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CN_KCS-603 Week: 3
UNIT-
Switching
A network is a set of connected devices. Whenever we have multiple devices, we have the problem of how to connect them to make one-to-one communication possible. One solution is to make a point-to-point connection between each pair of devices (a mesh topology) or between a central device and every other device (a star topology). These methods, however, are impractical and wasteful when applied to very large networks. The number and length of the links require too much infrastructure to be cost-efficient, and the majority of those links would be idle most of the time. A better solution is switching. A switched network consists of a series of interlinked nodes, called switches. Switches are devices capable of creating temporary connections between two or more devices linked to the switch. In a switched network, some of these nodes are connected to the end systems (computers or telephones, for example). Others are used only for routing. Figure shows a switched network.
Why is Switching Concept required? Switching concept is developed because of the following reasons: Bandwidth: It is defined as the maximum transfer rate of a cable. It is a very critical and expensive resource. Therefore, switching techniques are used for the effective utilization of the bandwidth of a network. Collision: Collision is the effect that occurs when more than one device transmits the message over the same physical media, and they collide with each other. To overcome this problem, switching technology is implemented so that packets do not collide with each other..
Advantages of Switching: Switch increases the bandwidth of the network. It reduces the workload on individual PCs as it sends the information to only that device which has been addressed. It increases the overall performance of the network by reducing the traffic on the network. There will be less frame collision as switch creates the collision domain for each connection.
We can then divide today's networks into three broad categories: circuit switched networks, packet-switched networks, and message-switched. Packetswitched networks can further be divided into two subcategories-virtual-circuit networks and datagram networks as shown in Figure.
CIRCUIT-SWITCHED NETWORKS
A circuit-switched network consists of a set of switches connected by physical links. A connection between two stations is a dedicated path made of one or more links. However, each connection uses only one dedicated channel on each link. Each link is normally divided into n channels by using FDM or TDM.
In circuit switching, the resources need to be reserved during the setup phase; the resources remain dedicated for the entire duration of data transfer until the teardown phase
Three Phases The actual communication in a circuit-switched network requires three phases: connection setup, data transfer, and connection teardown.
o The destination address is appended to the message. Message Switching provides a dynamic routing as the message is routed through the intermediate nodes based on the information available in the message.
o Message switches are programmed in such a way so that they can provide the most efficient routes.
o Each and every node stores the entire message and then forward it to the next node. This type of network is known as store and forward network.
o Message switching treats each message as an independent entity.
Advantages Of Message Switching
o Data channels are shared among the communicating devices that improve the efficiency of using available bandwidth.
o Traffic congestion can be reduced because the message is temporarily stored in the nodes.
o Message priority can be used to manage the network.
o The size of the message which is sent over the network can be varied. Therefore, it supports the data of unlimited size.
Disadvantages Of Message Switching
o The message switches must be equipped with sufficient storage to enable them to store the messages until the message is forwarded.
o The Long delay can occur due to the storing and forwarding facility provided by the message switching technique.
Packet Switching
o The packet switching is a switching technique in which the message is sent in one go, but it is divided into smaller pieces, and they are sent individually.
o The message splits into smaller pieces known as packets and packets are given a unique number to identify their order at the receiving end.
o Every packet contains some information in its headers such as source address, destination address and sequence number.
o Packets will travel across the network, taking the shortest path as possible.
o All the packets are reassembled at the receiving end in correct order.
o If any packet is missing or corrupted, then the message will be sent to resend the message.
o If the correct order of the packets is reached, then the acknowledgment message will be sent.
Approaches Of Packet Switching:
There are two approaches to Packet Switching:
Datagram Packet switching:
o Call request and call accept packets are used to establish a connection between the sender and receiver.
o When a route is established, data will be transferred.
o After transmission of data, an acknowledgment signal is sent by the receiver that the message has been received.
o If the user wants to terminate the connection, a clear signal is sent for the termination.
Efficiency
The efficiency of a datagram network is better than that of a circuit-switched network; resources are allocated only when there are packets to be transferred.
Delay There may be greater delay in a datagram network than in a virtual-circuit network. Although there are no setup and teardown phases, each packet may experience a wait at a switch before it is forwarded. In addition, since not all packets in a message necessarily travel through the same switches, the delay is not uniform for the packets of a message.
Switching in the Internet is done by using the datagram approach to packet switching at the network layer.
Advantages Of Packet Switching:
o Cost-effective: In packet switching technique, switching devices do not require massive secondary storage to store the packets, so cost is minimized to some extent. Therefore, we can say that the packet switching technique is a cost-effective technique.
o Reliable: If any node is busy, then the packets can be rerouted. This ensures that the Packet Switching technique provides reliable communication.
o Efficient: Packet Switching is an efficient technique. It does not require any established path prior to the transmission, and many users can use the same communication channel simultaneously, hence makes use of available bandwidth very efficiently.
Disadvantages Of Packet Switching:
o Packet Switching technique cannot be implemented in those applications that require low delay and high-quality services.
o The protocols used in a packet switching technique are very complex and requires high implementation cost.
o If the network is overloaded or corrupted, then it requires retransmission of lost packets. It can also lead to the loss of critical information if errors are nor recovered.
o Differences b/w Datagram approach and Virtual Circuit approach
Datagram approach Virtual Circuit approach
Node takes routing decisions to forward
the packets.
Node does not take any routing decision.
Congestion cannot occur as all the
packets travel in different directions.
Congestion can occur when the node is busy, and it does not allow other packets to pass through.
It is more flexible as all the packets are
treated as an independent entity.
It is not very flexible.
Multiplexing
It means multiple sources but one link. An alternative approach to it is Direct Point to Point Connection but it has a number of problems as it requires an I/O port for each device, a need line for each device, and also a large amount of wiring is needed if on different floors. But instead, if we use a multiplexer approach then all devices are connected to MUX and one line to host, the link carries multiple channels of information and a number of lines equal to the number of lines out.
Types of Multiplexers:
1. Frequency Division Multiplexing (FDM) –
The frequency spectrum is divided among the logical channels and each user has exclusive access to his channel. It sends signals in several distinct frequency ranges and carries multiple video channels on a single cable. Each signal is modulated onto a different carrier frequency and carrier frequencies are separated by guard bands. The bandwidth of the transmission medium exceeds the required bandwidth of all the signals. Usually, for frequency division multiplexing analog signaling is used in order to transmit the signals, i.e. more susceptible to noise.
b. Statistical Time Division Multiplexing: Time-division but on-demand rather than fixed, reschedule link on a per-packet basis and packets from different sources interleaved on the link. It allows the connection of more nodes to the circuit than the capacity of the circuit. Works on the premise that not all the nodes will transmit at full capacity at all times. It must transmit a terminal identification i.e destination id no. and may require storage. A statistical multiplexor transmits only the data from active workstations. If a workstation is not active, no space is wasted on the multiplexed stream. It accepts the incoming data streams and creates a frame containing only the data to be transmitted.
2. Wavelength Division Multiplexing –
It is the same as FDM but applied to fibers, only the difference is that here the operating frequencies are much higher actually they are in the optical range. There’s great potential for fibers since the bandwidth is so huge. Fibers with different energy bands are passed through a diffraction grating prism. Combined on the long-distance link and then split at the destination. It has got high reliability and very high capacity. It multiplexes multiple data streams onto a single fiber optic line. Different wavelength lasers (called lambdas) transmit multiple signals. Each signal carried on the fiber can be transmitted at a different rate from the other signals. Dense wavelength division multiplexing combines many (30, 40, 50, or more) channels onto one fiber. DWDM channels have a very high capacity and it keeps on improving. Coarse wavelength division multiplexing combines only a few lambdas. In this, channels are more widely spaced and are a cheaper version of DWDM.
Integrated Services Digital Network (ISDN)
These are a set of communication standards for simultaneous digital transmission of voice, video, data, and other network services over the traditional circuits of the public switched telephone network. Before Integrated Services Digital Network (ISDN) , the telephone system was seen as a way to transmit voice, with some special services available for data. The main feature of ISDN is that it can integrate speech and data on the same lines, which were not available in the classic telephone system. ISDN is a circuit-switched telephone network system, but it also provides access to packet-switched networks that allows digital transmission of voice and data. This results in potentially better voice or data quality than an analog phone can provide. It provides a packet-switched connection for data in increments of 64 kilobit/s. It provided a maximum of 128 kbit/s bandwidth in both upstream and downstream directions. A greater data rate was achieved through channel bonding. Generally, ISDN B-channels of three or four BRIs (six to eight 64 kbit/s channels) are bonded. In the context of the OSI model, ISDN is employed as the network in data-link and physical layers but commonly ISDN is often limited to usage to Q.931 and related protocols. These protocols introduced in 1986 are a set of signaling protocols establishing and breaking circuit-switched connections, and for advanced calling features for the user. ISDN provides simultaneous voice, video, and text transmission between individual desktop videoconferencing systems and group videoconferencing systems.
ISDN Interfaces: The following are the interfaces of ISDN:
- Basic Rate Interface (BRI) –
There are two data-bearing channels (‘B’ channels) and one signaling channel (‘D’ channel) in BRI to initiate connections. The B channels operate at a maximum of 64 Kbps while the D channel operates at a maximum of 16 Kbps. The two channels are independent of each other. For example, one channel is used as a TCP/IP connection to a location while the other channel is used to send a fax to a remote location. In iSeries ISDN supports a basic rate interface (BRl). The basic rate interface (BRl) specifies a digital pipe consisting of two B channels of 64 Kbps each and one D channel of 16 Kbps. This equals a speed of 144 Kbps. In addition, the BRl service itself requires an operating overhead of 48 Kbps. Therefore a digital pipe of 192 Kbps is required.
- Primary Rate Interface (PRI) – Primary Rate Interface service consists of a D channel and either 23 or 30 B
To support switched and non-switched applications
To support voice and non-voice applications
Reliance on 64-kbps connections
Intelligence in the network
Layered protocol architecture
Variety of configurations
Advantages of ISDN: ISDN channels have a reliable connection. ISDN is used to facilitate the user with multiple digital channels. It has faster data transfer rate. Disadvantages of ISDN: ISDN lines costlier than the other telephone system. It requires specialized digital devices. It is less flexible.
