Wireless Networks and Protocols

Ram DantuThis material is compiled from various sources including

several industrial presentations, university lecture notes etc.,

Why Wireless?

• Reduced cost• More flexibility for locating equipment (deployment ease)• User mobility • Reduced space needed for cabling• Provision for location information/location-based applications• More suitable for broadcasting

Limitations of Wireless

• Capacity reduction: reduced data rates, increased blocking• Spotty coverage• Variability in network performance• Energy constraints

The Wireless Media

Signal paths between a transmitter and a receiver can containphysical obstructions.

The presence of these multiple paths and/or physical obstructions implies fading.

The Wireless Media (continued)

Furthermore, fading can vary in time, frequency, and space (for example, as users move).

In addition to fading, wireless transmissions are susceptible to interference from other users and/or systems.

Consequently, the wireless media can significantly impact the transmission and reception of signals (much more so thantransmissions on wired networks).

Implications

Thus, network protocols designed assuming wired channel characteristics may not perform well in wireless scenarios.

➢ Wireless networks should be designed and studied for the wirelesschannel.

Considerations in Design and Study

• How are networks used?• What are the capabilities of the network elements?• What information is available to network elements?• How to jointly exploit the information and capabilities of elements?• And so on

Centralized vs Non-centralized

Centralized wireless networks have two types of transceivers:access points (AP's) and user terminals.

AP's have wired connections to other AP's and/or larger wired networks.

This implies control is centralized at the AP's, i.e., user terminals communicate only to AP's -> a spoke-and-wheel network configuration

Non-centralized networks can also contain both AP's and user terminalsbut communication is not limited between AP and user terminals only.

Ad hoc, peer-to-peer connections are possible, as is dynamic routing of information from source to destination.

Cellular

Services or Features: Voice and data through handheld phones.

Coverage Area: Continuous coverage. At times limited to metropolitan regions.

Limitations: Available bandwidth is very low for most data intensive applications.

Examples: AMPS, IS-136, IS-95, GSM, WCDMA, cdma2000, etc.

Wireless Local Area Networks (WLAN's)

Services or Features: Traditional LAN extended with wireless interface

Coverage Area: Used only in local environments

Limitations: Limited range.

Examples: IEEE 802.11 suite (a,b,g)

Global Positioning System (GPS)

Services or Features: Helps to determine the three-dimensional position, velocity, and time.

Coverage Area: Anyplace on earth.

Limitations: Cost.

Examples: GNSS, NAVSTAR, GLONASS

Satellite Based PCS

Services or Features: Voice paging and messaging.

Coverage Area: Almost anyplace on earth.

Limitations: COST!

Examples: Iridium, Teledesic.

TDMA Operation

• Fixed-assisgnment multiple access (FAMA): This assignment of capacity within the overall satellite channel is distributed in a fixed manner among multiple stations.

• Transmission in the form of repetitive sequence of frames– Each frame is divided into a number of time slots– Each slot is dedicated to a particular transmitter

• Earth stations take turns using uplink channel– Sends data in assigned time slot

• Satellite repeats incoming transmissions– Broadcast to all stations

• Stations must know which slot to use for transmission and which to use for reception

FAMA-TDMA Uplink

FAMA-TDMA Downlink

Home Networking

Services or Features: To connect different PC's in the house to share files and devices such as printers. Also to connect differentappliances in the house.

Coverage Area: Anywhere in the house.

Limitations: Limited to a home.

Examples: HomeRF.

This can also be categorized as a local area network.

Ad Hoc Networks

Services or Features: Group of users share data with each otherfor a short period of time.

Coverage Area: Equal to that of a LAN, but without fixed infrastructure.

Limitations: Limited range.

Examples: Bluetooth, defense applications. Battlefields, disasterlocations.

Sensor Networks

Services or Features: A large number of tiny (cheap, low-power) sensors with wireless capabilities. Collected data is transmitted back to a central processor.

Coverage Area: Relatively small terrain.

Limitations: Very limited range, scalability problems (under certaincircumstances).

Examples: Defense and civilian applications. Monitor inhospitableor inaccessible terrain.

A Simple Wireless NetworkMobile Data Set

PSTN

PacketNetwork

MobileVoice Unit

Base Transceiver System (BTS)

Base Station Controller

(BSC)

Mobile Switching

Center (MSC)

Packet Inter-Working Function

Challenge is to keep connection and not loose any data during handoff operation

The Components

• BTS– BTS consists of one or more transceivers placed at a single location.

The BTS terminates the radio path on the network side.

• BSC– Provides allocation and management of radio resources. – SDU: Selection and distribution unit. Also responsible for handoff

coordination

• MSC– Provides and controls mobile access to the PSTN. Interprets the

dialed number, routes and switches call to destination number. Also manages mobile’s supplementary services. Maintains a register of visitors operating within the coverage area of the MSC’s connected BTSs.

• PDSN: Packet data service node is basically a packet router.

MSC PDSN

BSC(SDU)

BSC(SDU)

BSC(SDU)

BTS BTS BTS BTS BTS

 

- Backhaul cost is by $$$/mile- 10-100 miles between BTS and BSC- Voice or data use one DS0 channel at a time- BTSs are located in the tower- BSC and MSCs are located

in the central office

Current Wireless Network Architecture

BSC

BTS

24xDs0in T1

PacketsTDMchannels

TDMchannels

Soft Handoff between two BTS

Handoffs == ( Hard || Soft )

Handoff: A handoff mechanism is needed to maintain connectivity as devices move, while minimizing disruptions to ongoing calls. This mechanism should exhibit low latency, incur little or no data loss, and scale to a large network.”

33.1

33.2

33.3

33.4

33.5

33.6

33.7

33.8

33.9

34

-85.2 -85 -84.8 -84.6 -84.4 -84.2 -84

Typical US city BTS Map

30x30 miles

Urban

Rural highway

2G/3G RAN Network (Traditional)

CO

BTS BTS BTS BTS BTS BTS

CO CO CO

BSC

Interoffice distance (costs per mile) cost + Fixed Cost

ChannelTerminationCost

ChannelTerminationCost

MSC

SDU and soft handoff

  

  

 

 

SDU - 1

WR-A

 

 

SDU-1 -

   

SDU - 2

WR-B

-

 

- 3 to 6 BTSs involved in soft handoff- SDU changeover due to weak signal from primary BTS- BTS forwards even corrupted radio frames to the SDU for selection

SDU - 2-

BTS-1

BTS-2

BTS-3

What happens when a cellular user turns on their phone?

Terminal scans control channels and locks on to strongest one.

If can’t find a strong enough signal, “no service”

With receiver turned to strongest control channel, terminalextracts important information from broadcast channel.

StrongestSignal

On interpreting this broadcast information, terminal turns on “roaming” sign, determines DCC, paging channels, etc.

Once this initialization is complete, mobile enters idle mode.

When no call in progress, terminal monitors paging messages in order to detect arrival of a new call.

Page for User with MIN XPaging Channel

(one of broadcastchannels)

User with Mobile Identification Number (MIN) X

Power consumed by radio receiver while it waits for pagingmessage has strong influence on standby time of terminal’sbattery.

Terminals can operate in sleep mode when no call in progress.

In this mode, terminal turns off its receiver for significant fractionof time. Wakes up for short period of time.

If there is paging message for terminal, BS schedules message toarrive during brief wake-up interval. This is synchronized by usinga hyperframe counter.

Paging messages arrive in SPACH blocks of superframe. Indicatesassigned traffic channels, etc.

How does system deal with roaming users?

Aside from connecting radio system to public/private wired networks,MSC also connects to two types of databases (DBs):

• Home Location Registers (HLRs)

• Visitor Location Registers (VLRs)

MSC

VLRHLR

MSCPrivate, Public,

ResidentialNetworks

BS

BS

BS

BS

HLR contains subscription information of a set of terminals and also records terminal’s current location and status. Information about each terminal is accessed using mobile identification number.

A call request for a terminal arrives at terminal’s home MSC.

Home MSC interrogates HLR in order to determine location of terminal. MSC then coordinates actions to page terminal in visited system.

VLR stores information about terminals currently in service area.

This information is used to setup calls initiated by cellular usersAnd to deliver calls directed to cellular users. Service area of VLRSpans coverage areas of one or more MSC’s.

Mobility Management

• Mobility management procedures begin when system detects visting terminal.

• When mobile is in coverage area of base, it will at some point send either a registration message or origination message (when making a call) to BS.

• BS will inform MSC which will register user’s presence at VLR.

• VLR notifies terminal’s HLR of terminal’s location.

• Whenever terminal that does not have a call in progress enters a new location area, it sends a registration message to local base station.

• Location areas are clusters of contiguous cells.

• When a call arrives for a terminal, system pages terminal only in the location area where it last registered.

• Registration tradeoff: if terminal registers often, it is easier to locate the terminal but it uses up resources.

How are Handoffs Coordinated?

Mobile Assisted Handoff (MAHO)

A terminal tuned to a digital traffic channel relies on MAHO to moveIts call to a new BS.

With MAHO terminal with call in progress monitors quality of Signal on active traffic channel.

During intervals in each frame when it is not required to transmit/Receive information on active traffic channel, terminal measuresStrength of signals received from surrounding BS.

Terminal reports measurements to its own BS on SACC.

MSC and BS control procedure by transmitting “measurementOrder” and “stop measurement order” messages to terminalsWith calls in progress.

“Measurement order” messages identify active channels inSurrounding cells. Terminal tunes to these channels andObserves signal strengths.

Terminal measures bit error rate (BER) and received signalStrength indication (RSSI).

MAHO monitors quality of signal received at terminal as well asAt BS. Can react promptly to signal-quality problems.

When signal at another BS exceeds current for some period Of time, MSC coordinates between two BS to switch user toNew BS. Switch requires terminal to tune into new channel.

Access TechniquesAccess Techniques

FREQUENCY

TIME

User 3

User 2

User 1

TDMA

TIME

FREQUENCY

CODE

CDMAo r “Spread Spectrum”

User 3

User 2

User 1

FREQUENCY

TIME

FDMA

1 2 3

Spatial Diversity

Cellular Systems

• Early Mobile Radio Systems– single high powered transmitters– good coverage, but impossible to reuse same frequency (e.g., Bell

Mobile System ‘70 -- max 12 calls over thousand sq. Miles)– it was impossible for the FCC to allocate new spectrum with the

growing demand

Cellular Systems

• Cellular Concept– technique of using a fixed a number of channels to serve an arbitrary large number of

subscribers by reusing channels throughout the coverage area– high system capacity in a limited spectrum– many low powered transmitters (small cells)– each base station allocated a portion of the spectrum– neighboring base stations assigned different groups of channels

Modeling radiation pattern

• Which one is the best model?

R RR R R

Desirable Topologiescluster size (N = 3)

Desirable Topologies

(N = 4)

Desirable Topologies

(N = 5)

Desirable Topologies

N = 7