6: Wireless and Mobile Networks6-1 89-850 Communication Networks: Wireless and Mobile Communication...

transcript

6: Wireless and Mobile Networks 6-1

89-850 Communication Networks: Wireless and Mobile Communication Networks

Prof. Amir HerzbergBIU, Dept. of CS

From ch.6 of Kurose and Ross, 3rd edition; and [KMK], ch. 8.

Background: Wireless and Mobile Networks

# wireless (mobile) phone subscribers now exceeds # wired phone subscribers!

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

Internet telephony: a reality, an earthquake Two important (but different) challenges

Wireless link: no CD (e.g. hidden-terminal), reliability, security

Mobility of computers and users; provisioning Plus: limited computing power and energy

Wireless and Mobile Communication Networks: Outline6.1 Introduction

Wireless 6.2 Wireless links,

characteristics 6.3 IEEE 802.11

wireless LANs (“wi-fi”)

Sensor and personal-area networks

Mobility 6.5 Principles:

addressing and routing to mobile users

6.6 Mobile IP 6.7 Cellular networks 6.8 Mobility and

higher-layer protocols

6.9 Summary

Elements of a wireless network

network infrastructure

wireless hosts laptop, PDA, IP phone run applications may be stationary

(non-mobile) or mobile wireless does not

always mean mobility

Elements of a wireless network

network infrastructure

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

Wireless network characteristicsLower Signal/Noise ratio (cf. wired networks)Limited, shared spectrum: orthogonal signals

(FDMA/CDMA/TDMA) or `collisions as noise`

Hidden terminal problem B, A hear each other B, C hear each other A, C can not hear each

A’s signalstrength

C’s signalstrength

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

interfering at B

Wireless Link CharacteristicsDifferences from wired link ….

Energy and computing-power limitations Decreased signal strength

• Obstacles and hidden-terminal problem• Collision detection hard or impossible

More noise• Interference from other sources• Multipath propagation different delays

interferences between paths or (multipath) fading

Lower signal/noise Higher bit error rate

…. more “difficult”

Hi/Low BER States Model Wireless links often have two BER states

High, Low E.g., due to (multipath) fading

Model by two-state Markov model:

Good Badg

Simplify: all packets Ok in `Good`, fail in `Bad`

Ad-hoc, sensor and personal-area networks

6.9 Summary

IEEE 802.11 Wireless LANs802.11WirelessLANs

5-6 GHz2.4-5 GHz

(unlicensed)

Up to 11Mbps

802.11blo-cost, good propagation;

but slow, interferences

Up to 54 Mbps

802.11a 802.11g

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

versions

802.11 LAN: Infrastructure and Ad-Hoc modes Infrastructure mode: wireless

host communicates with base station base station = access point

(AP) Basic Service Set (BSS) (aka

“cell”) contains:• wireless hosts (mobiles)• access point (AP): base

station Extended Service Set (ESS)

• One or more BSS • Connect by LANswitch or DS• DS=Distribution System

Ad hoc mode: hosts only• =Independent BSS (IBSS)

Internet

LAN switch

router

802.11 Media Access Coordination (MAC)

Point Coordination Function (PCF) Only in Infrastructure mode Access point coordinates transmissions Allows: bounded delay , QoS

Distributed Coordination Function (DCF) Ad-hoc or Infrastructure mode All are peers Like Ethernet, uses CSMA: random access, carrier sense Unlike Ethernet: Ack, no Collision Detection Optional: use RTS/CTS (Request/Clear To Send) No bound on delay (starvation possible) Our focus

IEEE 802.11 DCF Like Ethernet, uses CSMA:

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

Unlike Ethernet: Ack, no Collision Detection no collision detection – transmit all frames to completion ACK: to detect loss without collision detection

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 And… loss may be due to (higher) error rate of wireless

Goal: avoid collisions: CSMA/CA (Collision Avoidance)

802.11 DCF MAC Protocol: CSMA/CA[simplified]

802.11 sender1 if sense channel idle then

- transmit entire frame (no Colli. Detect)

2 if sense channel busy then - start random backoff timer- timer counts down while channel idle- transmit when timer expires

802.11 receiverif frame received OK then return ACK else ignore (no NACK!)SIFS: Short Inter-Frame Space – max time

to begin Ack [e.g., 16μsec in 802.11a]

sender receiver

SIFS(e.g.16s)

802.11 DCF MAC Protocol: CSMA/CA

802.11 sender (when trying to send)

1 if sense channel idle for DIFS then - transmit entire frame (no CD)[DIFS>SIFS+2Tprop for priority to ACK]

2 if sense channel busy then - count down the backoff timer- – but only while channel idle- transmit when timer expires

- If ACK, reduce backoff range by 1- if no ACK, double backoff range, select

time randomly from range, repeat 2

802.11 receiverif frame received OK - return ACK (within SIFS)

sender receiver

SIFS(e.g.16s)

Next MPDU

Contention Window

Defer Access Backoff after Defer

Src found channel idle for DIFS send data Other sender found channel busy

Wait for DIFS idle time, then exponential backoff delay [slot=Tprop]

802.11 DCF MAC OperationData Frames and their ACK

Two Additional Mechanisms in 802.11

PCF (Point Coordination Function) Polling to coordinate senders, e.g. to ensure

QoS SIFS < PIFS < DIFS (priorities!)

DCF with RTS/CTS mechanism Can’t detect collision while sending… Collision for long packet is wasteful RTS (Request to Send): request to reserve

channel to send long packet w/o collisions CTS (Clear to Send): approve RTS Optional mechanism

RTS/CTS [optional in 802.11 MAC] Sender sends small request-to-send (RTS)

RTSs may collide with each other (but are short) Include indication of length of packet transmission

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

sender transmits data frame other stations defer transmissions for time

specified in CTS Q: if you hear RTS only (no CTS), should you wait?

Avoid data frame collisions completely using small reservation packets!

Collision Avoidance: RTS-CTS exchange

RTS(A)RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

Q: Defer on RTS, CTS or both? Idea 1: RTS contains length, defer till

A B C D

CTSCTS

ACK ACK

end Problem: maybe not granted?

Idea 2: defer only on CTSRTS

A B C D

CTSCTS

ACK ACK

Idea 2: defer only on CTS What if unheard?

A B C D

CTSCTS

ACK ACK

Idea 2: defer only on CTS What if unheard?

Solution: Defer by CTS

• By length in CTS Defer by RTS…

But only 2 DIFS ! Ok if A hears either

recipient or sender

A B C D

CTSCTS

ACK ACK

802.11 DCF MAC Example

ACKData

RTS CTS Data ACK

backoff period

node defers; backoff counter frozen

Internetrouter

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

802.11 frame

R1 MAC addr AP MAC addr

dest. address source address

802.3 frame

802.11 addressing & `switching`

AP identified in 802.11frame (Unlike regular switch!!)

hub or switch

H1 BBS 2

802.11: mobility within same subnet

router H1 remains in same

IP subnet: IP address can remain same

Works fine for hub Switch: which AP is

associated with H1? self-learning switch

will see frame from H1 and “remember” port to reach H1

Solution: when H1 joins, AP2 sends switch a packet from H1

MAC Management, Beacons and Traffic Indication Map (TIM) 802.11 has several MAC management frames

(Re/De)Association req/response, Authentication… Beacon (sent periodically by AP)

Timestamp, Beacon Interval, Capabilities, SSID, Rates, Parameters, Traffic Indication Map (TIM)

Allows host to select AP (or host can send probe) TIM: list of (associated but sleeping) hosts with

packets queued at the access point. Even sleeping hosts (sometimes) listen to Beacon

• To check incoming messages in TIM, get broadcasts• Sleeping to save energy when idle

Ad Hoc Networking

Ad hoc networks no base stations transmit to other nodes within

link coverage nodes organize themselves into a

network: route among themselves Supported in 802.11 but still

many open issues, research WANET: Wireless Ad-Hoc NETwork MANET: Mobile Ad-Hoc Net (they

move, too!)

Mradius ofcoverage

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 Evolved from Bluetooth

Sensor Networks A special interesting type of Ad-Hoc network… Idea: distribute low-cost `sensors` to perform

measurements, even do actions Applications:

Weather forecasts, natural disaster warnings Detection of physical damages (leakage, fire,…) Military applications: intelligence, smart mines

Properties… Wireless Random location Low cost, energy

Which Transmission Range? When using AP/Bases, nodes must

reach it Large transmission range

But in sensor networks, WANET? Smaller transmission range Saves energy, allows spectrum reuse

(cellular?) But: requires routing, forwarding by nodes

Connectivity, Topology, Routing…

Assume nodes distributed uniformly in area [?] One dimensional (line), two (surface), three (space)

Let n be number of nodes Let r(n) be transmission range of node Questions:

Probability that all/most nodes are connected Probability that (almost) entire area is `covered` by

nodes [connected to `base`/`edge`] Number (and cost) of nodes

Routing, scheduling, broadcast protocols for nodes• Using minimal resources (energy, storage)• Minimize collisions

Sensor Network Tasks/Protocols Routing, forwarding, broadcast Neighbor/topology discovery, organization

E.g. setup spanning tree for efficient broadcast Optimization tasks

• Optimize communication• Load balancing (also to save energy)

Location measurement Clock synchronization

Use to save energy (listen only once per interval) Distributed computation

E.g. to detect image Handling mobility (MANET)

6.9 Summary

What is mobility?

spectrum of mobility, from the network perspective:

no mobility high mobility

mobile wireless user, using same access point

mobile user, passing through multiple access point while maintaining ongoing connections (like cell phone)

mobile user, connecting/ disconnecting from network using DHCP.

Covered in [KR], not in [KMK]

Mobility: Vocabularyhome network: permanent “home” of mobile(e.g., 128.119.40/24)

Permanent address: address in home network, can always be used to reach mobilee.g., 128.119.40.186

home agent: entity that will perform mobility functions on behalf of mobile, when mobile is remote

wide area network

correspondent

Mobility: more vocabulary

Care-of-address: address in visited network.(e.g., 79.129.13.2)

wide area network

visited network: network in which mobile currently resides (e.g., 79.129.13/24)

Permanent address: remains constant (e.g., 128.119.40.186)

Foreign agent: entity in visited network that performs mobility functions on behalf of mobile.

correspondent: wants to communicate with mobile

How do you contact a mobile friend:

I wonder where Alice moved to?

Consider friend frequently changing addresses, how do you find her? search all phone books?

call her parents? expect her to let you

know where he/she is?

Mobility: approaches

Let routing handle it: routers advertise permanent address of mobile-nodes-in-residence via usual routing table exchange. routing tables indicate where each mobile

located no changes to end-systems `breaks` routing (aggregation), allows MITM

Let end-systems handle it: indirect routing: communication from

correspondent to mobile goes through home agent, then forwarded to remote

direct routing: correspondent gets foreign address of mobile, sends directly to mobile

Mobility: approaches

Let routing handle it: routers advertise permanent address of mobile-nodes-in-residence via usual routing table exchange. routing tables indicate where each mobile

located no changes to end-systems

let end-systems handle it: indirect routing: communication from

correspondent to mobile goes through home agent, then forwarded to remote

direct routing: correspondent gets care-of-address of mobile, sends directly to mobile

not scalable

to millions of mobiles

Breaksroute

aggregation

AllowsMITM

attacks

Mobility: registration

End result: Foreign agent knows about mobile Home agent knows location of mobile

wide area network

home network

visited network

mobile contacts foreign agent on entering visited network

foreign agent contacts home agent home: “this mobile is resident in my network”

Mobility via Indirect Routing

wide area network

homenetwork

visitednetwork

correspondent addresses packets using home address of mobile

home agent intercepts packets, forwards to foreign agent

foreign agent receives packets, forwards to mobile

mobile replies directly to correspondent

Indirect Routing: comments Mobile uses two addresses:

permanent address: used by correspondent (hence mobile location is transparent to correspondent)

care-of-address: used by home agent to forward datagrams to mobile

foreign agent functions often done by mobile itself triangle routing: correspondent-home-network-

mobile inefficient when correspondent, mobile are in same network

Indirect Routing: moving between networks suppose mobile user moves to another

network registers with new foreign agent new foreign agent registers with home agent home agent update care-of-address for mobile packets continue to be forwarded to mobile

(but with new care-of-address) mobility, changing foreign networks

transparent: ongoing connections can be maintained!

Mobility via Direct Routing

wide area network

homenetwork

visitednetwork

41correspondent requests, receives foreign address of mobile

correspondent forwards to foreign agent

foreign agent receives packets, forwards to mobile

mobile replies directly to correspondent

Mobility via Direct Routing: comments

overcome triangle routing problem non-transparent to correspondent:

correspondent must get care-of-address from home agent what if mobile changes visited network?

wide area network

foreign net visited at session start

anchorforeignagent

new foreignagent

correspondentagent

correspondent

new foreignnetwork

Accommodating mobility with direct routing

anchor foreign agent: FA in first visited network data always routed first to anchor FA when mobile moves: new FA arranges to have

data forwarded from old FA (chaining)

Response (mobilecorresponding)

Triangle routing: mobile corresponding Using mobile host’s `home’ IP address Foreign network may block for `IP spoofing`

(egress filtering) Indirect via foreign: mobileFA corresp.

Requires FA (Foreign agent) to `spoof` Indirect via home: mobile

homecorresp. Overhead… but works

Direct: mobile corresponding Use temporary IP address (and mobile IP)

6.9 Summary

Mobile IP

RFC 3220 has many features we’ve seen:

home agents, foreign agents, foreign-agent registration, care-of-addresses, encapsulation (packet-within-a-packet)

three components to standard: indirect routing of datagrams agent discovery registration with home agent

Mobile IP: indirect routing

Permanent address: 128.119.40.186

Care-of address: 79.129.13.2

dest: 128.119.40.186

packet sent by correspondent

dest: 79.129.13.2 dest: 128.119.40.186

packet sent by home agent to foreign agent: a packet within a packet

dest: 128.119.40.186

foreign-agent-to-mobile packet

Mobile IP: agent discovery agent advertisement: foreign/home agents

advertise service by broadcasting ICMP messages (typefield = 9)

R bit: registration required

H,F bits: home and/or foreign agent

RBHFMGV bits reserved

type = 16

type = 9 code = 0 = 9

checksum = 9

router address

standard ICMP fields

mobility agent advertisement

extension

length sequence #

registration lifetime

0 or more available Care-Of-Addresses (COA)

0 8 16 24

Mobile IP: registration example

visited network: 79.129.13/ 24 home agent

HA: 128.119.40.7 f oreign agent

COA: 79.129.13.2 COA: 79.129.13.2

I CMP agent adv. Mobile agent MA: 128.119.40.186

registration req.

COA: 79.129.13.2 HA: 128.119.40.7 MA: 128.119.40.186 Lifetime: 9999 identification:714 ….

registration req.

COA: 79.129.13.2 HA: 128.119.40.7 MA: 128.119.40.186 Lifetime: 9999 identification: 714 encapsulation format ….

registration reply

HA: 128.119.40.7 MA: 128.119.40.186 Lifetime: 4999 Identification: 714 encapsulation format ….

registration reply

HA: 128.119.40.7 MA: 128.119.40.186 Lifetime: 4999 Identification: 714 ….

6.9 Summary

Mobile Switching

Center

Public telephonenetwork, andInternet

Mobile Switching

Center

Components of cellular network architecture

connects cells to wide area net manages call setup (more later!) handles mobility (more later!)

covers geographical region base station (BS) analogous to 802.11 AP mobile users attach to network through BS air-interface: physical and link layer protocol between mobile and BS

wired network

Multiple operators (providers)

correspondent

MSC MSC

wired public telephonenetwork

different cellular networks,operated by different providers

recall:

Handling mobility in cellular networks

home network: network of cellular provider you subscribe to (e.g., Sprint PCS, Verizon) home location register (HLR): database in

home network containing permanent cell phone #, profile information (services, preferences, billing), information about current location (could be in another network)

visited network: network in which mobile currently resides visitor location register (VLR): database with

entry for each user currently in network could be home network

Public switched telephonenetwork

mobileuser

homeMobile

Switching Center

HLR home network

visitednetwork

correspondent

Mobile Switching

Center

GSM: indirect routing to mobile

1 call routed to home network

home MSC consults HLR,gets roaming number ofmobile in visited network

home MSC sets up 2nd leg of callto MSC in visited network

MSC in visited network completescall through base station to mobile

Mobile Switching

Center

old BSSnew BSS

old routing

newrouting

GSM: handoff with common MSC

Handoff goal: route call via new base station (without interruption)

reasons for handoff: stronger signal to/from new

BSS (continuing connectivity, less battery drain)

load balance: free up channel in current BSS

GSM doesn’t mandate why to perform handoff (policy), only how (mechanism)

handoff initiated by old BSS

Mobile Switching

Center

old BSS

GSM: handoff with common MSC

new BSS

1. old BSS informs MSC of impending handoff, provides list of 1+ new BSSs

2. MSC sets up path (allocates resources) to new BSS

3. new BSS allocates radio channel for use by mobile

4. new BSS signals MSC, old BSS: ready

5. old BSS tells mobile: perform handoff to new BSS

6. mobile, new BSS signal to activate new channel

7. mobile signals via new BSS to MSC: handoff complete. MSC reroutes call

8 MSC-old-BSS resources released

home network

Home MSC

correspondent

anchor MSC

MSCMSC

GSM: handoff between MSCs

anchor MSC: first MSC visited during cal call remains routed

through anchor MSC

new MSCs add on to end of MSC chain as mobile moves to new MSC

Or: optional path minimization step to shorten multi-MSC chain

home network

Home MSC

correspondent

anchor MSC

MSCMSC

(b) after handoff

GSM: handoff between MSCs

anchor MSC: first MSC visited during cal call remains routed

through anchor MSC

new MSCs add on to end of MSC chain as mobile moves to new MSC

IS-41 allows optional path minimization step to shorten multi-MSC chain

Wireless, mobility: impact on higher layer protocols

logically, impact should be minimal … best effort service model remains unchanged TCP and UDP can (and do) run over wireless,

mobile … but performance-wise:

packet loss/delay due to noise, collisions, handoff

TCP interprets loss as congestion, will decrease congestion window un-necessarily

delay impairments for real-time traffic limited bandwidth of wireless links

Summary

Wireless wireless links:

capacity, distance channel impairments CDMA

IEEE 802.11 (“wi-fi”) CSMA/CA reflects

wireless channel characteristics

cellular access

Mobility principles: addressing,

routing to mobile users home, visited networks direct, indirect routing care-of-addresses

case studies mobile IP mobility in GSM

impact on higher-layer protocols

6: Wireless and Mobile Networks6-1 89-850 Communication Networks: Wireless and Mobile Communication...

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