4.9 Fundamentals Of Communication & Networking

Serial Data Transmission

Serial Data Transmission

Single bits are sent one after another along a single wire. Only one line is required to transmit data in one direction. (Two lines are required for two-way communication, plus a common signal ground wire).

Parallel Data Transmission

Several bits (usually one byte) are simultaneously sent along separate lines or channels. Risk of skew when bits arrive out of sync.

Serial Connectors

Smaller and cheaper than alternatives. Most common type is USB, which a computer contains ports for.

Advantages Of Serial Transmission

• Uses less conducting wires

• Reduced cost

• Supports long distance communication

Disadvantages Of Serial Transmission

• Slower than parallel transmission.

Parallel Connectors

Ribbon of several small cables used primarily for connecting internal components.

Advantages Of Parallel Transmission

• Data is sent faster than serial transmission

• Easier to program

• More bits can be sent at one time

Disadvantages Of Parallel Transmission

• Requires more transmission channels

• Requires more wires

• Data can become out of sync

• More costly

• Only used for short distances

• Cross talk

• Skew

Skew

Transmitted bits travel at slightly different speeds in the wires in parallel cables due to the individual wires having slightly different properties.

Crosstalk

Occurs when the signals of one wire affect the signals on an adjacent wire due to electromagnetic interference. Becomes more pronounced as the speed of transmission (frequency) increases.

Synchronous Transmission

A type of serial data transmission in which data is organized into groups or blocks of data that are transferred at regular, specified intervals (usually the internal clock pulse). There are no gaps in the transmission.

Asynchronous Transmission

A type of serial data transmission in which data is sent when it is ready to be sent without being synchronized. Each byte is proceeded by a start bit and ends with a stop bit or stop period (a short time gap between each set of bits).

Advantages of Asynchronous Transmission

It is cheap and an effective form of serial transmission well suited to low-speed connections such as keyboard and mouse.

Disadvantages Of Asynchronous Transmission

Relatively slow due to the increased number of bits being sent.

Requirement of Start & Stop Bits

The two bits need to be opposite of each other.

Latency

Measure of delay in communication between two devices over a network.

Protocol

A set of rules governing the exchange or transmission of data between devices.

Baud Rate

The rate at which signals on a wire may change.

Bit Rate

The number of bits transferred or received per unit of time. Baud Rate x Number Of Bits Per Signal.

Bandwidth

The maximum amount of data that can pass from one point to another in a unit of time

Propagation Latency

Length of time taken for a signal to reach it's destination.

Transmission Latency

The time taken from when the first bit leaves the transmitter to when the last bit is recieved.

Processing Latency

Time taken for routers to process the data packet's header.

Ping Test

Measures latency by determining the time it takes a given network packet to travel from source to destination and back.

Local Area Network (LAN)

Two or more computers that exchange data, confined to a small geographic area usually one building.

Advantages Of LAN

• Faster communications

• More secure as it is not reliant on the internet.

• Is able to identify all devices connected to it.

• Allows for sharing internet and software.

Disadvantages Of LAN

• Quite expensive to install

• Does not cover a large geographical area.

• Requires maintenance

• LAN can spread malware.

• Servers can crash.

Wide Area Network (WAN)

Computers and peripherals connected together over a large geographical area and exceeds a single site, like the internet.

Advantages Of WAN

• Applies over a much larger area

• Allows sharing for sharing of software and resources

• Allows for centralised data

• Assured up-time

Disadvantages Of WAN

• Expensive to setup

• More security issues that hackers can use and intercept

• Maintenance issues

• Server crashes can potentially lose data.

• High bandwidth required

• Fully reliant on public and private infrastructure

Network Toplogy

The arrangement of the various computing devices which make up a computer network

Bus Network

A network consisting of a central backbone cable to which all network devices are attached. Each end of the backbone is connected to a device or terminator, preventing data from bouncing back. Each node is passive, and data is sent in one direction at a time.

Advantages Of A Bus Network

• Inexpensive to set up

• Devices easily added

• Good for small networks

Disadvantages Of A Bus Network

• Main cable is a point of failure

• Limited cable length

• Performance degrades with heavy use, owing to data "collisions"

• Poor security

Carrier Sense Multiple Access with Collision Detection (CSMA/CD)

A protocol that detects when two nodes are attempting to transmit simultaneously. Both nodes cease transmission and wait a random amount time before reattempting. Used with wireless networks.

CMSA/CA Using RTS/CTS

When data is being sent, a 'request to send' (RTS) is transmitted to the wireless access point. If no other devices are being used, then a 'clear to send' (CTS) is transmitted to the node, and the node transmits the data. If a CTS has not been recieved, the node waits a random amount of time before transmitting another RTS. This method allows hidden nodes to stay hidden by only communicating with the wireless access point.

Hidden Node Problem

A situation on a wireless network in which a node on one side of a coverage area is too far apart from and therefore invisible to nodes on the other side of the coverage area. This situation prevents nodes from collaborating to prevent collisions.

Star Network

Each device is connected to a central device such as a hub or switch.

Advantages Of A Star Network

• Easier to isolate problems

• Good performance

• More secure if a switch is used as data is only sent to the recipient.

Disadvantages Of A Star Network

• Expensive for switch/hub and cabling

• Central node is a point of failure.

Switch

Sends each communication to the specific computer it is intended for. It holds all of the MAC addresses for each device connected to it and uses these to direct packets of data to the correct device.

Hub

Broadcasts the data it receives to all devices connected to its ports.

Network Interface Card

A card installed in a computer that allows you to connect to a network (can be Ethernet, telephone, or wireless)

Media Access Control (MAC)

Given to a network adapter when it is manufactured. It is hardwired or hard-coded onto your computer's network interface card (NIC) and is unique to it. Six pairs of hex digits seperated by colons, with the first being the manufacture code, while the second half is the device code.

Physical Topology

Defines how the devices are physically connected.

Logical Topology

Defines how the devices communicate across the physical topologies. E.g. A network wired in a star topology can behave logically as a bus Network by using a bus protocol and appropriate physical switching.l

Client

A computer that requests data stored or resources provided by a server.

Server

A powerful computer which provides services or resources required by any client.

Client-Server Network

A network that uses centrally administered computers, known as servers, to enable resource sharing for and to facilitate communication between the other computers on the network.

Features Of A Client-Server Network

• A central server used to manage security

• Some files stored on the central server

• Some processing tasks are performed by the server.

• Clients issue requests to the server for services such as email, file storage, backup and printing.

• Suitable for many different types of organisation, large or small.

• Can require specialist IT staff to adminster the network.

Advantages Of Client-Server Network

• Easier to upgrade the server than to upgrade every device

• Centralised back-up

• No access to other people's files (advantage depending on the situation)

• Used in small, medium and large companies.

Disadvantages Of Client-Server Network

• Requires maintenance and a specialist team

• Server is a point of failure.

• Can be expensive

Peer-to-Peer Network (P2P)

A computer network that relies on the computing power and bandwidth of the participants in the network rather than a centralized server

P2P File Sharing

A practice in which individuals can obtain music, video, and other types of files from other users on a network (BBC iPlayer uses P2P networking); sometimes the files are shared without authorization from the copyright holder.

Cloud Computing

use of web services to perform functions that were traditionally performed with software on an individual computer; i.e. Flickr, Google Docs, etc.

Advantages Of Cloud Computing

• Allows collaboration on work

• Back-ups can be made up easily.

• Software and data is accessible from any computer.

• Software and data do not take up space on the user's hardrive

Disadvantages Of Cloud Computing

• Requires constant internet connection.

• May require a subscription

• Hackers could attack it.

• Relies on other organisations, for security.

Wi-FI

Wireless local area network that uses radio signals to transmit data

The Purpose of Wi-Fi

Allows devices to connec to each other via wireless technology, which is particularly useful for small portable devices that don't have enough ports.

Requirements For A Wireless Network

• Internet Service Provider (ISP)

• Modem & Wireless Router (Often Combined)

• Device with a network interface card (NIC).

Service Set Identifier (SSID)

A network name that wireless routers use to identify themselves. Can be hidden, making it harder to detect.

Wireless Protected Access (WPA)

Provides sophisticated data encryption and user authentication.

Wireless Protected Access 2 (WPA2)

A stronger security protocol that is more secure by increasing the level of encryption.

MAC Address Whitelist

Identifies authorised sites/users/machines that are allowed to use the network. Any devices with a MAC address not identified will not be authorised access.

Internet

A global network connecting millions of computers, making it possible to exchange information. A network of inter-connected networks.

World Wide Web (WWW)

An electronic collection of information posted on websites. It includes text, audio, video, and graphics. It is accessed via the internet.

Backbone

A set of dedicated connections that connect several large networks at various points on the globe. Each of these points are then connected to other regional networks, usually controlled by ISPs

Internet Service Provider (ISP)

An organisation that provides services for accessing and using the internet. Internet services include internet access, internet transit, domain name registration, web hosting, etc.

Internet Protocol (IP)

A set of rules responsible for disassembling, delivering, and reassembling packets over the Internet.

IP Address

A dotted quad number that identifies every computer that sends or recieves data on a network and on the internet. Each device needs to uniquely identified so that data can be sent to the correct destination.

IPv4 vs IPv6

IPv4: 32-bit number: 4 billion addresses; four sets of numbers marked off by periods IPv6: 128-bit addresses, able to handle up to 1 quadrillion addresses; almost an unlimited number of addresses

Uniform Resource Locator (URL)

A full internet address that identifies hypertext documents.

Domain Name

The part of a network address that identifies it as belonging to a particular domain and identifies the type of site it is (.gov, .edu, .com).

Types Of Domain Names

.co - indicates a type of organisation. .com - indicates a commercial organisation. .gov - indicated it is part of a government facility .ac - indicates it is an acedemic institution. .sch - school .org - identifies an organisation that is not a buisness. .net - company providing an internet service. .uk - indicates it is registered within the UK

Domain Name Server (DNS)

Dedicated computers with an index of domain names and their corresponding IP addresses. When a computer queries a DNS server for a domain name, the server returns an IP address that the computer can use to send a message to it.

Resolving An IP Address

If a Domain Name Server does not have a record for a domain, it either:

• handles the DNS query so that it can eventually deliver an IP address or a 'DNS address could not be found' error message.

• Refers to a DNS server authoratitive (e.g. .uk) and follows this and subsquent referrals to successively lower level Domain Name Servers.

Internet Registries

Organisations who allocate and administer domain names and IP addresses There are 5 global internet registries that work togethor to maintain a database of address assignments that ensure IP addresses and domain names are unique.

Five Regional Internet Registers

• Africa Network Information Center (Africa)

• American Registry For Internet Numbers (US, Canada, several parts of the Carribean and Antarctica)

• Asia Pacific-Network Information Center (Asia, Austrailia, New Zealand and neighbouring countries)

• Latin America And Carribean Network (Latin America and parts of the Carribean region)

• Réseaux IP Européens Network Coordination Centre (Europe, Russia, Middle East, and Central Asia)

Packet Switching

A method of transmitting packets of data via the quickest route on a network. Packets are routed via the least congested route, which makes it more secure as the packets are not all on the same routes.

Data Packet

Data that is broken up into chunks of data for transmitting over a network, and it is assembled again at the recieving end. Increases network efficiency and reliability. Each one contains a sequence number, a source, a destination and a checksum. The ideal compromise size is 500-1500 bytes.

Header

Information that appended to the beginning of a data packets. Includes:

• MAC addresses of the sender and reciever

• IP address of the sender and reciever

• What protocol should be used

• The packet number or sequence number.

Body

The part of the data packet that contains the actual data being transmitted.

Footer

Appended at the end of a data packet, and contains a checksum.

Sender's Address

Identifies where the packet was sent from, and therefore where the response should be sent to.

Reciever's Address

Identifies the packet's intended recipient, allowing it to be routed to the correct device.

Time To Live

Holds the number of hops (number of routers) a packet can go through before the transmission is dropped.

Sequence Number

Contains the number of packets in a message, and identifies the packet's position in relation to the others.

Checksums

An error detection method using an algorithm to calculate the sum of bytes in a data packet transmission, and this calculated sum is also sent with the transmission. The receiving computer recalculates the sum with the transmission it received, and if the values of the sums don't match with the one appended to the transmission, an error has occured or the data packet has been altered.

Routing

The process of directing packets of data between networks. Routers forward data packets from one network to another.

• Each router stores data about the available routes to the destination node

• It looks up the destination IP address in its routing table to find the best router to which to forward the packet. Each forward is known as a 'hop'.

• Routers continue to forward the packet until it reaches its destination node.

Gateway

A bridge between two networks that use different protocols. It removes header data from a packet and reapplies it using the correct the format of the new network. Can sometimes be combined with a router into one integrated device.

Firewall

A part of a computer system or network that is designed to block unauthorized access while permitting outward communication. Either hardware or software that opens certain ports so that only certain traffic is allowed to pass through.

Packet Filtering

A process that uses various fields in a packet's IP and TCP headers to decide what to do with the packet. A technique that examines the contents of packets and rejecting them if they do not confrom to specfied rules. Firewall software is used to examine the packets to ensure it does not contain unauthorised data, and the header can be checked to see if it is from a recognised source.

Stateful Inspection

Examines the contents of a packet before deciding whether to allow it through the firewall. It remembers actions for futer decisions.

Proxy Server

A server that acts as an intermediary between a user and the Internet. It makes a web request on behalf of your own computer, hiding the true request IP addresses from the recipient.

Functions Of A Proxy Server

• No direct connection between device on LAN and the internet.

• Evaluate requests to ensure they come from a legitimate source

• Anonymous surfing

• Filter undesirable online content

• Log user data with their requests

• Provide a cache of previously visited sites to speed access.

Encryption

Process of converting readable data into unreadable characters to prevent unauthorized access.

Symmetric Encryption

An encryption method whereby the same key is used to encode and to decode the message. The key needs to be transferred to the communicating devices so they know how to encrypt and decode the messages. It is a relatively fast form of encryption and is used for home and small office Wi-Fi networks, mainly a pre-shared key.

Man In The Middle Attack

A Hacker placing themselves between a client and a host to intercept communications between them. If they have the key for the symmetric encryption they are using, they can access all the communications.

Asymmetric Encryption

An encryption method which uses a mathmatically linked public and private key to encrypt and decrypt data. The sender encrypts data using the recipient's public key, and the encrypted data is sent to the recipient. The recipient then can only use their own private key to decrypt the message. This prevents man in the middle attacks as it requires access to the public and private key.

Private Key

A code used to encrypt/decrypt data that is only known by one user but is mathematically linked to a corresponding public key.