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Discusses wireless network topologies defined by the IEEE (Part II)

Exam: Cisco 640-721 - Implementing Cisco Unified Wireless Networking Essentials (IUWNE)

Let us first try to understand the topologies outlined by IEEE before proceeding further. The topologies defined by 802.11 include:

  • Ad hoc mode: This is the mode which is used for direct communication between two computers. This is a very basic type of network where there is no need for any central communications device but the two devices communicate using a network name known as SSID with the channel acting as a radio parameter. It is also known as BSS or the Basic Service Set which defines the area of interaction between the two devices. Since the devices are independent of any third device, it is also known as Independent BSS or IBSS for short. Whenever two devices are within connecting range and detect each other, the network is established. The radio in a computer is similar to an access point and works in half duplex mode where it can only send or receive at one point of time.
  • Infrastructure mode

    In this mode applicable to wireless networks an Access Point or AP acts like a hub or bridge where radio waves are used instead of cables (as in wired networks) and the AP acts as an equivalent of hub or bridge in wired networks.

    Let us look at the reasons for the same:

    • There is a single radio that cannot receive and send the signals at the same time. This is where the AP acts like a hub and performs operations that are half duplex.
    • AP's are considered to have intelligence like that of a bridge. The AP maintains a MAC address forwarding database.

    Although there is a difference between an AP and a bridge in that wireless frames are quite complex compared to their standard Ethernet counterparts which have only source and destination MAC addresses, while the wireless frames have multiple MAC addresses.

    Wireless frames can travel over multiple hops, it includes a receiver address which is also the MAC address of the station to which the frame is being sent to. While using a network that uses a repeater, mesh or work bridge a transmitter address has to be used which represents the address of the device that is being used for forwarding traffic.

    An Access Point is just another type of a wireless station and though all radio devices on a wireless network are called stations, only the Access Point functions as an infrastructure device.

    Let us take a look at some of the typical terms associated with wireless networking. A wireless cell or BSA or Basic Service Area refers to area covered by the Access Point. Depending on the function of the AP, the AP has an Ethernet connection to an 802.3 LAN.

An Access Points acts to bridge the wireless and Ethernet (wired) portions of the networking namely the 802.11 and 802.3 devices respectively. The latter is connected to the Ethernet port of the Access Point and acts as a path to the wireless LAN controller.

Before being forwarded to the wired network which is known as distribution system, the traffic passes through the client. A client uses the distribution system for accessing the internet, printers and file servers and everything else that is available on a wired network.

There can be different reasons for wanting to connect more than one AP's to the LAN. It could be motivated by the desire to have better coverage, swift movement from one AP to another while staying on the same LAN and provide saturation of AP's.

The process of shifting from one AP to another is known as roaming. The AP cell must overlap for roaming to be successful. They are required to overlap, so that a client can view both the AP's and as the RSSI from the AP being used diminishes, the RSSI from the other AP increases. When this happens the client makes an attempt to associate itself with the AP with a stronger signal.

Service Set Identifiers

The moment one tries to connect a system to a wireless network, names of available networks appear on the screen. The network is identified through the SSID or Service Set Identifier which is a combination of the MAC address and the network name. SSID can be defined in various ways as listed below.

A Basic SSID means that AP which only offers its services to a unique network.

A Multiple BSSID refers to the situation where more than networks are available on the AP and is said to be a virtual AP. It uses same hardware for multiple networks. Situations of collision can occur in this case due to the presence of common hardware and frequency range.

It can be understood from the above discussion that in case the facility of roaming has to exist, the different wireless cells must overlap and the Access Points must have the common SSID. In roaming, the SSID remaining same changes occur in the MAC address. The MAC address is will always change to the AP which has better signal.