CS 536 Park Packet radio: ALOHA Base Station Stationary F1 F1’ F1’ F1 F1 F1’ ALOHA Stationary Stationary Stationary Stationary −→ downlink broadcast channel F 1 −→ shared uplink channel F 1 ′ Ex.: ALOHANET • data network over radio frequency • Univ. of Hawaii, 1971; 4 islands, 7 campuses
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
CS 536 Park
Packet radio: ALOHA
Base Station
Stationary
F1
F1’
F1’
F1F1
F1’
ALOHA
Stationary
Stationary
Stationary
Stationary
−→ downlink broadcast channel F1
−→ shared uplink channel F1′
Ex.: ALOHANET
• data network over radio frequency
• Univ. of Hawaii, 1971; 4 islands, 7 campuses
CS 536 Park
• Norm Abramson
→ precursor to Ethernet
→ parallel to wired packet switching technology
• carrier frequency
→ uplink: 407.35 MHz; downlink: 413.475 MHz
• bit rate: 9.6 kb/s
• contention-based multiple access: MA
→ plain and simple
→ needs explicit ACK frames (stop-and-wait)
CS 536 Park
Wireless LAN (WLAN): infrastructure mode
Access Point
F1
F1
F1
F1F1
F1
MobileMobile
Mobile
Mobile
Mobile
Mobile
Mobile
WLAN: Infrastructure Network
−→ shared uplink & downlink channel F1
• basic service set (BSS)
→ “hot spot”
• SSID (service set identifier): name/label of BSS
• base station: access point (AP)
• mobile stations must communicate through AP
CS 536 Park
WLAN: ad hoc mode
F1
F1
F1
F1F1
F1
MobileMobile
Mobile
Mobile
Mobile
Mobile
MobileMobile
F1
F1
F1
F1
F1
WLAN: Ad Hoc Network
−→ homogeneous: no base station
−→ everyone is the same
−→ share forwarding responsibility
• independent basic service set (IBSS)
• mobile stations communicate peer-to-peer
→ also called peer-to-peer mode
CS 536 Park
WLAN: internetworking
Access Point
F1
F1
F1
F1F1
F1
MobileMobile
Mobile
Mobile
Mobile
Mobile
Mobile
Access Point
F1
F1
F1
F1F1
F1
MobileMobile
Mobile
Mobile
Mobile
Mobile
Mobile
Access Point
F1
F1
F1
F1F1
F1
MobileMobile
Mobile
Mobile
Mobile
Mobile
Mobile
WLAN: Extended Service Set
Distribution System
−→ internetworking between BSS’s through APs
−→ mobility and handoff
• extended service set (ESS): shared SSID
• APs are connected by distribution system (DS)
→ typically: Ethernet switch
CS 536 Park
How do APs and Ethernet switches know where to for-
ward frames?
→ spanning tree
→ IEEE 802.1 (Perlman’s algorithm)
Learning bridge: source address discovery
→ log source MAC address of incoming frames per inter-face
→ initially (or if unclear): broadcast
→ simple form of routing
→ adequate for small systems
Misconfiguration issues resulting in loops
→ modifications to spanning tree algorithm
CS 536 Park
Additional headache: mobility
−→ roaming
−→ how to perform handoff
−→ mobility management at link vs. network layer
−→ link layer handoff dominant (vs. Mobile IP)
Mobility between BSS’s in an ESS
• Association
→ registration process
→ AP sends out periodic beacon frame
→ mobile station (MS) associates with one AP
• Disassociation
→ upon permanent departure: notification
CS 536 Park
Handoff from old to new AP:
• Reassociation
→ movement of mobile from one AP to another
→ mobile initiated
→ e.g., AP’s signal strength is low
→ passive (beacon) or active (probe) scanning to findalternate AP
→ go through association process
• Handoff
→ inform new AP of old AP
→ forwarding of buffered frames from old to new APin ESS
CS 536 Park
IEEE 802.11b/g WLAN spectrum 2.4–2.4835 GHz:
→ 11 channels (U.S.)
→ 2.412 GHz, 2.417 GHz, . . ., 2.462 GHz
→ unlicensed ISM (Industrial, Scientific, Medical) band
→ global: 2.4–2.4835 GHz
→ up to 14 channels (e.g., Japan)
IEEE 802.11a: 5.15–5.35 GHz and 5.725–5.825 GHz
→ UNNI (unlicensed National Information Infrastruc-ture)
→ 23 non-overlapping channels
CS 536 Park
IEEE 802.11n: both 2.4 and 5 GHz
→ 2.4 GHz: backward compatible
→ also uses multiple antennae
→ called MIMO (multiple input multiple output)
IEEE 802.11ac: extension of n/g with more streams,wider bandwidth, 256-QAM
→ 802.11ax in the works with 1024-QAM, OFDMA
CS 536 Park
Non-interference specification for 802.11b:
• each channel has 22 MHz bandwidth
• require 25 MHz channel separation
−→ thus, only 3 concurrent channels possible
−→ e.g., channels 1, 6 and 11
−→ 3-coloring. . .
CS 536 Park
HAAS (Old CS Building):
First floor frequency reuse:
CS 536 Park
Second floor frequency reuse:
Ground floor frequency reuse:
CS 536 Park
IEEE 802.11 WLAN:
→ uses CSMA as MAC
However:
• 802.11a/g/n: use OFDM
→ single-user (not OFDMA)
→ e.g.: 801.11g uses 48 subcarriers
→ 312.5 KHz separation
→ 1st bit on subcarrier 1, 2nd bit on subcarrier 2, . . .
• 802.11b: uses DSSS CDMA
→ 11-bit chip sequence
→ single-user
Why not just use multi-user OFDMA or CDMA?
→ two main reasons
CS 536 Park
Empty room:
→ large lecture room
→ no obstructions
→ 802.11 WLAN hot spot (infrastructure mode)
→ how does indoor signal reception look like?
CS 536 Park
Signal strength reception at height 0.7 m:
→ approximately table height
→ carrier frequency: 5.32 GHz
-10 0 10 20 30 40
AP->stations (5.32GHz)
0 5 10 15 20 25 30 35 40 0
5
10
15
20
25
30
→ signal strength varies by distance
→ but also varies by location
→ connection to microwave oven?
CS 536 Park
Multi-path fading:
→ EM waves interfere constructively and destructively