. Example of devices that include a transmitter (and receiver):

  • Network Interface Cards (NICs) [aka. Ethernet card]
  • Switches
  • Routers
  • ADSL/cable modems
  • Mobile phones
  • Bluetooth devices

Synchronising the Exchange

Both the transmitter and receiver use a common clock so that transmission can be perfectly synchronised. This allows large amounts of packets to be transferred – while the transmitter is encoding, the receiver is decoding simultaneously.

Addressing and Routing

Data packets pass through many different and varied links, therefore the path that may look the ‘fastest/shortest route’ may not be the case. Optical fibre transmission will significantly increase the transmission speed thus a longer distance covered by optical fibre will be faster than a shorter copper-wire route.

Error Detection and Correction

Many protocols calculate checksumsor CRC (Cyclic Redundancy Check) values and include them with the headers or footers. When the packets are received, the values are recalculated to ensure the results are matching.

Security and Management

Protocols can restrict messages based on usernames and passwords, while others go further by encrypting messages during transmission. For example:

  • POP (Post Office Protocol) operates on most mail servers. To retrieve emails from a POP server the user must first be authenticated.
  • SSL (Secure Sockets Layer) uses a public key encryption/decryption system to secure critical data transfers (e.g. financial transactions).

The Client-Server Model

Role of the Client and the Server

  • The client is responsible for connecting and using the server’s services, for example printing; file storing/retrieving; web services; mail services.
  • The server is responsible for providing/distributing these services among all clients. It establishes a networked community to work cooperatively and effectively.

Thin Clients and Fat Clients

  • Thin clients: “dumb” computers/terminals on a network which don’t carry out the processing or file storage of the system. They only let the user physically enter data through an input device which is then transmitted to a centralised server. For example, ATM or airline booking kiosk.
  • Fat clients: “intelligent” computers/terminals which contain their own processing and memory capabilities (aka. workstations).

Examples of Clients

  • Web browsers: these can browse the internet themselves or go through a web/proxy server to locate the web pages. For example, when performing an internet banking transaction, a web browser is the client application that requests data from the bank’s web server. The bank’s web server then acts as a client to the bank’s DBMS server.
  • Mail clients: these can retrieve and send mail to and from a mail server (eg. MS Outlook).

Example of Servers

  • Print servers: a computer that manages all the printing tasks of the network.
  • Mail servers: a computer that stores and manages the sending and receiving of all email messages of users.
  • Web servers: a computer that manages all the internet connections from the network to the internet. Provides a cache for HTTP web pages and security using firewalls and a DNS (Domain Name System) to record every web page as a number.
  • Files server: a computer dedicated for the storage of files and applications.
  • Database server: DBMS (Database Management System) software can be run from a database computer.

Advantages/Disadvantages of the Client-Server Model

Advantages / Disadvantages
  • Sharing of hardware/software and file resources
  • Lower costs of software as a network/site licence can be bought
  • Software updates and backups are done simultaneously and automatically
  • Control over what users can access on the internet
/
  • Security needs greater measures to be controlled (eg. Firewalls, antivirus software, login passwords)
  • Software licences need to be kept on record
  • Expensive to buy, set up and maintain
  • IT staff will be required to maintain
  • Full/incremental backups are continuously needed

Transmitting and Receiving in Communication Systems

Transmission Media

Wired Transmission

  • Twisted pair: a cable made from two strands of insulated copper wire. These strands are twisted around each other to reduce electrical interference.

-Inexpensive and widely used

-Slower than coaxial/optical fibre

-Speed: 1MHz

-Unshielded Twisted Pair (UTP) is most common and is split up into three types (higher frequency  higher data transfer speeds):

  • Cat 6 – frequencies up to 250MHz
  • Cat 5e – frequencies up to 125MHz (most common, shown below; right)


  • Cat 3 – frequencies up to 16MHz (now used exclusively for telephone lines)
  • Coaxial cable: a cable made by encasing insulated copper wire in a protective coast of mesh, providing extra protection from electrical interference.

-More reliable data carrier

-Expensive and less flexible

-Standard coaxial cables can only carry one signal at a time

-Broadband coaxial cables can carry multiple signals at a time (eg. video AND audio)

-
Fast data transfer speeds: 1GHz

  • Optical fibre: a cable made of thin (hair width) clear glass fibre, surrounded by a layer of plastic.

-Data is sent as a pulse of light

-Light pulses aren’t prone to EM interference

-Most expensive to make/repair

-Provides greater security measures

-

The fastest and most reliable wired medium currently available: 75THz

Wireless Transmission

  • Microwave: a radio wave that transmits by line-of-sight from antenna to antenna.

-Range of 40km – 50km

-Faster than coaxial and copper telephone lines: 100GHz

-Transmission performance can be degraded due to weather or obstacles in line-of-sight

  • Satellite:uses microwaves to carry digital signals to and from ground based stations and other satellites in a geostationary orbit.

-Speed: 100GHz

  • Infrared: operates by sending a signal in a straight line.

-Obstacles will prevent transmission’

-Used for short distances (eg. TV remote)

-Doesn’t require an antenna

  • Bluetooth: uses microwaves over a short distance (ie. < 10m).
  • Mobile: uses microwaves and cell technology, can connect to the internet.

Network Topologies

Describes the way in which nodes are connected and the routes data can take within a network.

Star

  • A network where all nodes are connected to a centralised server/switch.
  • This is the most commonly used topology.
  • Every node has their own dedicated cable.
  • Requires more expensive cabling than other topologies.
  • If one node fails, the rest of the network still functions.
  • If the central server fails, the whole network fails.


Bus

  • All nodes are connected to a common cable known as a bus (aka. backbone).
  • A terminator at both ends of the bus stops old messages from recirculating.
  • Each node has its own MAC (Media Access Control) address
  • Each node waits for the bus to be clear before it sends a message to another node (if two nodes send data simultaneously, there will be a collision).
  • CSMA/CD (Carrier Sense Multiple Access and Collision Detection) is employed to counteract this problem.
  • A break in the bus will disable the entire network.

  • Bus topologies use the Ethernet structure.

Ring

  • A circular network where all nodes are connected in a ‘ring’.
  • Each node has its own MAC address.
  • Uses the Token Ring structure of sending/receiving messages.
  • If one node fails, the whole network fails.
  • A constant speed is set for the entire network.
  • There are no data collisions as data only travels in one direction.

  • This topology can be slow as each node has to read the same message until the correct receiving node is identified.

Wireless

  • Provides wireless coverage to clients.

Hybrid

  • Contains a combination of star, bus, ring or wireless in one network.
  • All hybrid topologies have a single transmission path between any two nodes.
  • This is sometimes necessary when connecting two networks.

-If two networks operate on the same protocols (eg. both use Ethernet), a BRIDGE is used to form the connection.

-
If two networks operate on different protocols (eg. one uses Ethernet, the other Token Ring), a GATEWAY is used to form the connection.

Mesh

  • Include more than one physical path between pairs of nodes.

  • This is the primary topology of the internet, where IP packets can travel on different paths to the receiver.

Hardware Components of Communication Systems

Hub

  • A “dumb” device which provides a central connection point for all other nodes.
  • When a hub receives a data packet from one node, it directs it towards all other nodes on the network.
  • Hubs make no attempt to identify the address of a node.

Switch

  • An “intelligent” device which sets up a direct connection between a sender and a receiver.
  • It identifies where the data needs to go and sends it to the correct node.
  • Switches can simultaneously send and receive data packets to multiple pairs of nodes.
  • Reduces network traffic, therefore faster data transfer speeds.

Router

  • Routers direct data between networks with similar protocols.
  • A router looks at the data’s destination and decides the quickest path for it to follow.
  • If the path breaks, a new path is automatically rerouted.
  • A router can translate one protocol into another, therefore overcoming hardware/software compatibility issues.

Modem

  • Abbreviation of ‘modulation demodulation’ (digitalanalogue, analoguedigital).
  • Modems are used to connect a computer to their local ISP (internet Service Provider) which will provide access to the internet via ADSL or cable connection.
  • Modems connect to a computer via USB or Ethernet.

Network Interface Card (NIC)

  • A card built into the motherboard which converts data between the computer and the network.
  • An NIC will negotiate access to the network, including collision detection (CSMA/CD).
  • Each NIC has its own MAC address.

Bridge

  • A device that connects two or more LAN networks which operate on the same protocol.
  • For example, both use Ethernet.

Gateway

  • A device that connects two or more LAN networks which operate on different protocols.
  • For example, one uses Ethernet, the other uses Token Ring.

Wireless Access Point (WAP)

  • A central node in a wireless LAN, acting as a hub.
  • Provides wireless coverage for other nodes trying to access the LAN wirelessly.
  • Sends/receives data to all wireless nodes.
  • Requires an encryption key to maintain a secure connection.
  • The WAP must be physically connected to the LAN to allow wireless nodes to connect with wired nodes in a network.

Repeater

  • Any device that receives a signal, amplifies it and then transmits the amplified signal down another link.
  • Used to increase the physical range of wired/wireless transmission mediums (eg. fibre optics, microwave signals).
  • Most WAPs can be used as a repeater to extend the range of a WLAN (Wireless LAN).

Characteristics of Network Operating Software

Network operating System (NOS)

  • The NOS is designed to support computers connected to a LAN.
  • The NOS is installed on at least one server in the LAN.
  • One vital NOS service is the authentication of users based on their username and passwords.
  • The network administrator has access to the NOS and can create policies for other users.
  • NOS tasks: network administrators are responsible for performing certain tasks:

-Administration: adds, removes and organises user accounts.

-File management: gives users access to remote HDDs on the network and provides an organised method of storing files within the network directory.

-Applications: shares applications among users.

-Resources management: printers and modems can be shared, manages system memory for thin clients.

-Security: monitors and restricts access to network resources.

Contrasting Between the Internet, Intranets and Extranets

Internet

The internet is a series of computers interconnected with each other on a public network (Wide Area Network – WAN).

Intranet

An intranet is a series of network computers within a local organisation. It is a private Local Area Network (LAN).

Extranet

An extranet is an intranet that is accessible from outside the LAN.

Other Information Process in Communication Systems

Collecting

Phone

A phone can be used to record a voice mail message and stored in memory to be accesses at a later time.

EFTPOS Terminal

A keypad/keyboard is used to collect data from the user (eg. pin number). The terminal then collects data from the main bank server.

Processing

Encoding/Decoding Analogue and Digital Signals

  • Encoding: a message is converted into a form suitable for transmission along the medium.
  • Decoding: a message is converted into a form suitable for subsequent information processes (usually into the original form).
  • Digital data: represented as binary digits; 1 and 0.
  • Analogue data: represented as a continuous variable physical quantity (ie. waves).
  • Types of conversions:

-Analogue data to analogue signal: for example, a microphone collects analogue sound waves and encodes them as an indefinitely variable electromagnetic wave.