Introduction to LAN/WAN
Medium Access Sublayer (Part III)
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Wireless LAN Protocols: MAC Sublayer and IEEE 802.11 Standard, Slides of Social Work
An introduction to wireless lan protocols, focusing on the medium access sublayer and the ieee 802.11 standard. Topics include hidden and exposed terminal problems, maca and macaw protocols, and the ieee 802.11 protocol stack and physical layer. Learn about the differences between wired and wireless networks and the importance of mac protocols in wireless communication.
Typology: Slides
2012/2013
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Introduction to LAN/WAN
Medium Access Sublayer (Part III)
Now, Where are We?
Introduction
Multiple Access Protocols
- contention
- collision-free
Ethernet
Wireless LAN Protocols
Bridges
Hidden Terminal Problem
(a) Hidden station problem. (b) Exposed station problem.
)
Stations have transmission range:
naïve to try pure CSMA
)
Problem: due to ranges, interference at receiver is whatmatters
)
Hidden terminal problem (no CSMA), fig A:
- A, C want to send to B,– A starts transmitting, C cannot hear (out of range)– C then transmits, interferes with B
Exposed Terminal Problem
(a) Hidden station problem. (b) Exposed station problem.
)
Exposed terminal problem (reverse of hiddenterminal), fig (b):
- A wants to transmit to B, C wants to transmit to D– Note: both transmissions can happen simultaneously since
there will be bad reception only in area between B and C
- A transmits, C senses channel and falsely thinks it can’t
transmit, doesn’t transmit
IEEE 802.11 WLAN Protocol
)
IEEE 802.11: Wireless LAN standard
- Protocol Stack– Physical Layer– MAC Sublayer Protocol– Frame Structure– Services
)
Possible configurations: with or without base station
)
Operate in free ISM bands (900MHz, 2.4GHz, 5.5GHz)
The 802.11 Protocol Stack
Part of the 802.11 protocol stack.
)
Physical layer conforms to OSI (five options)
- 1997: infrared, FHSS, DHSS– 1999: OFDM, HR-DSSS
)
Data Link layer split into two: LLC and MAC as before
The 802.11 Physical Layer
)
Previously FCC rule: must use SS in ISM bands
)
Dropped rule in 2002: two new high speed standards
)
High Rate-DSSS
- 802.11b– Up to 11 Mbps in 2.4GHz band– 11 million chips/sec, PSK (simply increase chip rate)
)
Orthogonal Frequency Division Multiplexing (OFDM)
- 802.11a, compatible with European HiperLAN2– Up to 54 Mbps in wide 5.5 GHz band– 52 channels: 48 for data, 4 for synchronization– Complex encoding (PSK to 18 Mbps, QAM above)
)
2001: 802.11g (OFDM in 2.4GHz band, up to 54 Mbps)
The 802.11 MAC Protocol
)
Two modes:
- Point Coordination Function (PCF) (with base station)– Distributed Coordination Function (DCF) (no BS)
)
DCF must be implemented, PCF optional
)
Two DCF options:
- Both use CSMA/CA (physical and virtual carrier sensing)– One without RTS-CTS– Other with RTS/CTS, (Based on MACAW)– Exponential backoff algorithm (like ethernet) if collision
(Collision Avoidance)
Fragmentation in The 802.11 MAC
A fragment burst.
)
High wireless error rate means very long packets haveslim chance of making it through
)
Solution: break packets up (fragmentation)
)
Fragments individually numbered and ACKed usingstop-and-wait (
k
before
k+
)
Sequence of fragments: fragment burst
The 802.11 MAC: PCF
)
PCF uses base station
)
Base station polls other stations for traffic
)
Good for deterministic (real-time, video, audio) traffic
)
Beacon sent periodically for synchronization
)
Stations can go to sleep to save battery
)
Base station stores packets for sleeping station
)
PCF and DCF can co-exist by using InterFrame Spacing(IFS)
)
IFS: after a frame is sent all stations must wait a certainamount of dead time before transmitting
The 802.11 Frame Structure
The 802.11 data frame.
802.11 Services
)
Conformant wireless LANs must provide nine services
-^
5 distribution, 4 station services
)
Distribution Services (managing cell membership)
-^
Association: connect to base station
-^
Disassociation: disconnect from base station
-^
Reassociation: handoff, moving from cell to cell
-^
Distribution: how base station routes packets (local or backbone)
-^
Integration: address translation between different WLANs
)
Intracell Services (activity within a cell)
-^
Authentication: secure join
-^
Deauthentication: secure leave
-^
Privacy: encryption (uses RC4, by Ronald Rivest)
-^
Data Delivery (heart of 802.11, already discussed)
Bridges
)
Connect different LANs at the
Data Link Layer
- Transparently, so LANs can stay the same– Network layer not looked at– Can transport IPv4, IPv6, IPX, or OSI packets
)
Routers do look at network (IP) header (more later)
Bridges
Operation of a LAN bridge from 802.11 to 802.3.
)
Two different LANs: two different packet formats
)
Everyone wants to keep their packet formats
)