3.9:Fundamentals of communication and networking

Table of Contents

1 Communication

Communication methods

Objectives:

  • Define serial and parallel transmission methods and discuss the advantages of serial over parallel transmission.
  • Define and compare synchronous and asynchronous data transmission.
  • Describe the purpose of start and stop bits in asynchronous data transmission.

Serial and Parallel Data Transmission

  • electronic data (0s and 1s) can be transmitted by two different methods:
    1. Serial transmission: data bits are sent one bit at a time over a single wire. Serial transmission used in networks, phones, keyboard and mouse.
    2. Parallel transmission: data bits are sent over multiple wires at the same time. It is faster but only works well over a short distance such as several meters. Buses inside a computer use parallel transmision. Printers are also can be connected via parallel cables within a limited distance.

Parallel_and_Serial_Transmission.gif

Serial and Parallel Data Transmission Comparision
  • Serial transmission is cheaper than parallel for two main reasons:
    1. less wires needed
    2. much less complex and size in transmitting and receiving interfaces
  • Serial transmission is much more reliable over long distance without suffering from "skew". "Skew" can develop in parallel transimission due to slightly different properties in each parallel wire, which could result in different bits travel at different speeds.
  • In parallel transmission, a signal transmitted on one wire could create an undesired effect in another wire, resulting in the phenomenon called "crosstalk". This isparticularly prominent when using high frequency.
  • Serial can transmit data at a higher frequency (high bit rate) without suffering "crosstalk".

Communication basics

objectives:

  • Define:
    • baud rate
    • bit rate
    • bandwidth
    • latency
    • protocol.
  • Differentiate between baud rate and bit rate.
  • Understand the relationship between bit rate and bandwidth.

bit rate

  • The number of bits can be transmitted serially per second.

baud rate

  • The number of symbols/signals per second transferred. A symbol/signal may encode more than one bit. For example, using 4 different frequencies, 4 different 2-bit patterns can be encoded with each frequency encoding either 00, 01, 10, or 11
  • It is also called symbol rate
  • Since one symbol can have more than one bit, bit rate will be higher than baud rate

bit rate = (baud rate) X (number of bits per symbol)

bandwidth

  • The amount of data that can be transmitted from one point to another in a given time period (usually a second).
  • Bandwidth is usually expressed in bits per second (bps)
  • Modern networks typically have speeds measured in the millions of bits per second (megabits per second, or Mbps) or billions of bits per second (gigabits per second, or Gbps).

Latency

  • Latency is the delay from the start of the transmission to the time the data transmitted arrive at the destination.
  • Latency is the wait time introduced by the signal travelling the geographical distance as well as over the various pieces of communications equipment.
  • Even fiber optics are limited by more than just the speed of light, as the refractive index of the cable and all repeaters or amplifiers along their length introduce delays.

protocol

  • Protocol is a set of standardised rules used for governing communications between devices.
  • Standardised rules allow communications between different devices possible.
  • Those rules can include:
    • speed
    • data format
    • error detection and correction
    • mode of transmission
    • physical connections, cabling
Synchronous and Asynchronous Transmission Modes

Synchronous Transmission sync-trans.jpg

  • A process where data is transferred in regular intervals that are timed by a clocking signal
  • It allows for a constant and reliable transmission for time-sensitive data, such as real-time video or voice.
  • Parallel transmission typically uses synchronous transmission.

Asynchronous Transmission async-trans.jpg

  • Signals are sent in an agreed pattern of bits and if both ends are agreed on the pattern then communication can take place.
  • Bits are grouped together and consist of both data and control bits.
  • If the signal is not synchronised the receiver will not be able to distinguish when the next group of bits will arrive. To overcome this the data is preceded by a start bit, usually binary 0, the byte is then sent and a stop bit or bits are added to the end.
  • In addition to the control data small gaps are inserted between each chunk to distinguish each group.
  • Each bit remains timed in the usual way. Therefore, at bit level the transmission is still synchronous (timed).
  • Asynchronous transmission is relatively slow due to the increased number of bits and gaps. It is a cheap and effective form of serial transmission and is particularly suited for low speed connections such as keyboard and mouse.

2 Networking

Network topology

Objectives

  • Understand:
    • physical star topology
    • logical bus network topology
    • differentiate between them
    • explain their operation
    • compare each (advantages and disadvantages).

Network Basics

  • Computers can be connected in different layouts by cables to form a network.
  • LAN or local area network interconnects computers within a limited area such as a residence, school, laboratory, or office building.
  • LAN is contrasted in principle to a wide area network (WAN), which covers a larger geographic distance and may involve leased telecommunication circuits, while the media for LANs are locally managed.
  • All devices on a network must have a network interface card (NIC) to be able to communicate with other devices.
  • Each NIC has a factory assigned unique code, MAC address (Media Access Control Address). MAC address is also called physical address.
  • MAC address is 48 bits long and is written in 12 hex digits, such as 32:00:1a:d2:2c:80
  • When a device wants to join an existing network, it broadcasts its MAC address.
Bus and Star Network Topology

Bus network topology

  • In this topology/layout, all devices(nodes) are connected to a single cable(the backbone cable).
  • The ends of the cable are connected to a device or a terminator.

bus_topology.gif

  • When a node needs to transmit data, it broadcasts to every other nodes. But only the intended recipient accepts and processes the message.
  • The traffic generated by each node has equal transmission priority.
  • In order for nodes to transmit on the same bus simultaneously, they use a media access control technology such as Carrier Sense Multiple Access (CSMA) or a bus master.
  • If two nodes try to transmit at the same time, a carrier sensing detects the other signals, and both nodes then wait for a random amount time before re-transmit.

Star network topology

  • In a star network, each node connects to a central device which can be a hub or a switch.
  • A node sends data to a hub, and then the hub broadcasts the message to all other nodes.
  • A switch keeps a record of the MAC address (media access control address) of each node on its network. It sends the data cecieved to the intended node.

Star_Topology.png

Star and Bus Topology Comparision

Advantages of a bus topology

  • Inexpensive to set up. It does not require as much cabling as a star topology and no need for a central node such a hub or switch.
  • Adding additional node is easy without interrupting the network.
  • It works well for small networks.

Disadvantages of a bus topology

  • Entire network shuts down if there is a break in the main cable.
  • Low security as all nodes can see all data transmitted.
  • Terminators are required at both ends of the backbone cable.
  • Network slowness increases when more devices are added into the network.
  • Difficult to identify the problem if the entire network shuts down.

Advantages of a star topology

  • Star networks are very reliable because if one computer or its connection breaks it doesn’t affect the other computers and their connections
  • Easy to add a node without interrupting other nodes.
  • Secure as each node sends data directly to the central node.
  • Each node can have different speed.
  • With mode nodes added, the overall performance will not be affected significantly.

Disadvantages of a star topology

  • More expensive as more cabling needed and also a central device is needed.
  • If the central node fails, no communication is possible for the whole network.
Physical and Logical Topology
  • A physical topology is how devices are actually interconnected with wires and cables. For example, in a star network that uses hubs rather than switches, the logical topology appears as if every node is connected to a common bus that runs from node to node. However, its physical topology is a star, in which every node on the network connects to a central hub.
  • A logical topology is how devices appear connected to the user.

Types of networking between hosts

Objectives

  • Explain the following and describe situations where they might be used:
    • peer-to-peer networking
    • client-server networking

peer-to-peer(P2P) network

  • A network of personal computers, each of which acts as both client and sever, so that each can exchange files and email directly with every other computer on the network.
  • Each computer can access any of the others, although access can be restricted to those files that a computer's user chooses to make available.
  • Each computer has equal status

Research on P2P network and piracy

  • Remember the music sharing service Napster? And the movie streaming service Popcorn Time? Do some research on those two services.How do they work? What does copy right work? How could Kazaa weather the legal store for so long?

kazaa-napster-protocol.gif

Client-Server network

  • A computer network in which one centralized, powerful computer (called the server) is a hub to which many less powerful personal computers or workstations (called clients) are connected.
  • Most computers are nominated as clients and one or more as servers.
  • The clients run programs and access data that are stored on the server.
  • In a network, there may be many servers, such as web server, mail server,database server, directory server, file server and printer server.

Client-server and P2P Comparision

Advantages of client-server network

  • Better security due to centrally managed server which is easy to deplore security measures such as secure connections and access control.
  • Data and hardware resources can be shared.
  • If regualarly backed up and good restore practice in place, data lose and down time is kept to the minimum.

Disadvantages of client-server network

  • It is expensive to set up initially
  • Dedicated server admin needed to manage the server(s) and the network.

Advantages of P2P network

  • Easy and cheap to set up and easy to maintain
  • Allows users to share resources such as a printer or a router
  • There is no central server to setup or maintain
  • Easy and cheap to set up

Disadvantages of P2P network

  • Malware, spy ware and virus can be easily transmitted.
  • Each computer will have its own back-up system if at all.
  • Can be used for piracy purposes.
  • Network reliability is difficulty to determine.

Wireless networking

Objectives

  • Explain the purpose of WiFi.
  • Be familiar with the components required for wireless networking.
    • wireless adapter
    • wireless access point
  • Be familiar with how wireless networks are secured.
  • Be familiar with the purpose of Service Set Identifier (SSID).

Wireless Network Basics

Wireless networking basics

'A wireless network uses radio waves, just like cell phones, televisions and radios do. In fact, communication across a wireless network is a lot like two-way radio communication. Here's what happens:

  • A computer's wireless adapter translates data into a radio signal and transmits it using an antenna.
  • A wireless router receives the signal and decodes it. The router sends the information to the Internet using a physical, wired Ethernet connection.

The process also works in reverse, with the router receiving information from the Internet, translating it into a radio signal and sending it to the computer's wireless adapter.' - from howstuffworks.com

A WiFi hotspot is simply an area with an accessible wireless network. The term is most often used to refer to wireless networks in public areas like airports and coffee shops.

Purpose of WiFi

  • WiFi or Wi-Fi is a wireless technology that enables a device such as a PC, laptop, smartphone, tablet, printer or media player (music or movie) to connect to a network via a wireless access point(WAP) so they can access or share network resources.
  • In 1999, the WiFi alliance was formed to establish international standards for WiFi interoperability.
  • A typical WAP can have a range of 90 meters outdoor and 50 meters indoor. The range is affected by many other factors.

Components required for wireless networking

  • A device needs a wireless network interface controller(could be a USB or PCI card).
  • A device with a wireless network interface controller is called a station.
  • All stations in the WAP range will listen to one radio frequency transmission.
  • A WAP can connect to a router or be integral part of a router.
  • The WAP tries to process all transmissions from all stations in its range as fast it can. Only one station can transmit data at a time. The more stations it has, the slower the communication becomes.

WiFi Security

Wireless Network Security

  • When you trying to join a new WiFi network, if it is secured (password protected), you will be asked to choose either WPA and/or WPA2 secure scheme.
  • WPA (WiFi Protected Access) and WPA2 (the newer generation set to replace WPA) are two security protocols developed by the WiFi Alliance.
  • WPA-personal is one mode of WPA which is widely used in home and small office. It does not require a centralised server to work. WPA-personal works by encrypt communication using a 256-bit key. The WiFi password you normally use to join a network is part of the 256-bit key.
  • WPA-enterprise is designed for enterprise networks and requires a RADIUS server. This requires a more complicated setup, but provides additional security.

SSID (Service Set Identification)

  • SSID is the name for a local WiFi network, such as BTHub6-XTJG.
  • SSID's purpose is to identify a WiFi network and also used for part of the encryption key.

SSIDs.png

CSMA/CA

CSMA/CA

  • Carrier Sense Multiple Access/Collision Avoidance is a protocol used in wireless transmission to prevent collisions before they occur.
  • Before transmission, a station listens for signals on the network to determine if the channel is free or another station is transmitting. If a signal is detected, then it waits for a random amount of time for the other station before re-listens again.
  • Each station may optionally use a Request to Send(from the station)/Clear to Send(from the WAP) (RTS/CTS) after determining there is no station currently transmitting. Using RTS/CTS can avoid the problem with hidden stations - those stations whose signals are not visible to the listening station but only to the WAP due to outside the communication range.

Csma_ca.svg

By jjgarcia.tsc - http://commons.wikimedia.org/wiki/File:Csmaca_algorithm.png, Attribution, https://commons.wikimedia.org/w/index.php?curid=12661157

Location-based Wi-Fi services

  • WiFi location can be thought as indoor GPS.
  • Wi-Fi-based positioning systems are used where GPS is inadequate due (typically) to signal blockage.
  • Retail businesses, hospitalities, healthcare and education are looking to use this technology to reach people.
  • Example uses of (Location based services) LBS:
    • location based advertisements and coupons
    • retail centres can optimise store layouts based on typical customer traffic paths.
    • business owners can charge more for premium storefront or high-view ad spots.
    • personalise customer services such as personalised greeting and check-in on arrival.
    • hospitcals can use LBS for indoor navigation, tracking staff and patients, location-based messaging, asset management, location analytics, and in integrating with other clinical systems.
    • In a university campus, LBS can provide users with points of interests, such as restaurants, student meeting places, lectures and events, geo-fencing lecture contents, and in campus traffic patterns.
  • The problem of Wi-Fi based indoor localization of a device consists in determining the position of client devices with respect to access points. Many techniques exist to accomplish this.
  • Privacy concerns on how user location data being used and how long being retained.

Research on LBS technologies and its legal implications

  • try to conduct some online research and answer the following questions.
    • What are some of the LBS examples in everyday life?
    • What technologies are in use to locate users?
    • What are the privacy concerns?
    • What laws are in place to address LBS issues?
    • What is your take on national security over personal privacy?