Circuit Switching vs. Packet Switching

[dhilipkumar]

Circuit Switching vs. Packet Switching

Some exam study guides would have you believe there is only one way to send data through a network:

packet switching. Fact is, there's at least one other way, circuit switching.While the majority of switched networks today get data across the network through packet switching, the concept of circuit switching should be no mystery to the average tech, let alone the tech wannabe. There are at least two good reasons to learn the difference. First of all, there is plenty of legacy hardware out there to support. Second, and perhaps more or at least very  important, it could well turn up on the test. If one question stands between you and passing, don't make this the one you miss.

In principle, circuit switching and packet switching both are used in high-capacity networks. In circuit-switched networks, network resources are static, set in "copper" if you will, from the sender to receiver before the start of the transfer, thus creating a "circuit". The resources remain dedicated to the circuit during the entire transfer and the entire message follows the same path. In packet-switched networks, the message is broken into packets, each of which can take a different route to the destination where the packets are recompiled into the original message.

All the above can be handled by a router or a switch but much of IT today is going toward flat switched networks. So when we're talking about circuit switching or packet switching, we are more and more talking about doing it on a switch.

Switched Networks

First, let's be sure we understand what we mean by a switched network. A switched network goes through a switch instead of a router. This actually is the way most networks are headed, toward flat switches on VLANs instead of routers. Still, it's not always easy to tell a router from a switch. It's commonly believed that the difference between a switched network and a routed network is simple binary opposition. T'ain't so.

A router operates at Layer 3 of the OSI Model and can create and connect several logical networks, including those of different network topologies, such as Ethernet and Token Ring. A router will provide multiple paths (compared to only one on a bridge) between segments and will map nodes on a segment and the connecting paths with a routing protocol and internal routing tables.

Being a Layer 3 device, the router uses the destination IP address to decide where a frame should go. If the destination IP address is on a segment directly connected to the router, then the router will forward the frame out the appropriate port to that segment. If not, the router will search its routing table for the correct destination, again, using that IP address.

Having talked about a router as being a Layer 3 device, think about what I'm about to say next as a general statement. I know there are exceptions, namely the Layer 3 switch. We're not going to get into that, not in this article.

A switch is very like a bridge in that is usually a layer 2 device that looks to MAC addresses to determine where data should be directed. A switch has other applications in common with a bridge. Like a bridge, a switch will use transparent and source-route methods to move data and Spanning Tree Protocol (STP) to avoid loops. However, switches are superior to bridges because they provide greater port density and they can be configured to make more intelligent decisions about where data goes.

The three most common switch methods are:

1. Cut-through - Streams data so that the first part of a packet exits the switch before the rest of the packet has finished entering the switch, typically within the first 12 bytes of an Ethernet frame.

2. Store-and-Forward - The entire frame is copied into the switch's memory buffer and it stays there while the switch processes the Cyclical Redundancy Check (CRC) to look for errors in the frame. If the frame contains no errors, it will be forwarded. If a frame contains an error, it will be dropped. Obviously, this method has higher latency than cut-through but there will be no fragments or bad frames taking up bandwidth.

3. Fragment-free Switching - Think of this as a hybrid of cut-through and store-and-forward. The switch reads only the first 64 bytes of the frame into buffer before forwarding it (think of a truck...

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[dhilipkumar]

Circuit Switching vs. Packet Switching

In Circuit Switching networks, when establishing a call a set of resources is allocated for this call. These resources are dedicated for this call, and can be used by any of the other calls. Circuit Switching is ideal when data must be transmitted quickly, must arrive in sequencing order and at a constant arrival rate. There for when transmitting real time data, such as audio and video, Circuit Switching networks will be used.

Packet switching main difference from Circuit Switching is that that the communication lines are not dedicated to passing messages from the source to the destination. In Packet Switching, different messages can use the same network resources within the same time period. Since network resources are not dedicated to a certain session the protocol avoid from waste of resources when no data is transmitted in the session. Packet Switching is more efficient and robust for data that is burst in its nature, and can withstand delays in transmission, such as e-mail messages, and Web pages.

Circuits as WAN connections battled successfully against packets for years because they guaranteed bandwidth, no matter what.

If you bought a T-1 circuit, the service provider nailed up 1.5Mbps from point A to point B, and that was your bandwidth come hell or high water.

Packets - first frame relay then IP - didn't do that. They shared the network capacity, and service providers typically oversold access lines, knowing that if all customers tried to use all the bandwidth they bought at the same time the network would be swamped. Everybody's traffic would suffer delays.

But frame relay service was relatively inexpensive compared with T-1s and it came in connections smaller than 1.5Mbps, usually in increments of 56Kbps. That meant sizing circuits to just handle demand, making business WANs more cost effective.

Plus frame relay services supported bursting - the instant delivery of more bandwidth than customers contracted for to handle moments of spikes in traffic. Because the networks were oversubscribed, sometimes the bursts were available, sometimes not. But over time, they were available enough of the time to make the service attractive.

Frame relay also offered virtual circuits - the logical carving up of access lines into lower bandwidth logical links tied to different sites. If a site needed connectivity to six others, it needed a single physical line subdivided into six virtual circuits. In the world of circuits, nothing was virtual. If one site in a corporate network needed to connect to six others, it needed six physical access lines.

As frame relay and later IP services matured, QoS was built into their standards so bandwidth could be guaranteed, just as it was in a circuit. That advance put packets over the top.
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[dhilipkumar]

Packet Switching vs. Circuit-Switching

Like frame relay, and unlike most local TCP/IP technologies, ATM is a circuit-based technology. That is, when a connection is established over ATM, a specific route, or virtual circuit, through the network is chosen, and that channel is maintained until the connection is closed (when the application ends).

This provides for efficiency in many cases, but also potentially wastes network resources, since bandwidth is allocated to a connection that may never use it.

Additionally, establishing the virtual circuit takes additional processing at connection-time, so short conversations may be slower over a circuit-switched network than a packet-switched network of the same speed.

In the packet-switched gigabit Ethernet TCP/IP world, data packets may travel through different paths to get from point A to point B during a conversation.