Computer Communication Network

Switched Network

A switched network is made up of a number of interconnected nodes known as switches. Switches are devices that allow two or more devices to form temporary connections when they are attached to the switch. Some of these nodes are connected to the end systems in a switched network (computers or telephones, for example). Others are solely used for routing purposes. A switched network is depicted in Figure 1. Diagram of a Switched Network

Figure : Switched Network

The switches are named I, II, III, IV, and V, and the end systems (communicating devices) are labeled A, B, C, D, and so on. Each switch has numerous links attached to it. Circuit switching, packet switching, and message switching have all been major switching technologies in the past. The first two are still in widespread usage today. In general communications, the third has been phased out, but it still has networking applications. The three basic kinds of today's networks are circuit-switched networks, packet-switched networks, and message-switched networks. As shown in Figure 2, packet-switched networks can be further separated into two subcategories: virtual-circuit networks and datagram networks.

Figure 2: Switched network taxonomy

Virtual-circuit networks have several traits in common with circuit-switched and datagram networks. Thus, circuit-switched networks are discussed first, followed by datagram networks, and ultimately virtual-circuit networks.

Circuit-Switched Networks

A simple circuit-switched network with four switches and four links is shown in Figure 3. Using FDM or TDM, each link is separated into n (n is 3 in the illustration) channels.

Figure 3: A circuit-switched network

A switch is directly connected to the end systems, such as computers or telephones. When end system A needs to communicate with end system M, system A must initiate a connection request to M, which must be approved by all switches as well as M. The setup phase is when a circuit (channel) is reserved on each connection, and the dedicated way is defined by the combination of circuits or channels. Data transfer can begin after the specialized path of connected circuits (channels) has been created. The circuits are tom down once all of the data has been transferred.

Several points must be emphasized here:

  • The physical layer is where circuit switching happens.

  • These resources, such as channels (bandwidth in FDM and time slots in TDM), switch buffers, switch processing time, and switch input/output ports, must be devoted before commencing communication and must stay so during the data transfer until the teardown phase.

  • Although there may be intervals of stillness, the data is provided by the source station and received by the destination station in a continual flow.

  • During data transport, there is no addressing involved. The data is routed by the switches according to their occupied band (FDM) or time slot (TDM).

  • During the setup process, end-to-end addressing is, of course, employed.

  • During the setup phase of circuit switching, resources must be reserved; these resources must be allocated for the duration of data transfer until the teardown phase.

There are three stages to the process:

  • Connection setup, data transfer, and connection teardown are the three phases of communication in a circuit-switched network.

  • Setup Creating dedicated channels between the switches is what phase connection setup entails. In Figure 3, for example, when system A needs to connect to system M, it sends a setup request to switch I that provides the address of system M. Switch I searches for a channel between itself and switch IV that can be used for this purpose. The request is subsequently sent to switch IV, which establishes a dedicated channel between itself and switch III. Switch III notifies system M of system A's current intentions.

  • An acknowledgement from system M to system A must be transmitted in the opposite direction as the following stage in the connection process. The connection is formed only once system A receives this acknowledgement.

Phase of Data Transfer
The two parties can transfer data after establishing a dedicated circuit (channels).

Phase of Tear down
A signal is delivered to each switch to free the resources when one of the parties needs to disconnect.

Circuit-switched networks, it may be argued, are less efficient than the other two types of networks since resources are distributed throughout the duration of the connection. Other connections cannot access these resources. When using a telephone network, people usually end the call when they are finished with their conversation. In computer networks, however, a computer can connect to another computer even if it has been idle for a long period.

Although a circuit-switched network is typically inefficient, the latency is negligible in this form of network. The data is not delayed at each switch during data transfer; the resources are allocated for the lifetime of the connection. Figure 6 illustrates the concept of delay in a circuit-switched network with only two switches.There is no waiting time at each switch, as seen in Figure 6. The overall delay is related to the time it takes to join the circuit, transfer data, and disconnect it.

Figure 6 : Delay in circuit switched network

The setup delay is made up of four components: the propagation time of the source computer request (slope of the first grey box), the request signal transfer time (height of the first grey box), the propagation time of the destination computer acknowledgment (slope of the second grey box), and the signal transfer time of the acknowledgment (height of the second grey box). The propagation time (slope of the coloured box) and data transmission time (height of the coloured box) combine to create the data transfer delay, which can be rather significant. The time it takes to dismantle the circuit is shown in the third box. We've demonstrated the case where the receiver requests disconnection, which results in the longest delay.

Datagram Networks

  • We need to convey messages from one end system to another in data communications. If the message is to be sent via a packet-switched network, it must be broken into fixed or variable-size packets.

  • The network and the governing protocol decide the size of the packet. There is no resource allocation for a packet in packet switching. This means that no capacity is reserved on the lines, and no processing time for each packet is scheduled.

  • Demand determines how resources are allocated.

  • On a first-come, first-served basis, the allocation is made. If there are other packets being processed when a switch gets a packet, regardless of the source or destination, the packet must wait.

  • This lack of reserve, like other systems in our daily lives, may cause delays. If we do not have a reservation at a restaurant, for example, we may have to wait.

  • There are no resource reservations in a packet-switched network; resources are allocated on demand.

  • Each packet in a datagram network is treated independently of the others. Even if a packet is part of a multipacket transfer, the network treats it as if it were a standalone transmission.Datagrams are the name given to packets in this approach. Normally, datagram switching takes place at the network layer.

  • Figure 7 depicts how four packets are delivered from station A to station X using the datagram technique. In a datagram network, switches are commonly referred to as routers.

  • All four packets (or datagrams) in this example are part of the same message, although they may take different paths to their destination. This is because the links may be carrying packets from other sources and may not have enough bandwidth to deliver all of the packets from point A to point X.

  • The datagrams of a transmission may arrive at their destination out of sequence, with various delays between packets, if this strategy is used. Due to a shortage of resources, packets may potentially be lost or dropped.

  • Connectionless networks are a term used to describe datagram networks.

Figure 7: Datagram Packet Switching

Routing Table

How are packets routed to their destinations in a datagram network if there are no setup or teardown phases? Each switch (or packet switch) in this form of network has a routing table that is based on the destination address. The routing tables are dynamic and updated on a regular basis. The tables keep track of the destination addresses and the forwarding output ports that relate to them. This differs from the table of a circuit-switched network, where each item is added during the setup process and discarded after the breakdown phase. A switch's routing table is shown in Figure 8.

Virtual-Circuit Network

Fig 10 :A virtual-circuit network

A virtual-circuit network combines the advantages of both circuit-switched and datagram networks. It possesses some of the traits of both.

  1. In addition to the data transfer phase, there are setup and takedown phases as in a circuit-switched network.

  2. As in a circuit-switched network, resources are assigned during the setup phase, whereas in a datagram network, resources are allocated on demand.

  3. Data is packetized, much like in a datagram network, and each packet has an address in the header. The address in the header, on the other hand, has local jurisdiction (it specifies the next switch and the channel on which the packet is sent), not end-to-end jurisdiction.

  4. All packets follow the same path specified during the connection, just like in a circuit-switched network.

  5. A virtual-circuit network is typically implemented in the data link layer, whereas a circuit-switched network is typically implemented in the physical layer and a datagram network is typically implemented in the network layer. However, this could change in the future.

​​​​​​​A virtual-circuit network is depicted in Figure 10. Switches in the network allow traffic to flow from sources to destinations. A

There are two types of addressing in a virtual-circuit network: global and local (virtual-circuit identifier).

Global Addressing
A source or a destination must have a global address, which might be unique within the network's scope or globally if the network is part of an international network.

Virtual-Circuit Identifier
The virtual-circuit identifier is the identifier that is actually utilized for data transfer. In contrast to a global address, this is a tiny number with only switch scope; it is used by a frame between two switches. A frame has a VCI when it arrives at a switch and a different VCl when it leaves. The VCI in a data frame changes from one switch to the next, as seen in Figure 11.

Figure 11 : Virtual-circuit identifier

There are three stages to the process.
In a virtual-circuit network, as in a circuit-switched network, a source and destination must go through three phases: setup, data transfer, and teardown. The source and destination use their global addresses to assist switches in creating connection table entries during the setup phase. The source and destination notify the switches to delete the associated entry during the teardown phase.

Phase of Data Transfer
All switches must have a table entry for this virtual circuit in order to move a frame from a source to its destination. Figure 12 demonstrates how a frame from source A reaches destination B and how its VCI changes during the voyage. The VCI is changed and the frame is routed by each switch.

Figure 12 : Source-to-destination data transfer in a virtual-circuit network

Setup Phase 
A switch creates an entry for a virtual circuit during the setup process. Assume that source A requires the creation of a virtual circuit to B. The setup request and the acknowledgement are both essential.

Request for Setup 
The source sends a setup request frame to the destination. The procedure is depicted in Figure 14.

  1. Source A sends switch 1 a setup frame.

  2. The setup request frame is received by Switch 1.
    It understands that a frame from point A to point B is sent out over port 3. During the setup phase, the switch works as a packet switch and has a routing table separate from the switching table. The switch allocates the incoming port (1) and selects an available incoming VCI (14) and departing port (15). (3). It hasn't yet discovered the outgoing VCI, which will be discovered during the acknowledgment stage. The frame is subsequently forwarded to switch 2 through port 3 of the switch.


Figure 14 :Setup request in a virtual-circuit network

  1. The setup request frame is received by Switch 2. The same things happen here as they do there.
    1st swap; the table's three columns are now complete: In this scenario, the inbound port is used (l),
    Outgoing port (66) and incoming VCI (66) (2).

  2. The setup request frame is received by Switch 3. Three columns have been completed once more:
    outgoing port (2), incoming VCI (22) and incoming port (2) (3).

  3. If Destination B is ready to accept frames from A, it receives the setup frame.
    It assigns a VCI, in this case 77, to the incoming frames from A. This VCI informs the destination that the frames originate from A and not from any other source.

Phase of Teardown  
After transmitting all frames to B, source A sends a special frame known as a teardown request. Destination B responds with a confirmation frame for disassembly. The relevant entry in each switch's table is deleted.

As previously stated, in a virtual-circuit network, resource reservations can be made during setup or on demand during the data transfer phase. In the first scenario, each packet experiences the same delay; in the second situation, each packet may have various delays.

Delay in Virtual-Circuit Networks

Figure 16 :Delay in a virtual-circuit network

There is a one-time setup delay and a one-time teardown delay in a virtual-circuit network. Individual packets do not have to wait if resources are allocated during the setup phase. The delay for a packet going between two switches in a virtual-circuit network is shown in Figure 16.
The packet passes through two switches on its way to its destination (routers). Three transmission periods (3T), three propagation times (3't), data transfer shown by sloping lines, a setup delay (which includes transmission and propagation in both directions), and a takedown delay are all shown (which includes transmission and propagation in one direction).
In each switch, we ignore the processing time. The entire amount of time spent waiting is
3T+ 3't + setup delay + teardown delay = total delay

Data Communications

We share information when we communicate. This sharing can take place on a local or remote level. Local communication takes place face to face between persons, whereas remote communication takes place over a distance. Telecommunication refers to long-distance communication (tele is Greek for "far"). It encompasses telephony, telegraphy, and television.
The term "data" refers to information given in whatever format the parties originating and using the data have agreed upon.
The exchange of data between two devices across a transmission channel such as a wire cable is known as data communications. The communicating devices must be part of a communication system that consists of a combination of hardware (physical equipment) and software for data communications to take place (programs).

The delivery, accuracy, timeliness, and jitter properties of a data communications system determine its effectiveness.

  1. Data must be delivered to the proper location by the system. Only the specified device or user can get data.

  2. Perfection Data must be delivered accurately by the system. Data that has been tampered with and left incorrect during transmission is useless.

  3. Accuracy of information. Data must be delivered quickly via the system. Data that arrives late is of no use. In the case of video and audio, timely delivery refers to delivering data as soon as it is created, in the same sequence as it is created, and with minimal delay.Real-time transmission is the term for this type of distribution.

    1. Jitter
      The variance in packet arrival time is referred to as jitter. It is the delivery of audio or video packets with an uneven delay. Assume that video packets are transmitted every 3D milliseconds. If some packets come with a 3D-ms delay while others arrive with a 4D-ms delay, the video quality will be inconsistent.

    2. Components
      There are five parts to a data transmission system (see Figure 1).

    3. The Message
      The information (data) to be delivered is the message. Text, numbers, photos, audio, and video are all common types of information.

    4. sender
      The device that sends the data packet is known as the sender. It might be anything from a computer to a workstation to a phone handset to a video camera.

    5. Recipient
      The device that receives the message is referred to as the receiver. It might be anything from a computer to a workstation to a phone handset to a television.

    6. Medium of transmission
      A message's transmission medium is the physical path it takes to get from sender to receiver. Twisted-pair wire, coaxial cable, fiber-optic cable, and radio waves are examples of transmission media.

    7. Follow the protocol
      A protocol is a set of rules that control the transmission of data.