Page 425 - From GMS to LTE

Wireless Local Area Network (WLAN) 411

6.6.1 IEEE 802.11b – 11 Mbit/s
The breakthrough of WLAN in the consumer market was triggered by the introduction
of devices compliant with the 802.11b standard, with a maximum speed of up to 11
Mbit/s. More recent PHYs described in the 802.11g, n and ac standards can achieve
even higher speeds with the same bandwidth requirement of 22 MHz. The different
PHYs are discussed later in detail. The following list shows some basic WLAN param-
eters and compares some of them to similar parameters of other systems.

WLAN maximum transmission power is limited to 0.1 W. Mobile phone power, on
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the other hand, is limited to 1–2 W. Global System for Mobile Communications
(GSM) and Universal Mobile Telecommunications System (UMTS) base stations
have a typical power output of 20 W per sector and frequency.
Each channel has a bandwidth of 22 MHz. Up to three APs can be used at close range
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in the ISM band without interfering with each other. GSM uses 0.2 MHz (200 kHz)
per channel, and UMTS uses 5 MHz.
Frame size is 4–4095 bytes. However, IP frames do not usually exceed 1500 bytes. This
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value is especially interesting for comparison with other technologies: A General Packet
Radio Service (GPRS) packet, as shown in Section 2.3, consists of four bursts of 114 bits
each and thus can only contain 456 bits. If coding scheme 2 for error detection and
correction is used, only 240 bits or 30 bytes remain for the actual packet. Therefore, an
IP packet can be transmitted over a single WLAN frame, but it has to be split into sev-
eral packets if it has to be transmitted over the air interface of a GPRS network.
Transmission time of a large packet depends on the size of the packet and the trans-
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mission speed. If a large packet with a payload of 1500 bytes is transmitted with a
speed of 1 Mbit/s, the transmission takes about 12 milliseconds. If reception condi-
tions are good and the packet is sent with a transmission speed of 11 Mbit/s, the same
transmission takes only 1.1 milliseconds. Note that the SIFS and the time it takes to
send a short ACK frame as confirmation have to be added to these values to calculate
the precise transmission time.
Time between a data frame and an ACK frame (SIFS) is 10 microseconds or 0.01
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milliseconds.
If a transmission error occurs, a backoff procedure is performed as described in the
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previous section. A backoff slot (of which 63 exist for the first retry) has a length of 20
microseconds or 0.02 milliseconds.
At the beginning of the frame, a preamble is sent, which notifies all other devices that
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the transmission of a frame is about to start. The preamble is necessary to synchro-
nize all listeners to the start of the frame. The preamble has a length of 144 microsec-
onds or 0.144 milliseconds.
The preamble mentioned in the list above is part of the Physical Layer Convergence
Procedure (PLCP) header, which is sent at the start of every frame. The PLCP header
also contains information about the datarate used for the subsequent MAC frame. With
the 802.11b standard, the MAC frame can be sent with a speed of 1, 2, 5.5 and 11 Mbit/s.
This flexibility is necessary, as devices experiencing bad radio conditions can only send
and transmit with a lower speed to compensate for unfavorable radio conditions with a
higher redundancy. In practice, the sender decides on its own which coding to use for a
frame. Some devices also offer the possibility for the user to manually lock the speed to
a fixed value (e.g. 5.5 Mbit/s). This helps in situations when the automatic speed selection