Transmission Media

Communication Media or Transmission Media is the means through which information in the form of signals usually move from one network device to another. There are different types but they can be categorized into one of two categories:

  1. Guided Transmission Media (Cables)
  2. Unguided Transmission Media (Wireless)

Cables

Unshielded Twisted Pair (UTP)

This consists of two copper wires covered with a thin plastic coating twisted together. The twist in the wires act as a means of reducing electrical interference from the wires and radiation of radio frequencies. UTP is broken down into five categories:

Categories

Category 1 (CAT 1): – Voice Only (telephone wire)

Category 2 (CAT 2):- Data to 4Mbps

Category 3 (CAT 3):- Data to 10Mbps (Ethernet)

Category 4 (CAT 4):- Data to 20Mbps (16Mbps Token Ring)

Category 5 (CAT 5):- Data to 100Mbps (Fast Ethernet)

Shielded Twisted Pair (STP)

Similar to the UTP with the addition of a sheet of aluminium foil shielding each wire pair. The shield is grounded and prevents interference and signal radiation.

Coaxial

This cable consists of a central conductor surrounded by a shield. Coaxial cables are highly resistant to interference and are able to support wide bandwidths. This cable carries signals farther than 500m.

Optical Fibre

Optical Fibres use pulses of light, rather than pulses of electricity, to send data. It is made of glass or plastic fibres. It consists of a small fibre core encased in a thin, light absorbent plastic or glass jacket referred to as its cladding. The cladding is enclosed by a thick plastic or Teflon. Optical Fibre comes in many forms:

  1. Multi-mode Fibre (Step index) – light is introduced at several angles. At some angles, the light escapes and is absorbed by the cladding. At other angles it is reflected down the length of the fibre.
  2. Multi-mode Graded Index Fibre – uses a glass fibre in which the index of refraction varies. Instead of reflecting signals, multi-mode fibre bends the signal as they approach the cladding.
  3. Single Mode Fibre – the diameter of the core is reduced so that only one coherent light signal will be transmitted.
  4. Breakout Cable – has several fibres (often 4), each with its own cladding and its own jacket bound together with a larger jacket.
  5. Distribution Cable – Breakout cable with a larger jacket.
  6. Loose Cable –  1-12 fibres float inside a loose tube filled with get

Unguided/Wireless Transmission Media

Unguided media transport electromagnetic waves without using a physical conductor. This type of communication is often called wireless communication. Signals are normally broadcast through free space and thus are available to anyone who has a device capable of receiving them.

TCP/IP Suite

Transmission Control Protocol (TCP) / Internet Protocol (IP)

TCP/IP is a suite of protocols that performs the transfer of data between two computers or groups of dissimilar computers.

TCP is responsible for verifying the correct delivery of data from client to server. TCP also adds support to detect errors or lost data and to trigger retransmission until the data is correctly and completely received.

IP is responsible for moving packets of data from node to node. IP forwards packets based on a four byte destination address (IP number).

Application Layer

A TCP/IP application is any network process that occurs above the Transport Layer. This includes all the processes that users directly interact with, as well as other processes that users are not aware of.

Protocols:

Transport Layer

The transport layer corresponds to the Session and Transport Layer of the OSI model. It establishes a secure session between two machines and breaks down packets or datagrams.

Protocols:

  • Transmission Control Protocol (TCP) – ensures that the user gets data exactly as it was sent
  • User Datagram Protocol (UDP) – this protocol does not perform end-to-end reliability checks

Internet Layer

This is equivalent to the Network layer of the OSI model. It is concerned with how packets are routed and solving congestion problems. This layer uses Internet Protocol (IP), which is a connectionless protocol and Internet Control Message Protocol (ICMP).

Network Interface/Access Layer

The Network Access layer is equivalent to the Data Link and Physical layers of the OSI model. It ensures that data transmitted between sender and receiver is correct. This layer also provides guidelines on how to move data bits between modems.

Communication between Nodes

When connection-oriented services such as HTTP, FTP and pure IP are used, a connection must be established between the source and destination. This connection is called a handshake. The handshake has three steps. For argument sake, lets call the device that initiated the handshake Device A, and the target or destination Device B.

Step 1: – Device A sends its TCP sequence number and maximum segment size to Device B.

Step 2:- Device B responds by sending its sequence number and maximum segment size to Device A.

Step 3:- Device A acknowledges receipt of the sequence number and segment size information. Device A begins transmission.

A normal connection between a user (Alice) and...

A normal connection between a user (Alice) and a server. The three-way handshake is correctly performed. (Photo credit: Wikipedia)

 

 

OSI Reference Model

The Open System Interconnect (OSI)

The OSI reference model was developed by the International Standard Organization (ISO) to establish a framework of standards for computer-to-computer communication. The OSI model allows hardware and software companies to develop their products to work within certain parameters and guidelines of the model. This allows products to work with other products that operate within these guidelines.

The “Open” in OSI means that the model deals with systems that are open for communication with other systems. The OSI model is also called a layered protocol because of the seven (7) layers that its comprised of.

Benefits of the OSI Model

  • A network layer can be replaced by a layer from another network vendor.
  • Networks can be upgraded easily by replacing individual layers
  • The user and network designer are not restricted to using the product (hardware/ software) of a specific vendor.

Layer Function

Layer 1: Physical

This layer defines the the standards that provide guidelines on how to move data bits between modems. This layer also specifies the electrical connections between the transmission medium and the computer.

Layer 2: Data Link

This layer is responsible for ensuring that the data transmitted between two locations on the network is accurate. This layer also controls access to the network and breaks up data to be sent into frames. The data link layer solves competition problems using Media Access Control (MAC) and Carrier Sense Multiple Access/Collision Detection (CSMA/CD). This layer also solves lost, duplicated or destroyed frames.

Layer 3: Network

The network layer is concerned with determining how packets are routed from source to destination. Routes (paths) can be hard-wired into the LAN or they can be determined at the time a packet is sent. This layer solves congestion problems and ensures that the line is not overloaded with packets while other lines are under-utilized.

Layer 4: Transport

The basic function of the transport layer is to accept data from the session layer, break them into smaller pieces and pass them on to the network layer. This layer also ensures that pieces of data arrive at the other end correctly.

Layer 5: Session

The session layer allows users on different machines to establish and terminate a session between them. A session is used to allow a user to log into a remote time-sharing system or to transfer a file between two machines. The session layer manages communication and keeps track of whose turn it is to talk.

Layer 6: Presentation

This layer ensures that the receiving computer understands the information sent to it. Different computers represent data differently and it is the responsibility of this layer to provide a standard encoding (data representation) to be used by computers in the network. Data is formatted using this standard encoding before it is presented to destination machines. This layer is also responsible for data compression and encryption.

Layer 7: Application

This layer facilitates user functions (e.g. File Transfer, Email) and provides guidelines for network services.

Network Topologies

Computers can be connected together in many different ways. The layout/topology of the network will influence how reliable the network is and how easy it is to access. There are two types of topologies:Wired and Wireless. The main physical wired topologies are: Star, Ring, Bus and Mesh.

STAR TOPOLOGY

In a Star configured network, all devices are connected to a central device called a hub. Nodes communicate across the network by passing data through the hub.

Advantages:

  1. New stations can be added easily
  2. A single cable failure won’t bring down the network
  3. Relatively easy to troubleshoot

Disadvantages:

  1. Single point of failure – if hub goes down, the network goes down
  2. Total installation cost may be high due to the number of cables needed for each node

RING TOPOLOGY

All devices are connected to one another in the shape of a closed loop, so that each device is connected directly to two other devices, one on either side of it. When a computer sends data, the data travels to each computer on the ring until it reaches it destination.

Advantages:

  • Growth of ring has minimal effect on performance
  • Each node is able to filter and amplify the data before sending it to the next node.
  • Can cover a larger area than star.

Disadvantages:

  • A break in the cable will shut down the network
  • Most expensive topology
  • One computer failure affects others
  • Difficult to add or remove stations

BUS TOPOLOGY

Image courtesy of networkgue.blogspot.com

Devices are connected to a single central cable called the bus or backbone. Using the BUS and destination address, data is transmitted from source to destination along the backbone.

Advantages:

  1. – Inexpensive and easy to install
  2. – You can add and subtract devices without affecting the network
  3. – Failure of one device does not affect the network
  4. – Requires less cabling than star

Disadvantages:

  1. Network shuts down if there is a break in the cable
  2. Terminators are needed at both ends of the cable
  3. Difficult to troubleshoot

MESH TOPOLOGY

Devices are connected with many redundant interconnections between nodes.

Advantages:

  1. Failure of one node doesn’t affect the network
  2. Easy to expand
  3. More secure

Disadvantages:

  1. Very expensive due to the number of cable that would be required
  2. Difficult to implement

Networking – Types of Networks

There are several types of computer networks, but the main ones are:

  1. Local Area Network (LAN)
  2. Wide Area Network (WAN)
  3. Metropolitan Area Network (MAN)

Local Area Network (LAN) – these are computers and devices that are connected in small and limited area like a school, the office a lab etc. The data transfer rates are the lowest of the three networks mentioned.

 

 

 

 

 

 

 

Wide Area Network (WAN) – A WAN consists of several LANs connected together over the internet. WANs covers the largest geographical area of all networks.

Wide Area Network Image courtesy of bhumika.ca

Metropolitan Area Network (MAN) – MANs fall between the LAN and WAN. They cover a larger area than the LAN, but are much smaller than a WAN. As the name suggests, MANs covers cities.

MAN image courtesy of transition.com

Advantages of Networks

  1. Speed – transferring files is done quickly in networks
  2. Cost – less software and hardware need to be purchased
  3. Easy Management – Software are installed in a central location (the server), so updates can be dispatched easily through the server
  4. Resource Sharing – All machines in the network are connected to the same resource (printer, storage server)

Disadvantages of Networks

  1. If the server goes down, stored information on it becomes inaccessible
  2. Resources connected to the network becomes inaccessible
  3. Possible risk of attack from virus and hackers
  4. As traffic increases on a network, the performance degrades
  5. Larger networks become more difficult to manage