14.2 Circuit/Packet Switching

Files and Resources

Specification

show understanding of circuit switching
- Benefits, drawbacks and where it is applicable
show understanding of packet switching
- Benefits, drawbacks and where it is applicable
- show understanding of the function of a router in packet switching
- explain how packet switching is used to pass messages across a network, including the Internet

Circuit Switching Vs Packet Switching (Explanation 1)

This extract was sourced from the TCPIP Guide

There are many methods for exchanging information between networked devices. There are also a number of ways of categorising and describing these methods and the types of networks that use them.

One fundamental way of differentiating networking technologies is on the basis of the method they use to determine the path between devices over which information will flow. In highly simplified terms, there are two approaches: either a path can be set up between the devices in advance, or the data can be sent as individual data elements over a variable path.

Circuit Switching

In this networking method, a connection called a circuit is set up between two devices, which is used for the whole communication. Information about the nature of the circuit is maintained by the network. The circuit may either be a fixed one that is always present, or it may be a circuit that is created on an as-needed basis. Even if many potential paths through intermediate devices may exist between the two devices communicating, only one will be used for any given dialog.This is illustrated in Figure 1.

Figure 1: Circuit Switching

In a circuit-switched network, before communication can occur between two devices, a circuit is established between them. This is shown as a thick blue line for the conduit of data from Device A to Device B, and a matching purple line from B back to A. Once set up, all communication between these devices takes place over this circuit, even though there are other possible ways that data could conceivably be passed over the network of devices between them. Contrast this diagram to Fig 2.

The classic example of a circuit-switched network is the telephone system. When you call someone and they answer, you establish a circuit connection and can pass data between you, in a steady stream if desired. That circuit functions the same way regardless of how many intermediate devices are used to carry your voice. You use it for as long as you need it, and then terminate the circuit. The next time you call, you get a new circuit, which may (probably will) use different hardware than the first circuit did, depending on what's available at that time in the network.

Data is passed between the connections in the order sent. Packets do not need to be re-sequenced as they pass in a linear, sequential manner.

Packet Switching

In this network type, no specific path is used for data transfer. Instead, the data is chopped up into small pieces called packets and sent over the network. The packets can be routed, combined or fragmented, as required to get them to their eventual destination. On the receiving end, the process is reversed—the data is read from the packets and re-assembled into the form of the original data. A packet-switched network is more analogous to the postal system than it is to the telephone system (though the comparison isn't perfect.) An example is shown in Figure 2.

Figure 2: Packet Switching

In a packet-switched network, no circuit is set up prior to sending data between devices. Blocks of data, even from the same file or communication, may take any number of paths as it journeys from one device to another. Compare this to Figure 1

Key Concept: One way that networking technologies are categorised is based on the path used to carry data between devices. In circuit switching, a circuit is first established and then used to carry all data between devices. In packet switching no fixed path is created between devices that communicate; it is broken into packets, each of which may take a separate path from sender to recipient.

Use of a router in packet switching

A router is critical in enabling packet switching. It is a networking device that forwards data packets between computer networks. Routers perform the traffic directing functions on the Internet. Data sent through the internet, such as a web page or email, is in the form of data packets. A packet is typically forwarded from one router to another router through the networks that constitute an internetwork until it reaches its destination node.

A router is connected to two or more data lines from different networks. When a data packet comes in on one of the lines, the router reads the network address information in the packet to determine the ultimate destination. Then, using information in its routing table or routing policy, it directs the packet to the next network on its journey.

The most familiar type of routers are home and small office routers that simply forward IP packets between the home computers and the Internet. An example of a router would be the owner's cable or DSL router, which connects to the Internet through an Internet service provider (ISP). More sophisticated routers, such as enterprise routers, connect large business or ISP networks up to the powerful core routers that forward data at high speed along the optical fiber lines of the Internet backbone.

In computer networking a routing table, is a data table stored in a router or a networked computer that lists the routes to particular network destinations, and in some cases, metrics (distances) associated with those routes. The routing table contains information about:

the topology of the network immediately around it.
The Network ID (it is connected to) IP and subnet masks
Routing metric
Interface (information about each directly connected network)

The construction of routing tables is the primary goal of routing protocols, such as RIP. Static routes are entries made in a routing table by non-automatic means and which are fixed rather than being the result of some network topology "discovery" procedure.

Each interface of the router (the individual physical connections made to it) connects to an IP network, the router looks the best match for the destination IP address in its routing table. After finding out the best match, it forwards the packet through that interface to that network. The same procedure is repeated until the packet reaches its destination.

Comparing Circuit Switching and Packet Switching

A common temptation when considering alternatives such as these is to ask which is “better”—and as usually is the case, the answer is “neither”. There are places where one is more suited than the other, but if one were clearly superior, both methods wouldn't be used.

One important issue in selecting a switching method is whether the network medium is shared or dedicated. Your phone line can be used for establishing a circuit because you are the only one who can use it—assuming you can keep that pesky sister/ brother/ father/ mother off the phone.

However, this doesn't work well in LANs, which typically use a single shared medium. If two devices were to establish a connection, they would “lock out” all the other devices for a long period of time. It makes more sense to chop the data into small pieces and send them one at a time. Then, if two other devices want to communicate, their packets can be interspersed and everyone can share the network.

The ability to have many devices communicate simultaneously without dedicated data paths is one reason why packet switching is becoming predominant today. However, there are some disadvantages of packet switching compared to circuit switching. One is that since all data does not take the same, predictable path between devices, it is possible that some pieces of data may get lost in transit, or show up in the incorrect order. In some situations this does not matter, while in others it is very important indeed. For example, browsing the web is not time critical; however, a conference call would fall apart if the video kept stopping to re-sequence packets or though packet delay.

While the theoretical difference between circuit and packet switching is pretty clear-cut, understanding how they are used is a bit more complicated. One of the major issues is that in modern networks, they are often combined. For example, suppose you connect to the Internet using a dial-up modem. You will be using IP datagrams (packets) to carry higher-layer data, but it will be over the circuit-switched telephone network. Yet the data may be sent over the telephone system in digital packetized form. So in some ways, both circuit switching and packet switching are being used concurrently.

Another issue is the relationship between circuit and packet switching, and whether a technology is connection-oriented or connectionless. The two concepts are related but not the same; the section next discusses this in much more detail.

Note: Note that the word “packet” is only one of several terms that are used to refer to messages that are sent over a network. Other terms you will encounter include frame, datagram, cell and segment.

Connection-Oriented and Connectionless Protocols

Previously, the way networking technologies based on whether or not they use a dedicated path, or circuit, over which to send data were detailed. Another way in which technologies and protocols are differentiated has to do with whether or not they use connections between devices. This issue is closely related to the matter of packet versus circuit switching.

Division of Protocols into Connection-Related Categories

Protocols are divided into two categories based on their use of connections:

- Connection-Oriented Protocols: These protocols require that a logical connection be established between two devices before transferring data. This is generally accomplished by following a specific set of rules that specify how a connection should be initiated, negotiated, managed and eventually terminated. Usually one device begins by sending a request to open a connection, and the other responds. They pass control information to determine if and how the connection should be set up. If this is successful, data is sent between the devices. When they are finished, the connection is broken.

Connectionless Protocols: These protocols do not establish a connection between devices. As soon as a device has data to send to another, it just sends it.

Key Concept: A connection-oriented protocol is one where a logical connection is first established between devices prior to data being sent. In a connectionless protocol, data is just sent without a connection being created. Circuit-switched networking technologies are inherently connection-oriented, but not all connection-oriented technologies use circuit switching.

Connection-Oriented and Connectionless Protocols in TCP/IP

Looking again at TCP/IP, it has two main protocols that operate at the transport layer of the OSI Reference Model. One is the Transmission Control Protocol (TCP), which is connection-oriented; the other, the User Datagram Protocol (UDP), is connectionless. TCP is used for applications that require the establishment of connections (as well as TCP’s other service features), such as FTP; it works using a set of rules, as described earlier, by which a logical connection is negotiated prior to sending data. UDP is used by other applications that don't need connections or other features, but do need the faster performance that UDP can offer by not needing to make such connections before sending data.

The principle of layering also means that there are other ways that connection-oriented and connectionless protocols can be combined at different levels of an internetwork. Just as a connection-oriented protocol can be implemented over an inherently connectionless protocol, the reverse is also true: a connectionless protocol can be implemented over a connection-oriented protocol at a lower level.

Cost

Packet switching is easier and more affordable than circuit switching. Since all the bandwidth can be used at once, packet switching is more efficient because it doesn't have to deal with a limited number of connections that may not be using all that bandwidth

Packet Switching Vs Circuit Switching - Explanation 2

This is an alternative explanation, taken from the Comparitech website

Understanding how devices connect to each other is one of the most important elements of networking. The more you know about how your network is tied together the better you are able to respond to performance issues and run in-depth troubleshooting.

Circuit Switching and Packet Switching are two of the main models used to facilitate connections within enterprise networks. In this article, we’re going to look at circuit switching vs packet switching to see how these two models differ from each other.

What is Circuit Switching?

Circuit Switching Diagram

Circuit switching is when a dedicated channel or circuit needs to be established before users can speak to each other on a call. A channel used in circuit switching is kept reserved at all times and is used once the two users communicate.

Circuit switching connections are classified as half-duplex or full-duplex. Half-duplex communications allocate one channel and full-duplex communications allocate two channels.

Circuit switching is most commonly used to sustain telephone systems so that whenever the phone is picked up the conversation can begin. Circuit switching is considered to be distinct from packet switching because it provides a physical path between the source and destination. Packet switching offers no such physical path for data packets that travel independently through a range of dynamic routes.

What is Packet Switching?

Packet Switching Diagram

Packet switching is different from circuit switching because there is no requirement to establish a channel. The channel is available to users throughout the data network. Long messages are broken down into packets and sent individually to the network.

These data packets are handled by datagram or virtual circuit. Datagram sends each packet of data individually and can travel any route. The problem with this method is that packets can arrive out of sequence or even be lost in transit altogether.

On the other hand, a virtual circuit establishes a predefined route before the data packets are transmitted. As a consequence routing decisions don’t need to be made for the packet path as they are with a datagram. Every packet contains a virtual circuit identifier (VCI) so that the packets can reach their destination address. Call requests and accept messages are used to identify the route before packets are in transit. Packet switching is used to sustain data and voice applications that don’t require real-time service.

Circuit Switching vs Packet Switching Comparison

Feature Circuit Switching Packet Switching

Dedicated Path Yes No

Path Formation Path dedicated for one conversation Route is established on a per packet basis of the conversation using datagram

Delay Call setup delay Packet transmission delay (call setup delay for virtual circuit)

Bandwidth Type Fixed Bandwidth Dynamic bandwidth

Overload Effects Stops call establishment Increases packet delay

The Advantages of Circuit Switching

The circuit switching model has become popular for many different reasons. One of the main reasons is that it decreases the amount of delay the user experiences before and during a call. Circuit switching is adept at making sure that delay is minimized during phone calls so that the callers can enjoy the best end-user experience possible. Packet switching is unable to sustain the same standard of service to users throughout the call.

Under circuit switching the call will be provided with consistent bandwidth, channels, and an ongoing data rate. As a consequence, the user is able to stay on the phone for longer periods of connection time without running into any performance issues. Circuit switching also has the advantage of making sure that data packets are delivered in their correct sequence. The lower level of delay makes sure that data packets are delivered in order so that all the content in the call can be understood.

The Disadvantages of Circuit Switching

Though circuit switching is ideal for voice communications it isn’t right for any other type of connections. The reason for this is that the channel is reserved for future conversations. As a consequence, circuit switching isn’t a good choice if you’re looking to use your resources efficiently. Dedicating one channel to a single service leaves it unavailable to other services. This disadvantage also doubles up to make circuit switching more expensive. It is expensive to provision an entire channel to one service and one individual routing path.

However, even for calls circuit switching still poses challenges. If active users exceed the number of channels you have available then calls will fail to take place or will be dropped in the middle. Administrators are under pressure to make sure that channels keep up. In addition, even setting up calls results in delay so the connection needs to be fully established before the conversation can take place.

The Advantages of Packet Switching

While packet switching may not be as suited to voice calls as circuit switching, it has a number of advantages that are hard to ignore. The main advantage that packet switching has over circuit switching is its efficiency. Packets can find their own data paths to their destination address without the need for a dedicated channel. In contrast, in-circuit switching network devices can’t use the channel until the voice communication has been terminated.

Packet switching is also reliable because it helps to eliminate packet loss. With packet switching, data packets can be resent if they don’t reach their destination. This isn’t the case for circuit switching which doesn’t have the means to send lost packets. As a result, packet switching is the more reliable method of the two because it ensures that packets reach their destination.

Packet switching also reduces the costs associated with running the network. Packet switching networks can transfer general network traffic and voice traffic across the network without the need for a dedicated channel. This saves you money because you don’t need to pay to have one channel available for voice communications.

The Disadvantages of Packet Switching

The biggest limitation of packet switching is that it is unsuitable for applications that require minimal latency. In a network that uses lots of voice calls circuit switching is a necessity because it is the only setup that delivers a high-quality end call. Packet switching can only provide a voice call experience that results in choppy audio that makes it difficult for the users to understand each other.

Similarly, though packet switching is able to resend lost data packets, this isn’t the case if the network becomes overwhelmed by traffic. If there is too much traffic then packets will be dropped in transit. The end result is the loss of important data. This risk is further increased by the lack of security protocols used to protect packets during data transmission. There is no IPsec to give packets that extra barrier of security against damage. Though packet switching reduces costs in a number of ways it is significantly expensive to implement. Packet switching relies on a range of complex protocols that must be managed from deployment onward.

Circuit Switching vs Packet Switching

Circuit switching and packet switching are undeniably two of the most widely-used techniques for transferring data across enterprise networks. Both of these two techniques have their own space within modern networking. Using circuit switching allows you to keep a channel established for high priority voice calls to give the users the best chance to communicate with each other. In contrast, packet switching uses a more malleable approach so that traffic can travel a variety of paths.

Which one is better depends on what you’re trying to achieve. If you want to ensure that voice calls are adequately supported so that delay is minimized then you’ll want to deploy circuit switching. However, if you need to sustain lots of services at once then you’ll want to use packet switching.

Circuit Switching vs Packet Switching FAQs

What is time-division switching?

Time-division switching is a method to allow several connections to travel along the same trunk line. It involves time-division multiplexing (TDM) which splits up a stream into segments and sends them out on the line at a specific interval. The receiving de-multiplexer uses a synchronized clock to detect the elements of each stream.

Which switching method reduces traffic congestion?

Congestion is a symptom of an overloaded network. Packet switching is more efficient than circuit switching because it ensures that more of the bandwidth of all cables are fully utilized. As it makes better use of resources, packet switching is more likely to reduce congestion than circuit switching.

What is a frame relay circuit?

A frame relay is a connection-oriented virtual circuit service. It is like a temporary rental of a line between two networks. Several conversations between different endpoints can be carried through the same connection over a connecting public line.

What is ISDN?

ISDN stands for Integrated Service Digital Network. It is a method of sending voice and data simultaneously over the circuit-switched public telephone system. Several devices can be connected to the same line and each sends voice or data at the same time over a number of “bearer channels” (“B” channels). The line also has a data channel (“D” channel), which carries connection administration information.

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