Wireless LAN Fundamentals
I guess everyone knows about wireless now days but just in case here are some fundamentals of wireless.
Wireless communications is basically a means of transmitting information by using radio waves.
Regulations were put into place to organise the the use of the RF spectrum. In order to use the spectrum you would normally need a licence. Regulations were a concern because wireless communications were used for emergency services and the military. The private sector had not right to use any of the frequencies.
Basically an international agreement allowed certain parts of the spectrum to be used without a licence. Proprietary solutions moved to controlled bands basically paying a fee for the right to use the spectrum segment. The use of industrial specific and medical ( ISM )bands were introduced. ISM bands were free to use but with risk of interference form other networks.
The IEE in 1997 defined the first 802.11 standard which basically describes how a signal would be sent over the 2.4GHZ ISM band.
Most of the protocols in use today were defined between 1997 and 2003.
Current Standards with Rated bandwidth
Wireless Network and Topologies
Wireless networks use a variety of technologies depending on the distance, number of devices, amount of information. The different technologies will now be explored;
Wireless Personal-area network (WPAN )
The WPAN is exists in a relatively small area that connects devices such as desktop PC's, Printers, Scanners, fax machines, tablets, etc etc.
- WPAN will have a short range. 5 to 10 meters.
- Bluetooth is an example of a WPAN protocol.
- Links are usually Peer to Peer
- typically runs in the 2.4 GHZ.
- Unlicensed so no need to pay.
Wireless Local area Networks (WLAN)
This is the big topic that the rest of this blog will be working on so we will keep this to a minimum.
- WLAN provide a larger range the WPAN uses the 2.4 GHZ and 5 GHZ
- usual 802.11 a/b/g/n/ac
- requires more power that WPAN
- Multiple users
- Much more Flexible
- Requires proper Design to work efficiently.
Wireless Metropolitan Area Networks (WMAN)
WMAN extend to a large geographic area such as a city or a suburb. Its applications vary from a point-to-point or a point-to-multipoint link. WMAN will typically require licencing for the frequency to be used. Unlicensed ISM bands can be used but these will have interference issues. Examples of WMAN are WiMAX.
- Usually licensed
- Unlicensed bands can be used but interference issues will occur
- typically bandwidth is reduced the longer the distance
- Long distance used by telccos with licence to provide T1 or T3 speeds.
Wireless wide-area networks (WWAN)
WWAN provide connectivity over a wide geographical area. WWAN's are networks used for mobile phone and data services and are operated by carrier companies. WWAN will typically use licensed frequencies.
- large coverage areas
- issues with bandwidth and number of users
- cost based on usage and duration
- Data cost on the amount of information transmitted.
- until recently 115Kbs was normal
- Global Syatem for Mobile Communications GSM
- 3G
- 4G
WIRELESS TOPOLOGIES
Ad HOC Networks
- Basic Service Set (BSS)
- Independent Basic Service Set (IBSS)
- Limited number of devices.
- More devices cause collisions and Organization issues.
- normally referred to as Peer-to-Peer network
Two wireless devices in range with each other just need to share a common set of parameters in order to be able to communicate and establish a WLAN. This is basically all it takes to create a wireless network. The first station defines the radio parameters and the connection name. The other stations just need to detect the connection and adjust their parameters to connect to the first station. This type of network is called an AD Hoc network.
A Basic Service Set (BSS)
As soon as two service stations in the 802.11 standard connect to each other via wireless they form a Basic Service Set (BSS). Since these workstations do not rely on any other devices they form what is called a Independent Basic Service Set (IBSS)
Ad Hoc networks are limited in functionality and as soon as you you have a few users it slows down and becomes restrictive.
INFRASTRUCTURE MODE
Most wireless networks now have central device present which decides the common parameters (such as radio parameters, Priority, range, authentication) for all devices to connect on. This Central Device is call an Access Point (AP). The AP is pretty much like an Ethernet Hub when relaying communication to wireless devices.
- AP functions as a translation Bridge between the 802.3 wired media and 802.11 wireless.
- Wireless traffic is a half Duplex environment.
- The AP provides a BSS
- The area that the AP provides a service to is called a Basic Service Area (BSA)
The basic service area is also called a
Cell
s
Infrastructure mode is a little more complex than the Ad Hoc. The AP is basically dedicated for connecting end stations. It is far better equipped to handle clients as its job is to connect clients and forward traffic. All stations will normally share the same frequency and only one station can transmit data at any given time. For this reason the wireless network is a half-duplex network. There are a lot of comparisons to the old Ethernet hubs however the AP does perform more complex functions. the AP is normally responsible for directing traffic to the wired network.
The wired section of the AP is called the
Distribution System (DS). When the distribution system links 2 or more AP,s or cells then this group is called an
Extended Service Set (ESS).
The
ESS is the most common scenario in most wireless networks. The ESS allows wireless stations from two separate areas of the network to communicate and with proper design will allow the station to
Roam from one cell to the other seamlessly. Roaming is basically when a wireless station disconnects from on AP and reconnects to the second without loosing its connection to the network. For Seamless roaming the cells must overlap but the two cells must also be on different frequencies.
Service Set Identifiers
When a client roams from one AP to the another AP wireless administrators have to use names which identify the wireless connection. This name is called a service set Identifier ( SSID ). Each of the neighbouring AP's have to offer the same type of connection and the parameters to enable the roaming. Each AP must be must use the same SSID. So when the client roams he connects to the same SSID. The SSID can have as many as 32 ASCII characters. The term WLAN is often goes hand in hand with the SSID. The AP identifies itself by associating its radio MAC address to the SSID string. The AP will generate a different MAC address for each SSID. The name of the MAC address associated to an SSID is referred to as the Basic Service Set Identifier (BSSID).
So the BSSID identifies the BSS that is determined by the AP coverage. The administrator can create several groups on the same AP for example corporate SSID and Guest SSID. The criteria by which a station is allowed to connect to one SSID or the other will be different but the AP will be the same. This configuration wehre we have multiple SSID is called a Multiple Basic Service Set Identifiers MBSSID's
WORK GROUP BRIDGES
Sometimes you need to locate AP's in places where it is not possible to run Ethernet cables or because of the environment . In this kind of situation wireless is the best way to provide network connectivity by using a special device called a Workgroup Bridge WGB.
- A WGB provides wireless connection from an Ethernet Port
- several devices can be connected to a switch
- Traffic from these devices is sent via wireless over the WGB
All the users connect to the switch as well as the WGB. The WGB then transmits data from the end devices via wireless to the AP.
REPEATERS
Sometimes we may have a large area such as a warehouse where installing an Ethernet cable is impossible or because the distance may exceed 100 meters. In this situation the solution will be to install a repeater. A wireless repeater is simply an AP that is not connected to the wired network. It is connected to another AP instead. For this set-up to work properly the overlap between the two AP,s must be at least 50%.
