From 47ab100f89a292a55cd4d101ce401466164f5454 Mon Sep 17 00:00:00 2001 From: Jet Hughes Date: Thu, 20 Oct 2022 10:17:04 +1300 Subject: [PATCH] vault backup: 2022-10-20 10:17:03 --- content/notes/23-wireless-mobile.md | 39 ++++++++++++++++++++++++++++- 1 file changed, 38 insertions(+), 1 deletion(-) diff --git a/content/notes/23-wireless-mobile.md b/content/notes/23-wireless-mobile.md index eccdf0521..435551014 100644 --- a/content/notes/23-wireless-mobile.md +++ b/content/notes/23-wireless-mobile.md @@ -57,10 +57,47 @@ Standards created and maintaned by IEEE - all use CSMA/CA for multiple access, and have base station and ad-hoc versions WLAN architecture -- wireless hosts communicates with base station (A) +- wireless hosts communicates with base station (Access point AP) +- Basic service set (BSS): set of devices that share the same service identifier + - wireless hosts + - AP +- in ad hoc mode: hosts only, no AP + +channels and association +- spectrum divided into channels at different frequencies + - admin chooses frequency for AP + - interference possible: channel can be the same as a neighboring AP +- a new host must assiciate with the AP (connect) + - listens for *beacon frames* + - contain AP name (SSID) and MAC + - may perform authentication + - run DHCP to get IP in AP's subnet # CSMA/CA +- avoid collisions +- CSMA sense before transmitting +- wifi: no collision detection + - hard due to: hidden terminal, fading, + - instead: CA collision avoidance +DCF (distributed coordination function) +- CSMA/CA with binary exponential backoff algorithm + +sender +- if sense channel idle for DIFS (DCF interframe space) then send data (no CD) +- if sense channel busy: + - start random backoff time + - timer counts down while channel idle + - transmit when timer expires + - if no ACK, increase random back of interval, repeat 2 + +reciever +- if frame recieved ok + - return ACK after SIFS (short interframe space) + - ACK needed due to hidden terminal problems + + + # Hidden terminal problem # Cellular networks