Wireless Local and Wireless Personal Area Network Communication in Industrial Environments - Networked Embedded Systems - page 641

usedfortheguardintervaltocombatmultipathpropagation.Forsynchronization,channel
estimation and equalization a training sequence is transmitted, which consists of  repeated
short and  repeated long OFDM symbols [Insc,vNAM + ].
DSSS-OFDM: This optional physical layer format of IEEE .g combines the DSSS PHY with
the OFDM PHY such that for the header DSSS is used while the payload employs OFDM
(including the OFDM preamble).

Table . compares the theoretical maximum throughput values TP for the DSSS and OFDM
IEEE . PHY versions for MAC data traffic. The maximum payload length is  bytes (with
has to include the -byte MAC header).  bytes is the common length of an Ethernet packet
(plus  bytes MAC header and check sum),  a typical length for a Web browsing packet and 
bytes the length of a TCP ACK. he throughput TP is calculated as follows:

× 

PayBytes

TP = . (.)

CW min


SIFS +

t slot

average back-off +

× t data packet + + t ACK

DIFS
 t slot

SIFS

+

The durations needed to transmit the data packet t data packet and acknowledgment t ACK vary depend-
ing on the physical layer chosen. For the DSSS mode they are calculated as follows:


ModRate
MAC header

 × ×  

ModRate .

 × PayBytes

t data DSSS = t PHYh + + ; t ACK DSSS = t PHYh + (.)

ModRate

For the OFDM physical layer in the  GHz bands Equation . needs to be calculated. For the OFDM
mode in the . GHz band, additional  µs signal extension have to be added to both. “Ceil” stands
for the next larger integer.

 +(  + )×  + 

PayBytes

t data OFDM a = t PHYh + t sym ceil (

 )

/  Mbps ×

ModRate

 +  ×  + 

t ACK OFDM a = t PHYh + t sym ceil (

 )

. (.)

/  Mbps ×

ModRate

For small payload sizes ( and  bytes), the maximum throughput values decrease drasti-
cally (. and . Mbps maximum at  Mbps modulation rate). When considering Ethernet packets
the highest theoretical throughput rates are . Mbps for IEEE .b and ./. Mbps for the
OFDM modes. Naturally, these wireless throughput rates are smaller than the wired ones (there -
 Mbps are possible), but at least for the higher modulation rates with  bytes Ethernet packets
the throughput values are reasonably good. Note that the real-life throughput values for IEEE .b
systems are still smaller than the theoretically possible ones: values around  Mbps have been mea-
sured [Mob,MZ]. his is because for the actual implementations used, higher protocol layers like
TCP/IP cause additional overhead and delays.
For security, IEEE . WLANs support several authentication processes which are listed in
the specification (none is mandatory). ∗ Table . lists the IEEE . standardization activities
involving IEEE ..

∗ Neither Bluetooth nor IEEE . is renowned for their security concepts, and both have been criticized. Nevertheless,
both systems have taken security aspects into consideration.