CMPT771 Wireless Media 1 Wireless Basics CMPT 771 Internet Architecture and Protocols.

CMPT771 Wireless Media 1

Wireless Basics

CMPT 771 Internet Architecture and Protocols


Wireless Basics

Two types of “links”: point-to-point

PPP for dial-up access point-to-point link between Ethernet switch and host

broadcast (shared wire or medium) traditional Ethernet Satellite 802.11 wireless LAN Cellular

Or lecture room


Multiple Access protocols

single shared broadcast channel two or more simultaneous transmissions by nodes:

interference only one node can send successfully at a time

multiple access protocol algorithm that determines how nodes share

channel, i.e., determine when node can transmit communication about channel sharing must use

channel itself! what to look for in multiple access protocols:


Ideal Mulitple Access Protocol

Broadcast channel of rate R bps1. When one node wants to transmit, it can send

at rate R.2. When M nodes want to transmit, each can

send at average rate R/M3. Fully decentralized:

no special node to coordinate transmissions no synchronization of clocks, slots

4. Simple


Channel Partitioning MAC protocols: TDMA

TDMA: time division multiple access access to channel in "rounds" each station gets fixed length slot (length =

pkt trans time) in each round example: 6-station LAN, 1,3,4 have pkt, slots

2,5,6 idle

Problem unused slots go idle


Channel Partitioning MAC protocols: FDMA

FDMA: frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band example: 6-station LAN, 1,3,4 have pkt, frequency

bands 2,5,6 idle

Problem unused transmission time in frequency bands go idle

frequ

ency

bands time


Channel Partitioning (CDMA)

CDMA (Code Division Multiple Access) Analogy

Public Key Encryption• Only the one holing the key can decrypt

MotivationSender – Mix information encoded with

“codes” of receiversReceiver – Decode mixed information

using its own code, and find that for itself


Channel Partitioning (CDMA)

CDMA (Code Division Multiple Access) used mostly in wireless broadcast channels (cellular,

satellite, etc) unique “code” assigned to each user; i.e., code set

partitioning all users share same frequency, but each user has own

“chipping” sequence (i.e., code) to encode data encoded signal = (original data) X (chipping sequence) decoding: inner-product of encoded signal and chipping

sequence allows multiple users to “coexist” and transmit

simultaneously with minimal interference (if codes are “orthogonal”)


CDMA Encode/Decode


CDMA: two-sender interference


CDMA: two-sender interference

Assume codes

(1,-1) for recv1

(1,1) for recv2

Data bit 1 for recv 1:

1·(1,-1)

Decoded using code for recv1:

1·(1,-1) ·(1,-1)

=(1,-1) ·(1,-1)=2

Decoded using code for recv 2: 1·(1,-1) ·(1,1)=(1,-1) ·(1,1)=0


Random Access Protocols

When node has packet to send transmit at full channel data rate R. no a priori coordination among nodes

two or more transmitting nodes -> “collision”, random access MAC protocol specifies:

how to detect collisions how to recover from collisions (e.g., via delayed

retransmissions)

Examples of random access MAC protocols: slotted ALOHA ALOHA CSMA, CSMA/CD (Ethernet), CSMA/CA (802.11 WLAN or

Wi-Fi)


Slotted ALOHA

Assumptions all frames same size time is divided into

equal size slots, time to transmit 1 frame

nodes start to transmit frames only at beginning of slots

nodes are synchronized if 2 or more nodes

transmit in slot, all nodes detect collision

Operation when node obtains fresh

frame, it transmits in next slot

no collision, node can send new frame in next slot

if collision, node retransmits frame in each subsequent slot with prob. p until success


Slotted ALOHA

Pros single active node can

continuously transmit at full rate of channel

highly decentralized: only slots in nodes need to be in sync

simple

Cons collisions, wasting slots idle slots nodes may be able to

detect collision in less than time to transmit packet – but sill waste a slot


Pure (unslotted) ALOHA

unslotted Aloha: simpler, no synchronization when frame first arrives

transmit immediately

collision probability increases: frame sent at t0 collides with other frames sent in [t0-

1,t0+1]


CSMA (Carrier Sense Multiple Access)

CSMA: listen before transmit: If channel sensed idle: transmit entire frame If channel sensed busy, defer transmission

No time slot – fully distributed Human analogy: listen and don’t interrupt others

Seems perfect – no collision !


CSMA collisions

collisions can still occur:propagation delay means two nodes may not heareach other’s transmissioncollision:entire packet transmission time wasted

spatial layout of nodes

note:role of distance & propagation delay in determining collision probability


CSMA/CD (Collision Detection)

CSMA/CD: carrier sensing, deferral as in CSMA collisions detected within short time colliding transmissions aborted, reducing channel

wastage collision detection:

easy in wired LANs: measure signal strengths, compare transmitted, received signals

difficult in wireless LANs: receiver shut off while transmitting

human analogy: the polite conversationalist


CSMA/CD collision detection


IEEE 802.11 Wireless LAN

802.11b (1999) 2.4-5 GHz unlicensed

radio spectrum up to 11 Mbps direct sequence

spread spectrum (DSSS) in physical layer

• all hosts use same chipping code

All use CSMA/CA for multiple access All have base-station and ad-hoc network

versions

802.11a (1999) 5-6 GHz range up to 54 Mbps

802.11g (2003) 2.4-5 GHz range up to 54 Mbps

802.11n (2009) 2.4/5GHz, MIMO OFDM (instead of

DSSS)


Base station approach

Wireless host communicates with a base station base station = access point (AP)

Basic Service Set (BSS) (a.k.a. “cell”) contains: wireless hosts access point (AP): base station


Ad Hoc Network approach

No AP (i.e., base station) wireless hosts communicate with each other

to get packet from wireless host A to B may need to route through wireless hosts X,Y,Z

Applications: “laptop” meeting in conference room, car interconnection of “personal” devices battlefield

IETF MANET (Mobile Ad hoc Networks) working group


IEEE 802.11: multiple access

Collision if 2 or more nodes transmit at same time

CSMA makes sense: get all the bandwidth if you’re the only one transmitting try to avoid collision if you sense another transmission


IEEE 802.11 MAC Protocol: CSMA

802.11 CSMA: sender- if sense channel idle for

DISF sec. then transmit entire frame

(no collision detection)-if sense channel busy

then binary backoff802.11 CSMA receiver- if received OK return ACK after SIFS (make sure no collision)


Enhancement: CSMA/CD ?

Collision detection doesn’t work: hidden terminal problem


Collision avoidance mechanisms

Problem: two nodes, hidden from each other, transmit complete

frames to base station wasted bandwidth for long duration !

Solution: small reservation packets nodes track reservation interval with internal

“network allocation vector” (NAV)


Collision Avoidance: RTS-CTS exchange

sender transmits short RTS (request to send) packet: indicates duration of transmission

receiver replies with short CTS (clear to send) packet notifying (possibly

hidden) nodes

hidden nodes will not transmit for specified duration: NAV


Collision Avoidance: RTS-CTS exchange

RTS and CTS short: collisions less likely, of

shorter duration end result similar to

collision detection IEEE 802.11 allows:

CSMA CSMA/CA: reservations polling from AP


IEEE 802.11 MIMO

Wireless Technology Evolution


Progress: 1G/2G Wireless Cellular Networks First generation - analog Second generation cellular network (1992-)

Technology: TDMA, TDMA hybrid FDMA• Systems: DAMPS(USA, IS-54), GSM

Technology: CDMA (Qualcomm)• Systems: CDMA(IS-95)

Pros (vs 1G) • Higher Frequency available, good Security, soft

Capacity (with CDMA), higher Capacity Cons (vs 3G)

• Speech Service, lower rate, no multimedia Service and no higher rate Service


Progress: 3G Wireless Networks

Third generation 3G (1996-) Objective

• truly anybody at any place to communicate with anyone at any time

Support Multimedia Service• 144kb/s(Outdoor and higher velocity ), 384kb/s(from

Outdoor to indoor, lower velocity), 2Mb/s(indoor) Standards

• Japan: WCDMA (TDD/FDD)• Europe: WCDMA (FDD model) and TD-CDMA(TDD). • America: CDMA-2000 (TDD)• China: TD-SCDMA(1998)


Progress: 4G Wireless Networks 4G Wireless Networks: Objective

Support interactive multimedia services: teleconferencing, wireless Internet, etc

Global mobility and service portability Higher rate, lower cost

What's New in 4G Entirely packet-switched networks All network elements are digital Higher bandwidths to provide multimedia services Tight network security.

3G vs 4G Circuit and packet switchiong/Entirely packet switched networks Combination of existing & evolved equipment/All digital elements Data rate (up to 2Mbps)/Higher rate (up to 100Mbps)

Technology MIMO-OFDM (Orthogonal Frequency division multiplexing) ?

WiMax

“Worldwide Interoperability for Microwave Access” WiMAX is a standards-based Broadband Wireless

Access fast broadband connections over long distances

Standard based: Wireless MAN by IEEE 802.16 802.16d: Fixed WiMAX (Jun. 2004) 802.16e: Mobile WiMAX (Nov. 2005) Industry to create interoperable complete ecosystem

WiMAX Forum (2001), Intel etc. involved Claims there are over 455 WiMAX networks deployed

in over 135 countries

LTE (Long Term Evolution)

LTE Air-interface Downlink OFDM Uplink SC-FDMA Mutiple Antenna Scheme (MIMO)

WiMaX vs LTE (4G and beyond)


More on Wireless Media

Power optimization Transmission power Circuit power

Joint source-channel coding Channel ?

Roaming Hand off Vertical hand off

…

CMPT771 Wireless Media 1 Wireless Basics CMPT 771 Internet Architecture and Protocols.

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