Increasing SIFS/DIFS?

From: Bruno Lopes F. Cabral (
Date: 2002-09-09 14:57:48 UTC

Hi there

is there a way to increase the SIFS/Slottime using hostap_cs?? the purpose is to extend a link beyond 15km

investigating pcmcia_cs for linux, seems only the raylink card driver has support to modify the defaults

any pointers would be greatelly appreciated

from Brazil


The issue is for a long link. The longer the link the more time it takes packets to travel through it. Light travels at 3E8 M/Sec. This equates to 3,000 KM/mSec; or 3KM/uSec.


* IEEE 802.11 Spec, section 15.3.2
*      - default values for the DSSS PHY MIB

#define DSSS_CWMin 31
#define DSSS_CWMax 1023
#define DSSS_SlotTime 0.000020 // 20us
#define DSSS_CCATime 0.000015 // 15us
#define DSSS_RxTxTurnaroundTime 0.000005 // 5us
#define DSSS_SIFSTime 0.000010 // 10us
#define DSSS_PreambleLength 144 // 144 bits
#define DSSS_PLCPHeaderLength 48 // 48 bits
#define DSSS_MinimumBandwidth 1000000.0 // 1Mbit/sec

For a reliable multi-station network, each station must be able to hear a packet being transmitted within the timing of a single 'SlotTime'. The default 'SlotTime' is 20 uSec.

But the standard says that stations must be able to contact the system within the SIFS (Shortest Inter Frame Sequence) for packets such as ACK or CTS. A station must have heard the end of a packet and transmit the packet within the SIFS period, and the SIFS must not have ended for any station (importantly the first station). The SIFS default is 10 uSec. But since there is the forward and reverse propergation this value is halved to 5 uSec which translates to about 15km maximum with the default values

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