Switched networks

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You may recall that Ethernet networks are based on a protocol called Carrier Sense Multiple Access with Collision Detection, commonly known as CSMA/CD. This means that data packets from the various devices connected to the network are broadcast over a common network backbone. If two or more devices send a packet at the same time, a collision occurs and the packets are not able to reach their destination.

If the packets collide, then each device will wait a random period of time to resend. The device will continue to resend the packet until it reaches the destination. While Ethernet is an effective means to transfer data between devices, it is inefficient, particularly as a network grows. Consider that a moderately loaded 100Mb/s Ethernet network will only provide about 25Mb/s throughput, which will deteriorate as traffic increases. Switched networks reduce the potential collision of packets by establishing a virtual connection between two devices attached to the network.

How it works

Network switches replace the passive hubs that were common to older traditional Ethernet installations. Passive hubs are simply a signal splitter that distributes or repeats, packets to all ports simultaneously. Network switches examine each packet and establish a dedicated connection between the two devices, similar to that of a traditional telephone connection — when you dial a number, you are routed to a specific destination based on the telephone number dialed; once the person on the other end picks up the phone, a virtual connection is established.

In many ways switches operate similar to that of a bridge that is operating at OSI Layer Two, which is the layer that deals with network addressing. When a packet arrives at a port of a network switch, the packets are examined to determine their source and destination. The switch will handle the packet in one of three ways:

  1. If the packet is local to the segment (i.e. connected to the originating device before the switch), it will be filtered (or ignored) and not retransmitted through the switch.

  2. If the destination address of the packet can be identified within the switch's database of addresses, it will forward the data to the proper port.

  3. If the destination address of the packet is unknown, it will be transmitted on all ports.

Network switches are based on two possible architectures: store and forward and cut through. Switches based on the store-and-forward method analyze the entire packet prior to sending them to a destination. This process takes more time and, more importantly, requires a great deal of hardware memory to prevent a bottleneck of data. Switches based on the cut-through method only read the destination address before forwarding to the proper port, making it a far more efficient method. Improvements in technology have increased the throughput of the store and forward-based switches to nearly that of cut through.

Switch speed

Switches are designed to handle a huge amount of data throughput. To put this into perspective, consider a 10-port switch rated at 100Mb/s. Add the maximum speed of each port: 100Mb/s × 2 (duplex operation) × 10 (ports) = 2Gb/s. Under fully loaded conditions, the theoretical bus speed must be capable of operating at 2Gb/s minimum. In the real world, the use of each port would not exceed 50 percent, and therefore the bus speed can be reduced to about 1Gb/s.

Whereas a hub sends data packets to every node on a network, a switch sends the packet to just the intended recipient.

If the bus speed of the switch is less than that required to sustain traffic at a 50 percent usage rate, then the switch is considered to be blocking, which means that the switch may not be able to pass data under those conditions.

Some manufacturers combine the cut-through and store-and-forward technologies into a switch. These hybrid designs incorporate features of both, permitting a higher level of traffic management.

New network switches are available that operate at Layer Two and Three, also called Layer Three switches. Layer Three switches add the benefit of operating at the network layer, permitting traffic to be switched based on IP addresses. These switches also identify the flow of traffic and are capable of switching those flows at the hardware level.

All of the Networks articles have been approved by the SBE Certification Committee as suitable study material that may assist your preparation for the SBE CBNT exam.

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