The book outlines here a number of common elements that need to be set on the AP, the radio card, or both.
Service Set Identifier (SSID)
- An alphameric value on APs and radio cards that distinguish a wireless LAN. It is the wireless LAN’s name.
- All APs intended to participate in the same wireless LAN should have the same SSID.
- You can set SSID so that it is not broadcast in beacon frame.
- This means that it will not appear in a radio cards scan list, thus requiring you to configure it manually.
- This is a moderate security benefit.
- A hacker who’s snooping wireless packets can determine the SSID from a captured association frame.
- Depending on the vendor, it’s possible to have several SSIDs broadcast by an AP, each one associated to a different VLAN.
- In ad-hoc mode, the first radio card includes an SSID in the beacon frame. Each ad-hoc radio card needs to have the same SSID to associate to the ad-hoc wireless LAN.
- The various 802.11 standards define various radio frequencies to be used.
- 802.11b/g defines 14 overlapping RF channels in the 2.4Ghz range, of which 1 – 11 can be used in the United States.
- Because b and g channels overlap, multiple wireless LANs in the same general area can co-exist best when set to channesl 1, 6 and 11, minimizing the effects of the overlapping frequencies during high-traffic times.
- 802.11a defines channels that do not overlap.
- Before installing a wireless LAN, a best practice is to perform an RF site survey..
- Some APs offer an automatic channel selection, where the AP listens to each RF channel and determines the best one to use for minimal interference.
- In an ad-hoc network, the first radio card pops up, with the radio channel configured by the user.
- The highest transmit power available is usually 100mW (one-tenth of a watt), and is usually the default setting.
- Setting power lower allows you to reduce the size of an AP’s radio cell, effectively reducing the number of radio cards that can associate to the AP. While you’ll need more APs to cover an area, you’ll have fewer cards per AP, improving throughput for each user. This design may come into play in a VoWiFi scenario, assuming you can keep roaming delays to a minimum.
- The data rate typically defaults to “auto”, allowing the card to determine the best data rate. A card in “auto” will use the highest rate the connection to the AP can sustain.
- 802.11b allows rates of 1, 2, 5.5 and 11Mbps.
- 802.11g goes beyond b to 54Mbps.
- In practice, a card can communicate at a lower data rate from further away.
- Manually setting the radio card to a lower data rate affects transmissions only. Faster reception is still possible.
- Radio cards have a low power mode intended to conserve battery power by as much as 20 – 30% in mobile devices.
- A radio card heading for night-night time will notify the AP via a power management bit set in the Frame Control field of a frame headed upstream. The tells the AP to buffer frames destined for the radio card. The AP will buffer until the card wakes up and requests its traffic.
- A radio card in sleep (power-save) mode wakes up from time to time to receive beacon frames from the AP. This must happen in order for the radio card to receive buffered frames. The AP will the radio card about buffered frames through the Traffic Indication Map (TIM).
- A radio card knows to wake up just before a beacon frame (that includes the TIM) is sent. The TIM includes the AID of the radio cards with waiting traffic.
- If the radio card has traffic, it wakes up, sends a power-save poll frame to the AP to kick off the send of the buffered data, and stays awake long enough to get his traffic.
- Once waiting traffic has been received, the radio card goes back to night-night time, IF there’s not a beacon frame corresponding to the delivery traffic indicatin map (DTIM). The DTIM is set by an AP, determining how many beacons will pass before multicast traffic is delivered by the AP. The radio card will stay awake long enough to receive multicast frames, if there are any.
- Throughput decreases in power-save mode.
RTS/CTS (Request To Send/Clear To Send)
- An 802.11 defined optional function intended to help with collisions from radio cards that may be close enough to an AP to associate with it, but too far from one another to hear each other’s transmissions.
- When RTS/CTS is enabled, a station that wants to transmit will send an RTS frame to the AP. If it’s okay to send, the AP will respond with a CTS that everyone will hear. The CTS has a duration, so that other stations know how long to be silent. This duration equals the time it will take the station to send the frame, and the AP to ACK it.
- RTS/CTS is invoked if the frame to be sent is larger than a defined threshold.
- RTS/CTS frames are, themselves, traffic. Thus, the administrator will need to evaluate carefully whether throughput increases using RTS/CTS or not, and tweak the threshold value accordingly. 750 bytes is a recommended starting point.
- 802.11 frames can be fragmented.
- A fragment is made up of MAC header, FCS, and fragment sequence number. Each fragment is ACKed separately.
- Only unicast frames can be fragmented.
- The receiver reassembles fragmented frames using the fragment sequence numbers.
- Fragmentation may be desirable if there’s a lot of RF interference, since collisions are less likely to happen if you’re pushing smaller frames into the air.
- You can set the byte size threshold that will trigger fragmentation. 750 bytes is a recommended starting point.
This next section is a small one I’m throwing onto the end of this posting, rather than making a new posting just for it. This covers the algorithm a transmitter uses before it can broadcast a wireless frame. There are similarities in the logic to ethernet.
Wireless Medium Access
- Distributed Coordination Function (DCF) – everyone fights for the medium
- The sender must check the network allocation vector (NAV) counter. This counter represents the amount of time that the previous frame on the wire needs to complete sending the frame. NAV must equal 0 before the frame can be sent. NAV is calculated by the frame length and data rate by the sender, who puts this value in the frame’s duration field. Stations seeing the frame look at the duration field, and use it to populate their NAV counter.
- Like ethernet, if a collision is detected, a random back-off timer is employed.
- A sending station can’t listen for collisions and send at the same time. Thus, the receiver needs to send an ACK, frame by frame. No ACK implies no receipt of frame, requiring a retransmission.
- Point Coordination Function (PCF)
- The AP polls stations, allowing them to send once polled.
- No known NICs or APs use PCF.
- 802.11e is working to allow for QoS over wireless medium, presumably upgradeable to your existing wireless gear firmware.