SSID

Introduction

Used by WiFi networking equipment, Service Set IDentifiers are unique names designed to help devices identify the correct network to connect to. In scenarios where more than one network is present in a given area, this allows devices to ensure that the data being transmitted and received is directed to the appropriate destination.
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Selecting an SSID

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Constraints

An SSID can consist of up to 32 alphanumeric characters and can contain letters, numbers or any common symbols (eg. !@#$%). The 802.11 specs allow any ASCII characters to be used in the SSID, although some networking hardware may have additional restrictions. It is important to note that Unicode characters are not currently supported, so non-roman alphabets may have issues when used in this field.
SSIDs are sensitive to both case (ie 'Network' is different from 'network') and whitespace characters (ie 'Network' is different from ' Network'). As such, for proper operation of the network users must be careful to enter this value in exactly the same manner on all devices. Any mistakes, regardless of how small, will result in the device failing to connect to the network.
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Defaults

It is generally a very good idea for users to avoid using the default SSID on any wireless networking devices that are in use. These SSIDs are very common, so if another user sets up another wireless network from the same vendor it is possible for devices to connect to the wrong network. This process is usually relatively simple, although the precise process depends on the specific hardware being used.
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Privacy Considerations

Note that the SSID is transmitted in an unencrypted form in the header of each and every packet transmitted over the network. Additionally, if SSID broadcast is enabled this name will be sent as part of a periodic beacon signal even when the network is not in use. As such, this value should not contain any information that users do not want to share with others (eg it should not be the same as any passwords).
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ESSIDs and Roaming

For larger wireless network installations, the same SSID can be configured on a group of access points that are connected to the same network. When configured in this way, WiFi devices consider the network as a single entity and will automatically move between the access points as required. This allows users to move around a large area without any interuption in their network connection - the roaming process operating transparently to the user.
For this system to work, all access points must share a single SSID and must be connected to the same wired network. It is also important to note that not all residential WiFi equipment has full support for this type of operation.
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SSID Broadcast

The basic 802.11 standard specifies that access points should periodically transmit a beacon packet to announce their presence. This helps devices to rapidly survey the available networking resources and select the network they would like to connect to. It also assists in the roaming process, as it allows devices to connect to the nearest access point at any given time.
Some newer networking devices allow users to disable this process in an attempt to improve security. As the AP is no longer broadcasting it's presence, the concept is that malicious users may not detect the network and thus keep the data stored on the network safe. This protection offered by this process is extremely limited, however, as the SSID is also transmitted with each data packet sent over the network.
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See Also

• WiFi - The set of wireless networking standards that the SSID is a part of.