What is an Ethernet Switch & How It Works

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Ethernet switches are the key element in most data networks from local data links to local area networks, and even wide area networks. They are used from home environments, right up to commercial networks and within the Internet.

Although networks switches could use any one of a variety of communications standards, Ethernet is by far the most widely used.

An Ethernet switch can be small, or very large - major networks are known to have failed when a network switch has gone down. This shows the key nature of network switches and in this case Ethernet switches within networks.

Switches are also used for industrial control, where robustness and carrier grade performance are required - naturally these industrial switches have a price to reflect this and are not normally used where standard switches can suffice.

What is an Ethernet switch

Network switches and in this case, Ethernet switches are key building blocks for any network. These switches connect multiple devices together when they are on the same network. The nodes on the network will include devices such as: computers, servers; wireless access points, printers, and and so forth.

The Ethernet switch enables the connected devices on the network to exchange data and talk to one another other.

Network switches are semi-intelligent devices that enable the data to be routed in the required direction so that when a packet of data is sent from the source to the destination it is routed correctly.

An Ethernet network switch is able to work with the MAC addresses of the devices connected to it. Using this information it is able to identify the computers or other units on each of its ports. In this way it is able to send the data packets to the relevant ports and hence to the right devices without flooding the network with unnecessary data.

Additionally an Ethernet switch is able to allocate the full bandwidth to each of its ports. This means that regardless of the number of devices operating, users will always have access to the maximum amount of bandwidth.

Managed & unmanaged Ethernet switches

When looking at the descriptions of Ethernet switches, it will be seen that some of them are referred to as unmanaged switches, whereas others are referred to as being managed switches. When selecting an Ethernet switch it is important to select the required type.

• Unmanaged switches:   An unmanaged network switch is the most common form of Ethernet switch. This type of Ethernet switch is design to be simply plugged into the network and then operate without any manual configuration. Unmanaged switches are typically for basic connectivity and they are often used in home or small office networks or wherever a few more Ethernet ports are needed, at a desk, in a lab, in a conference room, etc.

• Managed switches:   Managed switches give greater security with more features and flexibility because they can be configured to custom-fit the network. With this greater control, it is possible to better protect the network and improve the quality of service for those who access the network. The traffic can be prioritised so that the available bandwidth, etc is allocated to a given application, etc in the best way.

There are also a type of switch referred to as an industrial switch, and reference to them is sometimes seen in various forms of literature.

These industrial switches are typically used for industrial control, and similar applications. They are called industrial switches because they are used within industrial environments requiring a high level of robustness and tolerance of wide temperature ranges etc,.

In addition to this industrial switches feature carrier grade Ethernet performance because industrial and production environments need to have very high reliability in view of the costs of any disruption.

How Ethernet switches work - the basics

Ethernet switches are used to link various Ethernet devices together, relaying the data from one device on the data network such as a local area network to another. These devices may even be other Ethernet switches, or devices such as computers, servers, printers, etc.

When moving data around an Ethernet based network, data frames called Ethernet frames are used.

The switch uses these frames and relays them between the devices that are connected to them.

By moving Ethernet frames between the switch ports in this way, a switch is able to link the traffic carried by the individual network connections into a larger Ethernet network such as a local area network, etc.

Ethernet switches work by "bridging" Ethernet frames between different segments of a local area network.

The switch performs its function by copying the frames from one switch port to another according to the Media Access Control, MAC addresses int he frames.

It is worth noting that the concept of Ethernet bridging was defined in IEEE 802.1D, the IEEE standard for Metropolitan and Local Area Networks: Media Access Control Bridges.

The fact that there is a universally adopted standard for this operation means that Ethernet switches bought from different manufacturers can all interface together and work in any Ethernet based local area network, or even larger networks like Metropolitan Area Networks or Wide Area Networks.

The Ethernet frames are used to transport the data over the Ethernet cables, and lines between the nodes on the network. Essentially frames have a defined format and they are sequences of information, containing the data to be transported, but with additional data or information around it to manage the sending and receiving of the data successfully.

The format of the Ethernet frame includes a number of different elements as seen in the diagram:

• Header:   This consists of a preamble to enable the receiver to synchronise, a destination address, a source address and a Length / type indication.
• Payload:   This is the actual data to be carried around the local area network by the Ethernet frame.
• Trailer:   This consists of a Frame Check Sequence for error checking.

The basic format for these Ethernet frames has remained the same over the many versions of the Ethernet standard. With Ethernet data travelling over many local area networks and WAN networks, consisting of equipment utilising many different versions of the standard: 100Mbps Fast Ethernet, Gigabit Ethernet 1GE, 10 GE, etc, the frames provide an essential and constant format for this data, allowing backward compatibility.

Ethernet switch operation

One of the key aspects of Ethernet switches is that their operation should appear transparent to the devices with which they connect. Accordingly this type of operation is called "transparent bridging."

For transparent bridging, the Ethernet frames remain the same as they pass through the switch. As a result, the switch will automatically start to work without needing any configuration on the switch or the various nodes that are connected to the Ethernet network, and this makes the operation of the switch transparent to them all.

One of the key elements of an Ethernet switch is the address learning which it does. The device does not forward all frames onto all its ports - if it did this then the network would soon become overloaded. Instead the switch forwards the relevant frames to the relevant ports.

The switch achieves this by using the traffic forwarding rules defined in he IEEE 802.1D bridging standard.

Under this standard, the traffic forwarding is based around the switch learning where to send the data. The switch makes the decisions about where to forward the data based upon the 48 bit Media Access Control address contained within the Ethernet frame.

In order to route the data in the right direction, the switch learns which devices (called stations in the standard) are on which segments of the network. It does this by looking at the source addresses in the Ethernet frames it receives.

All Ethernet interfaces have a unique MAC address which is assigned during manufacture. Normally an Ethernet device will only accept frames that are directed to it, but in the case of an Ethernet switch, this is not the case because it handles frames that are intended for other devices - it runs in what is often termed "promiscuous mode."

As each frame is received on a given port, the software within the Ethernet switch looks at the source address contained within the frame and adds this to its table of addresses that it maintains within the memory of the switch. In this way it remembers which devices are attached to it, so that it can correctly route frames to the required destination.

The list is effectively a database and this enables the switch to make a packet forwarding decision for each frame that is received in a process called adaptive filtering.

This means that only the Ethernet frames intended for a particular device are sent along that path, and in this way the level of data being carried around the local area network is minimised.

Ethernet switch traffic management aspects

There are several aspects to the way in which an Ethernet switch handles the traffic. There are several features and capabilities that are key.

• Traffic filtering & forwarding:   With the addresses of the different stations or devices connected to the different ports of the Ethernet switch, it is able to forward the various frames it receives to the required destinations.

  As devices connected to the switch may not be ready to receive frames, or the switch itself may be busy, the different ports have the ability to hold frames until the system is ready fo them to be sent.

  Also, and frames received are forwarded on without any change, making the switch totally transparent to the stations on the local area network, or WAN network or whatever form of network it is.

• Frame Flooding:   As will sometimes happen, the Ethernet switch will receive a frame that is destined for a station or device that is new. When this happens the switch sends out a frame to all the ports, other than the one on which it received the frame with the unknown destination to discover the station which has the required address.

Ethernet switches are very commonplace items of IT equipment. There is a wide range of prices and capabilities ranging from those used for small home based local area networks, to very much larger ones used within large offices, businesses and large Metropolitan Area Networks and Wide Area Networks, and also to industrial switches used for industrial control.

Costs also vary widely dependent upon the capabilities, size and many other factors. However the choice available means that the right Ethernet switch can be bought for the job required.

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