Lecturer- Dr. Rama Rao

Data Communications and the Internet

Wireless Networks

Required Reading

Chapter 6, Computer Networking: A Top-Down Approach Featuring the Internet, James F. Kurose and Keith W. Ross, Addison-Wesley, third edition, 2005.

Local Area Networks (LANs)

LAN

products

fibertwisted pair

coaxair (wireless)

busStarringtree

FDDIEthernetToken ringWireless LAN

token passing polling

IEEEISO, ATM Forum

MACprotocols

standardbodies

topologies

transmissionmedia

Wireless Networks The boom in wireless communications in recent

years has been exponential and the wireless communications landscape is changing rapidly.

Number of wireless (mobile) phone subscribers now exceeds the number of wired phone subscribers !!

computer nets: laptops, palmtops, PDAs, Internet-enabled phone promise anytime untethered Internet access

Challenges/Issues Mobility management QoS Management Channel conditions Security

Elements of a wireless network

network infrastruc

ture

wireless hosts laptop, PDA, IP

phone run applications may be stationary

(non-mobile) or mobile wireless does not

always mean mobility

base station typically

connected to wired network

relay -responsible for sending packets between wired network and wireless host(s) in its “area” e.g., cell

towers 802.11 access points

Elements of a wireless network

network infrastructure

wireless link typically used to

connect mobile(s) to base station

also used as backbone link

multiple access protocol coordinates link access

various data rates, transmission distance

Comparison of selected wireless link standards

384 Kbps

56 Kbps

54 Mbps

5-11 Mbps

1 Mbps802.15

802.11b802.11{a,g}

IS-95 CDMA, GSM

UMTS/WCDMA, CDMA2000

.11 p-to-p link

2G

3G

Indoor

10 – 30m

Outdoor

50 – 200m

Mid rangeoutdoor

200m – 4Km

Long rangeoutdoor

5Km – 20Km

Wireless Networks

1001010.1 Mbps

Vehicle

Walk

Fixed

Walk

FixedIndo

or

O

utdo

or

GSM

, CD

MA

WLANWPAN

LAN

WAN3G2G

WPAN – Wireless Personal Area Network

Elements of a wireless network

Ad hoc mode no base stations nodes can only

transmit to other nodes within link coverage

nodes organize themselves into a network: route among themselves

Wireless Link CharacteristicsDifferences from wired link ….

decreased signal strength: radio signal attenuates as it propagates through matter (path loss)

interference from other sources: standardized wireless network frequencies (e.g., 2.4 GHz) shared by other devices (e.g., phone); devices (motors) interfere as well

multipath propagation: radio signal reflects off objects ground, arriving ad destination at slightly different times

…. make communication across (even a point to point) wireless link much more “difficult”

Wireless network characteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access):

AB

C

Hidden terminal problem B, A hear each other B, C hear each other A, C can not hear each othermeans A, C unaware of their

interference at B

A B C

A’s signalstrength

space

C’s signalstrength

Signal fading: B, A hear each other B, C hear each other A, C can not hear each other

interferring at B

Wireless Local Area Networks (WLANs)

WLANs-Motivation Mobile users’ typical demands of information access is

characterized by heavy data files and applications; WLAN can provide mobility and speed at the same time

In major structured hot spots such as airports and rail stations, the mobile radio infrastructure support of data communications seems to be inadequate and expensive

For office users, mobility, simple and low-cost network scalability, and high-speed access are advantageous factors

For home users, advantages of mobility without new wiring and at the same time high-speed access are the key issues

WLAN provides network flexibility: No infrastructure (ad hoc); a single-cell network (BSS); a cellular topology (ESS)

Use of unlicensed spectrum reduces the user’s costInformation extracted from Advanced Communication Networks- Elec 5509

Wireless LAN Requirements Same as any LAN

High capacity, short distances, full connectivity, broadcast capability

Throughput: efficient use wireless medium Number of nodes:Hundreds of nodes across multiple cells Connection to backbone LAN: Use control modules to connect to both

types of LANs Service area: 100 to 300 m Low power consumption:Need long battery life on mobile stations

Mustn't require nodes to monitor access points or frequent handshakes

Transmission robustness and security:Interference prone and easily eavesdropped

Collocated network operation:Two or more wireless LANs in same area License-free operation Handoff/roaming: Move from one cell to another Dynamic configuration: Addition, deletion, and relocation of end

systems without disruption to users

Wireless LAN

Why Wireless LANs?

Mobility

Flexibility

No cables, save cost

Fast installation

Problems:Limited bandwidth

Noisy channel

Multipath

Security

Power consumption

IEEE 802.11 Wireless LAN - WiFi

802.11b 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

widely deployed, using base stations

802.11a 5-6 GHz range up to 54 Mbps

802.11g 2.4-5 GHz range up to 54 Mbps

All use CSMA/CA for multiple access

All have base-station and ad-hoc network versions

802.11 LAN architecture wireless host communicates

with base station base station = access point

(AP) Basic Service Set (BSS) (aka

“cell”) in infrastructure mode contains: wireless hosts access point (AP): base

station• “Infrastructure WLAN”

ad hoc mode: hosts only• No central control, no

connections to the outside world.

BSS 1

BSS 2

Internet

hub, switchor routerAP

AP

Wireless Cells 802.11 has 11 channels Channels 1, 6, and 11 are

non-overlapping Each AP coverage area is

called a “cell” AP admin chooses

frequency for AP interference

possible: channel can be same as that chosen by neighboring AP!

host: must associatewith an AP

Wireless nodes can roam between cells

AP

AP

AP AP

AP AP

Channel 1

Channel 6

Channel 1Channel 11

Channel 6

Channel 1

IEEE 802.11: Physical Layer

Radio• Experience gained from

building mobile phone andwireless WAN networkscan be reused.

• Large coverage and deeppenetration.

• Very limited license-freefrequency bands.

• Very noisy.

Infrared• Cheap.• No licenses needed.• Interference by

sunlight, heat sources.• Low bandwidth.• Signals may be blocked

by many surroundingobjects.

IEEE 802.11: multiple access Like Ethernet, uses CSMA:

random access carrier sense: don’t collide with ongoing transmission

Unlike Ethernet: no collision detection – transmit all frames to completion acknowledgment – because without collision detection, you

don’t know if your transmission collided or not Why no collision detection?

difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

can’t sense all collisions in any case: hidden terminal, fading Goal: avoid collisions: CSMA/C(ollision)A(voidance)

IEEE 802.11 MAC Protocol: CSMA/CA802.11 sender1 if sense channel idle for DIFS then

- transmit entire frame (no CD)2 if sense channel busy then

- start random backoff time- timer counts down while channel idle- transmit when timer expires- if no ACK, increase random backoff

interval, repeat 2802.11 receiverif frame received OK

- return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

DIFS: Distributed Inter-frame Space

SIFS: Short Inter-frame Space

framecontrol duration address

1address

2address

4address

3 payload CRC

2 2 6 6 6 2 6 0 - 2312 4seq

control

802.11 frame: addressing

Address 2: MAC addressof wireless host or AP transmitting this frame

Address 1: MAC addressof wireless host or AP to receive this frame

Address 3: MAC addressof router interface to which AP is attached

Address 3: used only in ad hoc mode

framecontrol duration address

1address

2address

4address

3 payload CRC

2 2 6 6 6 2 6 0 - 2312 4seq

control

Type FromAPSubtype To

APMore frag WEPMore

dataPower

mgtRetry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

802.11 frame: moreduration of reserved transmission time (RTS/CTS)

frame seq #(for reliable ARQ)

frame type(RTS, CTS, ACK, data)

Hidden Station Problem

When A is transmitting aframe to B, since D is notin A’s coverage, D is ahidden station that Ddoesn’t sense a busychannel, thus D may starta transmission thatcollides with A’stransmission

Hidden Station Problem in Wireless LANs:

A BC D

A’s coverageB’s coverage

Exposed Station problem

While A is transmitting aframe to C, B senses a busychannel and concludes thatit may not transmit anyframe to D which isincorrect

Exposed Station Problem in Wireless LANs:

A BC D

A’s coverageB’s coverage

RTS/CTS

idea: allow sender to “reserve” channel rather than random access of data frames: avoid collisions of long data frames

optional; not typically used sender first transmits small request-to-send (RTS) packets

to AP using CSMA RTSs may still collide with each other (but they’re short)

AP broadcasts clear-to-send CTS in response to RTS CTS heard by all nodes

sender transmits data frame other stations defer transmissions

Avoid data frame collisions completely using small reservation packets!

Solution to Hidden/Exposed Station Problem

Collision Avoidance: RTS-CTS exchange

APA B

time

DATA (A)

reservation collision

defer

Next…..

Wide Area Network (WAN) Technology

Development of WANs - X.25 protocol X.25 is the standard protocol for the interface between an end

system and a packet-switching network. The X.25 protocol is a (ITU) recommendation that defines

connections of terminals and computers to packet-switching networks. X.25 is an example of a connection-oriented network, was the first

public data network. The protocol was standardized in 1976 and revised a number of times.

The CCITT (ITU) revised the standard in 1992 and boosted the speed to 2 Mbits/sec.

X.25 is a well-established (and now somewhat dated) packet-switching service traditionally used to connect remote terminals to host systems.

The service provides any-to-any connections for simultaneous users. Signals from multiple users on a network can be multiplexed through

the X.25 interface into the packet-switched network and delivered to different remote sites.

The X.25 interface supports line speeds up to 64 Kbits/sec.

Frame Relay Designed to be more efficient than X.25. Frame relay offers many of the same connection-oriented, packet-

switched network services as X.25, but with better performance. Frame relay is a metropolitan and wide area networking solution

that implements a form of packet-switching technology. It routes frames of information from source to destination over a

switching network owned by a carrier such as Telstra. The essence of frame relay is that it is a connection oriented

network with no error control and no flow control. Frame relay was developed to utilize the higher data rates and

lower error rates of contemporary networking facilities Developed before ATM (Asynchronous Transfer Mode) and larger

installed base than ATM. ATM now of more interest on high speed networks.

Frame Relay: Protocol Architecture

• Control Plane: establishment and termination of logical connections

• these protocols are between a scriber and the network

• User Plane: transfer of user data between subscribers

• these protocol provide end-to-end functionality.

Frame Relay: Control Plane Between subscriber and network. Separate logical channel used.

Similar to common channel signaling for circuit switching services.

At data link layer, LAPD (Link Access Procedure for Data services) protocol is used for: Provide reliable data link control. Error and flow control between user and network.

Frame Relay: User Plane Actual transfer of information between end users. LAPF (Link Access Procedure for Frame Mode Bearer

Services) protocol is used for: Frame delimiting, alignment and transparency. Frame multiplexing/demultiplexing using addressing field. Ensure frame is integral number of octets. Ensure frame is neither too long nor short. Detection of transmission errors. Congestion control functions.

User Data Transfer One frame type used for carrying user data.

No control frame. No in-band signaling. No sequence numbers.

No flow control and no error control.

Frame Relay: User Data Transfer

only have one frame type which carries user data

no control frames means no inband signaling no sequence numbers

flag and FCS function as in HDLC address field carries DLCIDLCI (Data Link Connection Identifier) has

local significance only

Frame Relay: Frame Formats

ATM a streamlined packet transfer interface similarities to packet switching

transfers data in discrete chunks supports multiple logical connections over a single physical

interface ATM uses fixed sized packets called cells with minimal error and flow control data rates of 25.6Mbps to 622.08Mbps ATM is a high-speed network technology that is

designed for both LAN and WAN use.

ATM: Protocol Architecture

Relation to Frame Relay and ATM Frame Relay and ATM have displaced X.25 for

enterprise WAN packet network connections Both are based on packet switching Frame Relay:

Is a very simple Layer 2 protocolHas low overhead

• Short 2-byte frame header• No numbering and ACKing

Operates at high speeds Its price is largely independent of distance Does not require special CPE hardware

Relation to Frame Relay and ATMATM is a Layer 1 and 2 protocolData is packed into an ATM frame, which

has no header The Layer 2 frame is segmented into cells The cells are transmitted across the provider

network• >10% overhead due to cell headers

ATM operates at high speeds Price is largely independent of distance

Relation to Frame Relay and ATM ATM requires special CPE equipment An ATM DTE can participate in many concurrent

virtual circuits ATM supports PVCs (permanent virtual circuit) and

SVCs (switched virtual circuit) ATM handles quality of service very well. It can

support voice and video as well as data. Many Frame Relay service networks are

implemented by converting incoming Frame Relay traffic to ATM This enables Frame Relay service providers to

offer quality of service that approximates ATM QoS

Next…….

M radius ofcoverage

S

SS

P

P

P

P

M

S

Master device

Slave device

Parked device (inactive)P

802.15: Personal Area Network less than 10 m diameter replacement for cables (mouse,

keyboard, headphones) ad hoc: no infrastructure master/slaves:

slaves request permission to send (to master)

master grants requests 802.15: evolved from Bluetooth

specification 2.4-2.5 GHz radio band up to 721 kbps

UWB (802.15.3) RFID (Radio Frequency Identification)

Broadband Wireless - WiMax WiMAX (Worldwide interoperability for Microwave Access) is

an exciting and unique technology that delivers high-speed, broadband fixed and mobile services wirelessly to large areas with much less infrastructure than is needed today.

What's WiMAX all about?

Next-generation wireless technology that delivers significant improvements in speed, throughput and capacity

Expected to be in widespread use within the next three to five years

Based on the IEEE 802.16 standards for wireless metropolitan-area networks (MANs) to extend coverage to today's existing networks

WIRELESS TECHNOLOGIES - COMPARISON

3G Cellular WLAN/802.11n WiMAX/802.16e

Coverage Ubiquitous Hotspot/Campus Zone/regionalQoS Controlled Shared spectrum ControlledMobility Full Very limited Fixed/FullRange Miles 100-500 ft MilesUser Speed 50–500 kbps 1-10 Mbps 1-10 MbpsArchitecture Hierarchical Flat, IP Flat, IP

The user’s view of a wireless world

The 802 Wireless Space

Next…….

Residential Area Networks

PSTN: Public Switched Telephone Network

ISDN: Integrated Service Digital Network

ADSL: Asymmetric Digital Subscriber

Cable modem: CATV

HFC: Hybrid Fiber Coax

Public Switched Telephone Network

• PSTN provides dedicated communication link between the computer and the server.

• Using an existing telephone line with a modem to provide a data communication between a computer in the residential area and a WAN server.

PSTNserver

56kbps

The Internet

home

• ISDN (Integrated Service Digital Network) provides dedicated end-to-end digital communication link between the computer and the server.

• Same as PSTN but the data communication is a digital one.

Integrated Switched Digital Network

PSTN

server

n*64kbps

The Internet

home

ISDN Features

PBX

LAN

Subscriber loop with ISDN channel

structure

Customer ISDN interface

ISDN Circuit switching network

Other networks

Packet switching network

Digital pipes to other networks and

services

...

...

...

ISDN• ISDN is intended to be a worldwide public telecom network to

replace existing PSTN.

• Narrowband ISDN: First generation specification (contributed to Frame relay).

• Broadband ISDN: Second generation specification (contributed to ATM) and supports very high data rate.

• Support of voice and non-voice applications using a limited set of standardized facilities.

• Support for switched and non-switched applications.

• Reliance on 64-kbps connections.

• Layered protocol architecture.

• Variety of configurations.

Asymmetric Digital Subscriber Line (ADSL)

• Modem technology designed to provide high-speed digital data transmission over ordinary telephone wire.

• Link between subscriber and a network (local exchange).– Local loop.

• Uses currently installed twisted pair cable.– Can carry broader spectrum (1 MHz or more).

• Asymmetric.– Greater capacity downstream than upstream.

• Frequency division multiplexing (FDM).– Lowest 25kHz for voice

• Plain old telephone service (POTS)– Use echo cancellation or FDM to give two bands.

• Range of up to 5.5km.

Asymmetric Digital Subscriber Line: Using existing telephone lines, but with a pair of special devices at the computer and the server.

ATU: ADSL Transmission Unit POTS: Plain Old Telephone Service

The Internet

server

PSTNPOTS

Home

16-640kbps

1.544M-6MbpsPCM

ATU-CATU-R

Asymmetric Digital Subscriber Line

Community Antenna TV (CATV)

Coax

Coax

CATVHead-End

Trunk splitter

…

Residential area

• Distribute broadcast television (and radio) programs.

• Based entirely on coaxial cable.

• TV programs are multiplexed using FDM.

Cable modem: Using part of the CATV bandwidth to provide data service.

CATV Bandwidth Usage

52MHz 550MHz 750MHz

frequency

Control

Video Channels

Delivered Bandwidth

Unused

Hybrid Fiber Coax (HFC)CATV

Head-End

server

TheInternet

fiber

coax

RF Amplifiers

Fiber Node…

Residential area• A network which incorporates both optical fiber along with coaxial

cable to create a broadband network• HFC provides a relatively high data rate for users.• The use of existing infrastructure leads to a cheap solution (even

cheaper than ADSL) for high data rate access to the Internetfrom residential areas.

• The cable is to be shared by all users. The data rate per user maydrop during peak hours.

Network Technologies

PSTN

Internet

Cellular Networks

LAN

PBX

WirelessData Networks

WLAN

CableNetworks

References Computer Networking: A Top-Down Approach Featuring the

Internet, James F. Kurose and Keith W. Ross, Addison-Wesley, third edition, 2005.

Computer Networks, 4/e, Andrew S. Tanenbaum, Prentice Hall. Data and Computer Communications, William Stallings, Prentice

Hall. www

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