Unit 1 Communication Switch Techniques.

 

What is Circuit Switching?

This method of switching establishes a dedicated communication path between the sender and receiver. Here the link is established in physical form between two stations present in the network. The link will be established, maintained and terminated for each communication session. The most common example of circuit switching is the Analog telephone network.

This method of switching provides a constant bit delay and fixed data rate channel between the sender and receiver. The full channel capacity is dedicated for the duration of a connection. When data is to be transferred from sender to receiver, firstly the sender sends a request to the switching station for the establishment of a connection. The receiver replies with an acknowledgment. After receiving the acknowledgment signal the sender starts the data transmission. This switching is commonly used for voice circuits. The public switched telephone network, Datakit, B channel of ISDN, Optical mesh network, etc are some of the examples of circuit-switched networks.

Circuit Switching Diagram

In this type of switching, there is a set of switches connected with physical links. Here once the dedicated path is established between the sender and receiver, it stays the same until one of the users terminates the connection. Fixed data is transmitted and this type of switching is highly used to transfer voice data. The network consists of the switching offices with permanent links between them. Whenever is connection requested the communication links are dedicated to the terminals forming the transmission route. This dedicated link is maintained until the connection is terminated. Other users will be able to use this link only when it is terminated by the sender or receiver.

There are three phases in the establishment of a circuit switching network. They are – circuit establishment, Data transfer and circuit disconnect.

Circuit Switching

 Circuit Establishment

This is the circuit setup phase. Here the link is established between the sender and receiver. When a connection has to be established between station A and station B, station A sends a connection request to node 1 through a dedicated link between station A and node1. Then node1 sends the request to all the nodes connected to it. The request is forwarded among the nodes finally forming a route to the Station B. Based on its status station B sends the acknowledgment if it is not busy. Thus a dedicated communication link will be established between Station A and Station B.

Differences Between Circuit Switching and Packet Switching

• In-circuit switching, data is transmitted continuously between the sender and receiver. Whereas in packet switching the data is packetized and sent independently over a shared network.
• When the connection is maintained in the circuit switching no other user can access that link, even when no data is been transmitted. Thus circuit switching provides high-quality service.
• In packet switching the bandwidth is shared by the users. Thus, the quality of the service is low in packet switching.
• Unlike circuit switching, the path is not reserved in the packet switching network. Packet switching supports store and forward transmission.
• No physical path is established for the packet switching network.
• Packet switching is more efficient compared to circuit switching.
• Packet switching infrastructure is less complicated compared to circuit switching.

Advantages and Disadvantages

Some of the advantages of circuit switching are as follows –

• It uses a fixed bandwidth.
• A dedicated communication channel increases the quality of communication.
• Data is transmitted with a fixed data rate.
• No waiting time at switches.
• Suitable for long continuous communication.

Some of the disadvantages of circuit switching are as follows-

• A dedicated connection makes it impossible to transmit other data even if the channel is free.
• Resources are not utilized fully.
• The time required to establish the physical link between the two stations is too long.
• As a dedicated path has to be established for each connection, circuit switching is more expensive.
• Even if there is no transfer of data, the link is still maintained until it is terminated by users. By this channel remains ideal for a long time thereby making circuit switching inefficient.
• Dedicated channels require more bandwidth

What is Packet Switching?

Definition: Packet switching refers to a set of protocols that uses a connection-less network switching approach to transmit the packets. In this switching, the messages are broken and grouped into small units called packets. These packets are transmitted individually across a digital network to reach its destination. Packets need not follow the same route to reach their destination. As all the packets arrive at a destination in a different order, the original message is recompiled by the destination itself. The packet switching diagram is shown below.

In this switching, packets have two parts – a header and a payload. The information in the header allows networking hardware/intermediate node to make sure that the packets are directed towards its destination, while the definite data is carried by the payload.

Each packet has a source and destination address to travel independently over a network with a variable bitrate. Packets are forwarded asynchronously by intermediate nodes because of congestion, queuing, and so on, and hence follows different routes. These packets arrive at the destination in a different order, and the destination ensures to reassemble the data of the same file.

The message consists of four packets – A, B, C, and D. Each packet consists of source and destination address and follows more than one route to reach the destination from source as shown in the figure below.

packet-switching. 

Advantages of Packet Switching Over Circuit Switching

This switching offers various benefits compared to circuit switching and they are listed below:

• It delivers the data to a destination by finding their own paths; circuit switching has a dedicated and predefined channel.
• It is highly reliable as missing packets are detected by destination; circuit switching does not have this option.
• It uses lesser bandwidth as packets are quickly routed towards the destination; circuit switching should have dedicated bandwidth.
• The channel in this switching is available for other transmissions as soon as packets are routed; circuit switching occupies the channel till the voice communication is completed
• It is cost-effective and easier to implement; circuit switching is expensive

Disadvantages of Packet Switching Over Circuit Switching

Despite offering various benefits, this switching offers disadvantages too, which are listed below:

• As the movement of packets is not synchronous in this switching, it may not be suitable in communication applications like voice calls; while circuit switching is highly suitable for voice calls
• Packets don’t move in an organized way, sequence numbers should be provided to identify each packet; circuit switching gives the highest priority for the channel to give the best experience to the users
• In this switching, complexity is high at each node as packets are routed over multiple paths to reach the destination, leading to loss of data or delay in delivering the packets; circuit switching makes sure there is no loss of data
• This switching needs additional and secure protocols to protect the data, leading to a significant rise in implementation costs; circuit switching has a dedicated channel for one service and one individual route.

Message switching techniques

Switched communication networks are those in which data transferred from source to destination is routed between various intermediate nodes. Switching is the technique by which nodes control or switch data to transmit it between specific points on a network. There are 3 common switching techniques:

1. Circuit Switching
2. Packet Switching
3. Message Switching

Message Switching –
Message switching was a technique developed as an alternate to circuit switching, before packet switching was introduced. In message switching, end users communicate by sending and receiving messages that included the entire data to be shared. Messages are the smallest individual unit.
Also, the sender and receiver are not directly connected. There are a number of intermediate nodes transfer data and ensure that the message reaches its destination. Message switched data networks are hence called hop-by-hop systems.

They provide 2 distinct and important characteristics:

1. Store and forward – The intermediate nodes have the responsibility of transferring the entire message to the next node. Hence, each node must have storage capacity. A message will only be delivered if the next hop and the link connecting it are both available, otherwise it’ll be stored indefinitely. A store-and-forward switch forwards a message only if sufficient resources are available and the next hop is accepting data. This is called the store-and-forward property.
2. Message delivery – This implies wrapping the entire information in a single message and transferring it from the source to the destination node. Each message must have a header that contains the message routing information, including the source and destination.

Message switching network consists of transmission links (channels), store-and-forward switch nodes and end stations as shown in the following picture:

Characteristics of message switching –
Message switching is advantageous as it enables efficient usage of network resources. Also, because of the store-and-forward capability of intermediary nodes, traffic can be efficiently regulated and controlled. Message delivery as one unit, rather than in pieces, is another benefit.

However, message switching has certain disadvantages as well. Since messages are stored indefinitely at each intermediate node, switches require large storage capacity. Also, these are pretty slow. This is because at each node, first there us wait till the entire message is received, then it must be stored and transmitted after processing the next node and links to it depending on availability and channel traffic. Hence, message switching cannot be used for real time or interactive applications like video conference.

Advantages of Message Switching –
Message switching has the following advantages:

1. As message switching is able to store the message for which communication channel is not available, it helps in reducing the traffic congestion in network.
2. In message switching, the data channels are shared by the network devices.
3. It makes the traffic management efficient by assigning priorities to the messages.

Disadvantages of Message Switching –
Message switching has the following disadvantages:

1. Message switching cannot be used for real time applications as storing of messages causes delay.
2. In message switching, message has to be stored for which every intermediate devices in the network requires a large storing capacity.

Applications –
The store-and-forward method was implemented in telegraph message switching centres. Today, although many major networks and systems are packet-switched or circuit switched networks, their delivery processes can be based on message switching. For example, in most electronic mail systems the delivery process is based on message switching, while the network is in fact either circuit-switched or packet-switched.

Unit 1 Communication Channel Wired Transmission....

 

Wired Transmission

What is it? 
Wired Transmission is a media that is used to transfer information over another network, such as a twisted pair cable, Coaxial cable and optic fibre.

Coaxial Cable

• It consists of two conductors, but is constructed differently to permit it to operate over a wider range of frequencies. These include the outer cylindrical conductor that surround a single wire conductor and an inner conductor which is held in place by either regularly spaced separating the rings.
  
• This cable can support between 10 to 100Mbps
• It can be cheaper for a physical bus topology because less cable will be needed.
• It can also support up to 370MHz
• The second layer of the cable or shield can help reduce the amount of outside interference. Covering this shield is the cable jacket.

Optic Fibre

What is optic fibre? 


A technology that uses glass or plastic fibers (threads) to transmit data. It is capable of transmitting messages modulated onto light waves. 

• Entry level fibre optic speed starts at 15 Mbps. For example, this means a 5 megabyte film will upload in 8 seconds, and a 10 megabyte film can upload in 16 seconds; or downloading 10 songs would take less than half a minute. This type of speed is typically 3 Mbps faster than the regular cable internet service

• Now with technology rapidly improving, the fastest fibre optic speed has approached around 100 Mbps. 

• Modern fibre cables can contain up to a thousand fibres in a single cable, with bandwidth potentially in terabytes per second. 

• Some companies are offering unlimited fibre optic broadband for around $300 a month with 100 Mbps speed…

• Fibre optics are known to be the most secure transmission media around as it uses light to transmit data rather than electricity currents.

• The reasons for its high security include: 
• that it’s immune to electronic interference, 
• it never loses signal 
• it can not be tapped into unless a person intercepts the cable physically. 

• A security issue relevant to fibre optics include fiber tapping- this is when the network is tapped into and signal is extracted from an optical fibre.

Telephone line ...

A telephone line or telephone circuit (or just line or circuit industrywide) is a single-user circuit on a telephone communication system. This is the physical wire or other signaling medium connecting the user's telephone apparatus to the telecommunicationsnetwork, and usually also implies a single telephone number for billing purposes reserved for that user. Telephone lines are used to deliver landline telephone service and Digital subscriber line (DSL) phone cable service to the premises. Telephone overhead lines are connected to the public switched telephone network.

What is Leased Line?

Leased Libe, also called a dedicated line, is a telecommunications service provided to businesses by telcos and long distance carriers that provides a permanent direct connection between two geographically separate local area networks (LANs).

Leased Line Service

Leased lines are dedicated circuits that the telco reserves for the exclusive use of the customer. They are permanently available, always active, and secure, and they have consistent quality of service (QoS) and a flat monthly fee. However, they are very expensive compared to dial-up lines, and businesses rarely use their full bandwidth capabilities except in short bursts.

Leased lines are a form of point-to-point connection. Your LAN is connected by bridges, routers, modems, and terminal adapters to the telco’s central office (CO), which sets up dedicated switches to connect you to the destination LAN. The presence of dedicated switches is what makes leased lines so expensive. You would use a leased line to connect a Microsoft Exchange server to the Internet, for example. Since the leased line is always on, there is no connection delay when users try to access the server for their 

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Switched Line

Definition - What does Switched Line mean?

A switched line is a communications link established through the use of switching equipment which allows a connection to be established between two transmission devices. The switched line provides a temporary connection between two user terminals or machines and only lasts for a certain period. Switched lines minimize the line costs while maintaining the advantages of interconnected systems. They are less expensive compared to leased lines and are often appropriate when there is low traffic, as well as for connecting several remote sites.

Advantages of a switched line are:

• Low cost, especially if there is low usage or traffic between terminals
• Provides means to access and connect multiple distant machines
• Flexibility since many machines offering different services can be accessed
• Once a breakdown occurs on a connection to a facility, the user or machine can redial and obtain an alternative route to the facility.

What is broadband?

Broadband is a high-speed internet connection which allows you to enjoy everything the internet has to offer.

Before broadband, internet access was achieved with ‘narrowband’ dial-upconnections that were very slow by today’s standards. Broadband is much quicker and allows us to do more on the internet.

How fast is broadband?

What makes a connection fast enough to be called “broadband”? There is no standard definition, and what qualifies as a broadband service varies across the world.

In the UK we have access to a wide variety of services with different levels of performance. Almost every home and business can get access to a fixed-line connection of at least 2Mbps (megabits per second), but most can get speeds in excess of 24Mbps.

Not sure what speed you need, or want to know what you’re currently getting? Our guide to broadband speeds can help.

How does broadband work?

An internet service provider (ISP) supplies the service and equipment to get your home or business online. Some ISPs operate their own infrastructure, but this is a complex and expensive endeavour so most pay for wholesale access to a network which they can then resell.

Most of the home broadband services you’ll find on Broadband Genie come into your home via a fixed line, which will very likely be either an Openreach (BT) telephone line or a Virgin Media network connection.

Broadband service does not always require a line though. There is also mobile broadband using mobile phone networks, satellite internetwhich connects to orbiting relays, and wireless broadband providers using long-range Wi-Fi transmissions.

Unit 1 Networking Topology..

 

Computer Network Topology – Mesh, Star, Bus, Ring and Hybrid

BY CHAITANYA SINGH | FILED UNDER: COMPUTER NETWORK

Geometric representation of how the computers are connected to each other is known as topology. There are five types of topology – Mesh, Star, Bus, Ring and Hybrid.

Types of Topology

There are five types of topology in computer networks:

1. Mesh Topology
2. Star Topology
3. Bus Topology
4. Ring Topology
5. Hybrid Topology

Mesh Topology

In mesh topology each device is connected to every other device on the network through a dedicated point-to-point link. When we say dedicated it means that the link only carries data for the two connected devices only. Lets say we have n devices in the network then each device must be connected with (n-1) devices of the network. Number of links in a mesh topology of n devices would be n(n-1)/2.

Advantages of Mesh topology

1. No data traffic issues as there is a dedicated link between two devices which means the link is only available for those two devices.
2. Mesh topology is reliable and robust as failure of one link doesn’t affect other links and the communication between other devices on the network.
3. Mesh topology is secure because there is a point to point link thus unauthorized access is not possible.
4. Fault detection is easy.

Disadvantages of Mesh topology

1. Amount of wires required to connected each system is tedious and headache.
2. Since each device needs to be connected with other devices, number of I/O ports required must be huge.
3. Scalability issues because a device cannot be connected with large number of devices with a dedicated point to point link.

Star Topology

In star topology each device in the network is connected to a central device called hub. Unlike Mesh topology, star topology doesn’t allow direct communication between devices, a device must have to communicate through hub. If one device wants to send data to other device, it has to first send the data to hub and then the hub transmit that data to the designated device.

Advantages of Star topology

1. Less expensive because each device only need one I/O port and needs to be connected with hub with one link.
2. Easier to install
3. Less amount of cables required because each device needs to be connected with the hub only.
4. Robust, if one link fails, other links will work just fine.
5. Easy fault detection because the link can be easily identified.

Disadvantages of Star topology

1. If hub goes down everything goes down, none of the devices can work without hub.
2. Hub requires more resources and regular maintenance because it is the central system of star topology.

Bus Topology

In bus topology there is a main cable and all the devices are connected to this main cable through drop lines. There is a device called tap that connects the drop line to the main cable. Since all the data is transmitted over the main cable, there is a limit of drop lines and the distance a main cable can have.

Advantages of bus topology

1. Easy installation, each cable needs to be connected with backbone cable.
2. Less cables required than Mesh and star topology

Disadvantages of bus topology

1. Difficultly in fault detection.
2. Not scalable as there is a limit of how many nodes you can connect with backbone cable.

Ring Topology

In ring topology each device is connected with the two devices on either side of it. There are two dedicated point to point links a device has with the devices on the either side of it. This structure forms a ring thus it is known as ring topology. If a device wants to send data to another device then it sends the data in one direction, each device in ring topology has a repeater, if the received data is intended for other device then repeater forwards this data until the intended device receives it.

Advantages of Ring Topology

1. Easy to install.
2. Managing is easier as to add or remove a device from the topology only two links are required to be changed.

Disadvantages of Ring Topology

1. A link failure can fail the entire network as the signal will not travel forward due to failure.
2. Data traffic issues, since all the data is circulating in a ring.

Hybrid topology

A combination of two or more topology is known as hybrid topology. For example a combination of star and mesh topology is known as hybrid topology.

Advantages of Hybrid topology

1. We can choose the topology based on the requirement for example, scalability is our concern then we can use star topology instead of bus technology.
2. Scalable as we can further connect other computer networks with the existing networks with different topologies.

Disadvantages of Hybrid topology

1. Fault detection is difficult.
2. Installation is difficult.
3. Design is complex so maintenance is high thus expensive.