Wireless Networks Instructor: Fatima Naseem Computer Engineering Department, University of Engineering and Technology, Taxila.

Wireless Networks

Instructor: Fatima Naseem

Computer Engineering Department, University of Engineering and Technology, Taxila

Lecture # 13

Broadband Wireless

Broadband Wireless Background

Deregulation of telephone system in many countries

Competitors are now allowed to offer local voice & Internet service

Running Fiber, coax, or cat 5 UTP to millions of homes & businesses is Prohibitively expensive

Solution Broadband Wireless

Broadband Wireless Access (BWA)

Technology for high-speed connection over the air Uses radio waves to transmit and receive data BWA is a point-to-multipoint system Made up of base station and subscriber equipment. Base station uses an outdoor antenna to send and

receive high-speed data and voice to subscriber equipment

What is WiMAX?

Prior Attempts: LMDS & MMDS Local Multipoint Distribution Service (1998) 1.3 GHz around 28 GHz band (Ka Band) 28 GHz Rain effects⇒ Multi-channel Multipoint Distribution Services (1999-2001) 2.1, 2.5-2.7 GHz Band Not affected by rain⇒ Issues: Equipment too expensive, Roof top LoS antennas,

short range (LMDS) or too small capacity (MMDS)

WiMax (Worldwide Interoperability for Microwave Access) – IEEE 802.16

Based on Wireless MAN technology WiMAX standard consist of Fixed system (IEEE 802.16d-2004 Air Interface standard) Mobile system (IEEE 802.16e) Define specifications for the PHY & MAC layer

PHY layer specs Frame structure, OFDMA, modulation, and coding

MAC layer specs Data and control plane, sleep mode for the terminals

Key Features of WiMAX Works on many bands: 2.3 GHz, 2.5 GHz, 3.5 GHz, … Scalable Can use any available spectrum width: 1.25 MHz to 28 MHz Strong security Open technology like WiFi Reach and mobility like Cellular but much higher data rates

High data rate, up to 70Mbps Long distance, up to 50kms Mobility, up to 120 to 150 km/hour

Data rate vs Distance trade off using adaptive modulation. 64QAM to BPSK Offers non-line of site (NLOS) operation Strong QoS Guaranteed services for data, voice, and video

WiMAX

WiMAX ≠ IEEE 802.16 Worldwide Interoperability for Microwave Access 420+ members including Semiconductor companies,

equipment vendors, integrators, service providers. Like Wi-Fi Alliance

Narrows down the list of options in IEEE 802.16 WiMAX forum lists certified base stations and

subscriber stations from many vendors

Effect of Frequency Higher Frequencies have higher attenuation, e.g., 18 GHz has 20 dB/m more than 1.8 GHz Higher frequencies need smaller antenna Antenna > Wavelength/2, 800 MHz 6”⇒ Higher frequencies are affected more by weather Higher than 10 GHz affected by rainfall 60 GHz affected by absorption of oxygen molecules Higher frequencies have more bandwidth and higher data rate Higher frequencies allow more frequency reuse They attenuate close to cell boundaries. Low frequencies

propagate far. Mobility Below 10 GHz

WiMax- Type of Service Line-of-sight A fixed dish antenna points straight at the WiMAX tower from a

rooftop or pole The line-of-sight connection is stronger and more stable, so it's

able to send a lot of data with fewer errors Line-of-sight transmissions use higher frequencies, with ranges reaching a possible 66 GHz Non-line-of-sight A small antenna on your computer connects to the WiMAX tower WiMAX uses a lower frequency range -- 2 GHz to 11 GHz

Comparison of 802.11 with802.16 Why devise a new standard? Why not just use 802.11 Reasons: 802.16 provides service to buildings & buildings are not mobile.

Do not migrate from cell to cell often 802.16 runs over a part of city, distances involved can be several

kilometers In 802.16, each cell has more users than 802.11cell & these

users are expected to use more BW than a typical 802.11. so more spectrum needed

802.16 works in a 10-to-66 GHz range but these millimeter waves have different properties then the longer waves in ISM bands

Strongly absorbed by water Line of sight communication

Data rate vs. Mobility

Why not WIFI

Scalability Relative Performance Quality of Service Range Coverage Security

The 802.16 Protocol Stack

Differences with 802.11 Transmission sublayer is used to hide the different technologies from

data link layer Security sublayer MAC sublayer is completely connection oriented Services-specific convergence sublayer takes place of LLC Provides support for both connectionless & connectionoriented

protocols

IEEE 802.16 PHYs

The 802.16 Physical Layer Base station has multiple antennas Each antenna pointing at different sector of surrounding terrain

due to line of sight requirement for millimeter waves Each sector has his own users & independent of surrounding

sectors De-Merits Signal strength in millimeter band falls off sharply with distance

from base station SNR drops with distance from base station Solution Employs three different modulation schemes Based on how far subscriber station is far from base station

The 802.16 Modulation Close-in subscribers QAM-64 is used with 6 bits/baud Medium-distance subscribers QAM-16 is used with 4 bits/baud Distant subscribers QPSK is used with 2 bits/baud Example: for 25 MHz of spectrum QAM-64 gives 150Mbps QAM-16 gives 100Mbps QPSK gives 50 Mbps Farther the subscriber is from base station, lower the data rate Transmission environment is shown pictorially on next slide

The 802.16 TransmissionEnvironment

The 802.16 Multiplexing TDD: Time Division Duplexing Base station periodically send out frames, each containing time

slots; Refer next slide 1st ones are for downstream traffic Guard frames are used to switch direction Number of time slots devoted to each direction can be changed

dynamically to match the BW in each direction to the traffic Downstream traffic is mapped into time slots by base station. Upstream is more complex & depends upon on quality of service Use of Hamming codes to do FEC in physical layer

Frames and time slots for TDD

802.16 Frame Structure

Mobile WiMAX Frame

Frame Structure DL Preamble: Time and frequency synchronization Frame Control Header (FCH): MAPs lengths, modulation and

coding, usable subcarriers Downlink MAP: Burst profile (time, frequency, modulation,

coding) to each user Uplink MAP: Burst profile for transmission from each user. MAPs can be compressed Contention-based region: Ranging, bandwidth request,

besteffort data Ranging Channel:

Closed loop frequency, time, and power adjustments Channel quality indicator channel (CQICH) Ack Channel: subscriber stations

Initially, 5 ms frames only.

Subscriber Initialization

The 802.16 MAC SublayerProtocol … Security sublayer Encryption is used to keep secret all the data Only payloads are encrypted; the headers are not MAC sublayer common part MAC frames occupy an integral number of physical

layer time slots Each MAC frame is composed of subframes, 1st two

of which are downstream & upstream maps These maps tells what is in which time slot & which

time slots are free Upstream channel allocation is closely tied to the

quality of service issue

The 802.16 MAC SublayerProtocol: Service Classes

All services in 802.16 are connection-oriented Each gets one of the above classes of service,

determined when the connection is setup Design is very different from that of 802.11 or Ethernet Standard defines two forms of BW allocation

per station & per connection Per station case

Subscriber station aggregates the needs of all users in the building & makes collective requests for them

Per connection case Base station manages each connection directly

The 802.16 Frame Structure

(a) A generic frame (b) A bandwidth request frame

EC bit: tells whether payload is encrypted Type field: identifies frame type CI field: presence or absence of final checksum EK field: which of encryption keys is used (if any) Length field: complete length of the frame including header Connection ID: tells which connection this frame belongs to Header CRC: checksum over header only For more details, please consult the standard

WiMAX MAC: Key Features Flexible and Extensible - Same MAC for all current and future

PHYs Modular: Several optional features. Negotiable SS/BS features Multiple Topologies: PTP, PMP, mesh Multiple Antenna Technologies: Adaptive Antennas, MIMO Multiple Protocol Payloads: ATM, Packets (IP or Ethernet), W or

w/o header suppression Flexible Retransmission Policies: ARQ, HARQ TDD and FDD Support Variety of Subscribers: Several per subscriber or per connection

parameters Integrated QoS Security

Base Station and Subscriber Stations Base Station (BS): Controls the entire system, frame

size, scheduling, admission control, QoS, Ranging, clock synchronization, power control and handoff.

All traffic goes through BS Subscriber Station (SS): Find BS, Acquire PHY

synchronization, Obtain MAC parameters, Generate bandwidth requests, make local scheduling decisions, follow transmission/reception schedule from BS, perform initial ranging, maintenance ranging, power control

Mobile Station (MS): Mobility management, Handoff, Power Conservation

Framing and Duplexing Burst = n MPDUs with per burst CRC Burst Profile: Modulation type, FEC, preamble type, guard time Downlink Interval Usage Code (DIUC): Identifies burst profile DL Channel Descriptor (DCD): Describes DL PHY. Broadcast

periodically by BS. Frame duration, Defines DIUCs. Uplink Interval Usage Code (UIUC): Identifies UL burst profiles UL Channel Descriptor (UCD): Describes UL PHY.

Connections and Service Flows

Service Flows = Higher layer flows Each Service flow has a connection Extra connections for management and control 16-bit CID 65,535 connections⇒ Each station has many connections with BS:

Initial Ranging CID Basic CID Primary Management CID Secondary Management CID: Higher layer Multicast Polling CID: Bandwidth requests

IEEE 802.16 – QoS Classes Connection oriented: All traffic is assigned a connection Five Service Classes:1. Unsolicited Grant Service (UGS): CBR traffic, e.g., voice

Specified throughput, delay, and delay jitter

2. Enhanced Real-Time Polling Service (ertPS): Silence suppressed voice. On/off UGS.

3. Real-Time Polling Services (rtPS): rtVBR, e.g., streaming video. Specified peak and average throughput, delay and delay jitter.

4. Non-Real-Time Polling Service (nrtPS): nrtVBR, Specified peak and average throughput

5. Best Effort (BE); No throughput or delay guarantees

ARQ Allows selective repeat (Stop and Wait, go back n ) ARQ block size negotiated at connection setup Depends upon the Type of Service (ToS), expected delay, etc ARQ block cannot be fragmented A fragment may contain blocks from multiple SDUs

IEEE 802.16 Protocol Structure CS: All functions that are specific to a higher layer protocol

Classify SDUs based on MAC address, VLANs, priorities Assigns Service Flow ID (SFID) and a connection identifier Optional payload header suppression (PHS)

CPS: Fragmentation and reassembly of large MAC SDUs Packing and unpacking of several small MAC SDUs QoS control, Scheduling Bandwidth request Automatic repeat request (ARQ

Benefits OF WiMax

Speed Faster than broadband service Wireless Not having to lay cables reduces cost Easier to extend to suburban and rural areas Broad Coverage Much wider coverage than WiFi hotspots

ENDOF LECTURE 13