Saturday, October 11, 2014

Learning Wi-Fi P4

Wi-Fi Quality of Service (QoS)

There are plans to incorporate quality of service (QoS) capabilities in Wi-Fi with the adoption of the IEEE 802.11e standard. The 802.11e standard will include two operating modes, either of which can be used to improve service for voice:
  • Wi-Fi Multimedia Extensions (WME)- Mandatory.
  • Wi-Fi Scheduled Multimedia (WSM)- Optional.

Wi-Fi Multimedia Extensions (WME):

This uses a protocol called Enhanced Multimedia Distributed Control Access (EDCA), which is Extensions an enhanced version of the Distributed Control Function (DCF) defined in the original 802.11 MAC.
The enhanced part is that EDCA will define eight levels of access priority to the shared wireless channel. Like the original DCF, the EDCA access is a contention-based protocol that employs a set of waiting intervals and back-off timers designed to avoid collisions. However, with DCF all stations use the same values and hence have the same priority for transmitting on the channel.
With EDCA, each of the different access priorities is assigned a different range of waiting intervals and back-off counters. Transmissions with higher access priority are assigned shorter intervals. The standard also includes a packet-bursting mode that allows an access point or a mobile station to reserve the channel and send 3- to 5-packets in sequence.

Wi-Fi Scheduled Multimedia (WSM):

True consistent delay services can be provided with the optional Wi-Fi Scheduled Multimedia (WSM). WSM operates like the little used Point Control Function (PCF) defined with the original 802.11 MAC.
In WSM, the access point periodically broadcasts a control message that forces all stations to treat the channel as busy and not attempt to transmit. During that period, the access point polls each station that is defined for time sensitive service.
To use the WSM option, devices must first send a traffic profile describing bandwidth, latency, and jitter requirements. If the access point does not have sufficient resources to meet the traffic profile, it will return a busy signal.

Wi-Fi Security

Security has been one of the major deficiencies in Wi-Fi, though better encryption systems are now becoming available. Encryption is optional in Wi-Fi, and three different techniques have been defined. These techniques are given here:

Wired Equivalent Privacy (WEP):

An RC4-based 40-or 104-bit encryption with a static key.

Wi-Fi Protected Access (WPA):

This is a new standard from the Wi-Fi Alliance that uses the 40 or 104-bit WEP key, but it changes the key on each packet. That changing key functionality is called the Temporal Key Integrity Protocol (TKIP).

IEEE 802.11i/WPA2:

The IEEE is finalized the 802.11i standard, which is based on a far more robust encryption technique called the Advanced Encryption Standard. The Wi-Fi Alliance designate products that comply with the 802.11i standard as WPA2.
However, implementing 802.11i requires a hardware upgrade.

Wi-Fi Network Services

The picture has become somewhat confused as service providers started using Wi-Fi to deliver services for which it was not originally designed. The two major examples of this are wireless ISPs and city-wide Wi-Fi mesh networks.

Wireless ISPs (WISPs):

One business that grew out of Wi-Fi was the Wireless ISP (WISP). This is the idea of selling an Internet access service using wireless LAN technology and a shared Internet connection in a public location designated a hot spot.
From a technical standpoint, access to the service is limited based on the transmission range of the WLAN technology. You have to be in the hot spot (i.e. within 100m of the access point) to use it. From a business standpoint, users either subscribe to a particular carrier's service for a monthly fee or access the service on a demand basis at a fee per hour. While the monthly fee basis is most cost effective, there are few intercarrier access arrangements so you have to be in a hot spot operated by your carrier in order to access your service.

City-Wide Mesh Networks:

To address the limited range, vendors like Mesh Networks and Tropos Networks have developed mesh network capabilities using Wi-Fi's radio technology.
The idea of a radio mesh network is that messages can be relayed through a number of access points to a central network control station. These networks can typically support mobility as connections are handed off from access point to access point as the mobile station moves.
Some municipalities are using Wi-Fi mesh networks to support public safety applications (i.e. terminals in police cruisers) and to provide Internet access to the community (i.e. the city-wide hot spot).

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