Local-Area Networks and LAN Switching - CCIE Routing and Switching Exam Certification Guide - page 126

Wireless LANs
WLANs provide the capability to access internetworking resources without having to be wired
to the network. WLAN applications include inside-building access, LAN extension, outside
building-to-building communications, public access, and small-office home-office (SOHO)
communications. For the CCIE written test, focus on the IEEE 802.11b standard.

The first standard for WLANs is IEEE 802.11, approved by the IEEE in 1997. IEEE 802.11
implemented WLANs at speeds of 1 Mbps and 2 Mbps using Direct Sequence Spread Spectrum
(DSSS) and Frequency Hopping Spread Spectrum (FHSS) on the physical layer of the OSI
model. DSSS divides data into separate sections, and each section is sent over different
frequencies at the same time. FHSS uses a frequency hopping sequence to send data in bursts.
With FHSS, some data is transmitted at frequency 1; then the system hops to frequency 2 to
send more data, and so on, returning to transmit more data at frequency 1.

Current implementations use the IEEE 802.11b standard. The IEEE 802.11b standard is
referred to as high-rate and provides speeds of 11, 5.5, 2, and 1 Mbps. An interoperability
certification exists for IEEE 802.11b WLANs called Wireless Fidelity (Wi-Fi). The Wi-Fi
certification is governed by the Wireless Ethernet Compatibility Alliance (WECA). IEEE
802.11b uses DSSS and is backward-compatible with 802.11 systems that use DSSS. The
modulation techniques used by IEEE 802.11b are as follows:
•

Complementary Code Keying (CCK) at 5.5 and 11 Mbps

•

Differential Quadrature Phase Shift Keying (DQPSK) at 2 Mbps

•

Differential Binary Phase Shift Keying (DBPSK) at 1 Mbps

A description of each modulation technique is outside the scope of the CCIE written test and is
not covered in this topic.

Service Set Identifier (SSID)
WLANs use a SSID to identify the network name of the WLAN. The SSID can be 2 to 32
characters in length. All devices in the WLAN must have the same configured SSID to
communicate.

WLAN Access Method
The IEEE 802.11 MAC layer implements carrier sense multiple access with collision avoidance
(CSMA/CA) as an access method. With CSMA/CA, each WLAN station listens to see if a
station is transmitting. If there is no activity, the station transmits. If there is activity, the station
uses a random countdown timer. When the timer expires the station transmits.