Chapter 6

www.ischool.drexel.edu

INFO 331Computer Networking

Technology II Chapter 6

Wireless Networking

Glenn Booker1INFO 331 chapter 6


Wireless & Mobile Networks

• The number of mobile devices has grown immensely in the last few years– 34 million cell phones worldwide as of 1993– 2 billion [ITU] as of 2005, many Internet-aware– 4 billion by the end of 2008

• Distinguish between wireless connectivity and the mobility that affords – Some wireless devices are stationary

2INFO 331 chapter 6


Wireless & Mobile Networks

• Challenges for this context include– Establishing and maintaining a wireless

connection– Handing off a wireless client from one part of

the network to another

• Some terminology– Wireless host is the end user’s device

connected to the network– Wireless links are analogous to the wired

variety

3INFO 331 chapter 6


Terminology

– A base station communicates with the wireless hosts; e.g. cell towers for cell phones, and access points for wireless computers

• Base stations connect to the rest of the network, either through wired or other wireless links

• Infrastructure versus ad hoc mode– When a wireless host connects in infrastructure

mode, it relies on the network for address resolution, routing, etc.

– In ad hoc mode, the host performs those functions

4INFO 331 chapter 6


Terminology

• When a host changes from one base station to another, the change of attachment is a handoff

• Can categorize wireless networks by the number of wireless hops (one or more), and whether it uses infrastructure (e.g. a base station)– Single hop, with infrastructure – is typical of a

local wireless connection to a wired network

5INFO 331 chapter 6


Terminology

– Single hop, no infrastructure – like Bluetooth or ad hoc 802.11 networks

– Multi-hop, with infrastructure – needs a wireless relay to get to the wired world, like a wireless mesh network

– Multi-hop, no infrastructure – typically has mobile nodes as well as hosts; MANETs (mobile ad hoc networks) and vehicle versions, VANETs are in this category

6INFO 331 chapter 6


Wireless Links

• If a simple wired Ethernet link is replaced by a wireless connection– The hub or switch would be replaced by an

access point– The host needs a wireless network card– The Ethernet cable goes in the closet

• So how does this affect service?

7INFO 331 chapter 6


Wireless Links Problems

• Key impacts of changing to wireless are– Decreasing signal strength with distance from

the access point– Interference from other sources in the same

frequency range– Multipath propagation – signals can bounce

around, giving echoes (like talking at edge of Grand Canyon)

• This results in much higher, and more variable, bit error rates (BER)

8INFO 331 chapter 6


Wireless Links Problems

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

AB

C

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 f ading: B, A hear each other B, C hear each other A, C can not hear each other

interfering at B

A B C


space


A B CA B C


space


Signal f ading: B, A hear each other B, C hear each other A, C can not hear each other

interfering at B

Images from Kurose’s slides

9INFO 331 chapter 6


CDMA

• Last term we covered three approaches to sharing links (multiple access)– Channel partitioning (TDM and FDM)– Random access protocols (ALOHA & CSMA)– Taking turns protocols (polling or token ring)

• Here we need another type of multiple access protocol – Code Division Multiple Access (CDMA)

10INFO 331 chapter 6


CDMA

• In CDMA, the original data stream is multiplied by a code which changes much faster than the data, the chipping rate– In the following example, for every bit of

incoming data, the code has eight values (11101000)

– The data*code product is sent over the link– The receiver undoes the code, and recovers

the original signal



CDMA Example

slot 1 slot 0

d1 = -1

1 1 1 1

1- 1- 1- 1-

Zi,m= di.cm

d0 = 1

1 1 1 1

1- 1- 1- 1-

1 1 1 1

1- 1- 1- 1-

1 1 11

1-1- 1- 1-

slot 0channeloutput

slot 1channeloutput

channel output Zi,m

sendercode

databits

slot 1 slot 0

d1 = -1d0 = 1

1 1 1 1

1- 1- 1- 1-

1 1 1 1

1- 1- 1- 1-

1 1 1 1

1- 1- 1- 1-

1 1 11

1-1- 1- 1-

slot 0channeloutput

slot 1channeloutputreceiver

code

receivedinput

Di = Zi,m.cmm=1

M

M

slot 1 slot 0

d1 = -1

1 1 1 1

1- 1- 1- 1-

d1 = -1

1 1 1 1

1- 1- 1- 1-

1 1 1 1

1-1- 1-1- 1-1- 1-1-

Zi,m= di.cm

d0 = 1

1 1 1 1

1- 1- 1- 1-

d0 = 1

1 1 1 1

1- 1- 1- 1-

1 1 1 1

1-1- 1-1- 1-1- 1-1-

1 1 1 1

1- 1- 1- 1-

1 1 1 1

1-1- 1-1- 1-1- 1-1-

1 1 11

1-1- 1- 1-

1 1 11 1 11

1-1- 1- 1-

11

1-1-1-1- 1- 1-1-1- 1-1- 1-1-

slot 0channeloutput

slot 1channeloutput

channel output Zi,m

sendercode

databits

slot 1 slot 0

d1 = -1d1 = -1d0 = 1d0 = 1

1 1 1 1

1- 1- 1- 1-

1 1 1 1

1- 1- 1- 1-

1 1 1 1

1- 1- 1- 1-

1 1 1 1

1-1- 1-1- 1-1- 1-1-

1 1 1 1

1- 1- 1- 1-

1 1 1 1

1-1- 1-1- 1-1- 1-1-

1 1 1 1

1- 1- 1- 1-

1 1 11

1-1- 1- 1-

1 1 1 1

1- 1- 1- 1-

1 1 1 1

1-1- 1-1- 1-1- 1-1-

1 1 11

1-1- 1- 1-

1 1 11 1 11

1-1- 1- 1-

11

1-1-1-1- 1- 1-1-1- 1-1- 1-1-

slot 0channeloutput

slot 1channeloutputreceiver

code

receivedinput

Di = Zi,m.cmm=1

M

M

Di = Zi,m.cmm=1

M

M



CDMA

• So how does this help??– Interfering signals add onto the signal you

want to receive– If the code is chosen properly, the desired

signal can be picked out of the sum of your signal plus garbage

• It’s kind of like being able to follow one conversation in a crowded room



802.11 LAN Protocols

• The WiFi or 802.11 protocols are used for local wireless networks

• 802.11a and 802.11g are most common– Both provide service at up to 54 Mbps– 802.11a operates at 5.8 GHz– 802.11g operates at 2.4 GHz

• All use CSMA/CA as medium access protocol, have the same frame structure







• All 802.11 protocols can slow themselves down for longer distances, or to deal with interference

• All can use infrastructure or ad hoc mode

• They differ at the physical layer




• Both 2.4 (for .11b and g) and 5.8 GHz (.11a) frequency ranges have disadvantages– 2.4 GHz has more interference from cell

phones and microwave ovens– 5.8 GHz needs more power for a given

distance, and suffers more from multipath propagation




• Notice each band is a range of frequencies (technically 2.4 – 2.485 and 5.1 – 5.8 GHz); typically have 11 channels in that range




• 802.11n is being standardized– Uses two or more antennae to send and

receive, and should be over 200 Mbps

• What wavelength are the 802.11 bands? = c = 3E10 cm/s = c/

• For 2.4 GHz, = 3E10 cm/s / 2.4E9 s-1 = 12.5 cm• For 5.8 GHz, = 3E10 cm/s / 5.8E9 s-1 = 5.2 cm



802.11 Architecture

• A basic service set (BSS) is an access point (base station) and one or more wireless hosts

• The access points for various BSSs are connected to each other via hubs, switches, or routers

• Every wireless adapter has a 6 byte MAC address, and the access point has a MAC address– Again, MAC addresses are managed by IEEE



802.11 LAN Architecture

BSS 1

BSS 2

I nternet

hub, switchor routerAP

AP

BSS 1

BSS 2

I nternetI nternet

hub, switchor routerAPAP

APAP



802.11 Architecture

• In infrastructure mode, the access points are essential elements

• In ad hoc mode, there are no access points, and wireless devices communicate independently– This could be used to network with another

laptop directly, for example– The outside world isn’t visible in ad hoc mode



Channels & Association

• In infrastructure mode, need to associate with an access point before data can be sent or received

• Each access point is given a Service Set Identifier (SSID), and channel– The SSID is a readable name, like ‘sixflags’– Channels 1-11 are available, but only

channels 1, 6, and 11 are non-overlapping



It’s a jungle out there!

• A Wi-Fi jungle is when you can choose from multiple access points (APs), possibly using the same channels– Could occur downtown, where many cafés

and local networks could intersect

• How tell the networks (APs) apart?– Each AP sends beacon frames periodically,

with the AP’s SSID and MAC address– You choose which AP with which to associate



Passive vs Active scanning

• When access points broadcast their presence, and you merely look for their beacon frames, this is passive scanning

• A wireless host can also broadcast a signal to look for APs, this is active scanning



After association

• Once an AP has been selected for association, generally DHCP is used to get an IP address, find DNS servers, etc.

• To be allowed to associate, might have to authenticate the host– Can specify which MAC addresses are

allowed to associate– May require logging into the network, verify

identity with a Radius or Diameter server26INFO 331 chapter 6


802.11 Multiple Access Control

• Ethernet has been very successful – Recall it used CSMA/CD – carrier sense

multiple access with collision detection– Wait for a pause in traffic before transmitting,

and sense when a collision occurs

• 802.11 uses a variation of this – CSMA/CA– Collision avoidance instead of detection– Also adds link-layer acknowledgement &

retransmission (ARQ)27INFO 331 chapter 6


802.11 Collision Avoidance

• Why no collision detection?– It requires ability to send and receive at the

same time - here the received signal is weak compared to the sent signal, so it’s expensive to make hardware to do this

– The hidden terminal problem and fading make it impossible to detect all collisions

• So 802.11 always transmits a full frame – Unlike Ethernet, it won’t stop mid-transmission



802.11 ARQ

• To transmit data from sender to receiver:– Sender waits a short time period DIFS

(distributed inter-frame spacing)– Sender transmits the data using CSMA/CA– Data gets to receiver– Receiver validates integrity of data with CRC– Waits a time SIFS (short inter-frame spacing)– The receiver sends an ACK



802.11 ARQsender receiver

DI FS

data

SI FS

ACK

sender receiver

DI FS

data

DI FSDI FS

datadata

SI FS

ACK

SI FS

ACKACK



802.11 ARQ

• 802.11 uses CRC to check for bit errors– You recall the cyclic redundancy check, right?

• If channel is busy when a transmission is ready– Wait a random time of idle channel, and

transmit when the channel is idle; don’t count down when the channel is busy

– Why? This avoids collisions when multiple hosts are waiting for a clear channel



802.11 ARQ

• So in wireless communication, it’s all about AVOIDING COLLISIONS!

• If the source doesn’t get an ACK within some time, it retransmits

• If some number of retransmissions aren’t ACKed, discard the frame



802.11 Reservation Scheme

• There is an optional scheme to avoid collision even when there are hidden hosts

• It’s very polite – each host asks for permission to transmit– Sort of like the polling protocols

• Sender sends a Request To Send (RTS) frame to the AP

• AP broadcasts a Clear To Send (CTS) frame to reserve use of channel by that sender



802.11 Reservation Scheme

• Sender then transmits exclusively during that time period – other hosts know from getting the CTS to be quiet

• This is very effective at avoiding collisions, but has time overhead to exchange RTS and CTS messages– Often used for sending large data files– May establish threshold, so only files larger

than threshold are allowed to use RTS/CTS34INFO 331 chapter 6


802.11 point-to-point

• Using directional antennae, the 802.11 protocols can be used up to 80 kilometers of distance– This was done in India, for example



802.11 Frames

• A frame in 802.11 consists of 34 bytes of header and trailer, plus 0 to 2312 bytes of data (payload)– Data generally limited to 1500 bytes due to

Ethernet limit– Data is usually an IP datagram or ARP packet



802.11 Frame Fields

– Frame control (2 B, shown on next slide)– Duration (2 B) for timeout or CTS period– Address 1 (6 B) MAC of destination node– Address 2 (6 B) MAC of transmitting node– Address 3 (6 B) MAC of router leaving this BSS– Sequence control (2 B) just like in TCP– Address 4 (6 B) used only for ad hoc networks– Payload (data) (0-2132 B)– CRC code (4 B) [size verified here]



802.11 Frames

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

duration of reserved transmission time (RTS/ CTS)

f rame seq #(for reliable ARQ)

f rame type(RTS, CTS, ACK, data)

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt


2 2 4 1 1 1 1 1 11 1

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt


2 2 4 1 1 1 1 1 11 1

duration of reserved transmission time (RTS/ CTS)

f rame seq #(for reliable ARQ)

f rame type(RTS, CTS, ACK, data)

bits

bytes



802.11 Frame Fields

• The sizes for frame control parts are in bits (total 16 bits = 2 bytes)– The Type field also distinguishes association

frames from normal data frames– WEP is an encryption mode

• The duration field can be the timeout interval, or time for a clear to send (CTS)

• Address 3 is critical for communicating across wireless networks



802.11 Frame Fields

• Sequence numbers are also used to tell multipath echoes apart, in addition to detecting retransmissions

• Address 4 is only used for ad hoc networks

• The CRC field (4 B, not 2) is particularly important, since there is a large chance of bit errors

• We’ll ignore the other fields for now



Mobility within IP subnet

• If a host moves between BSS’ within the same subnet (i.e. they are not connected by a router), it’s relatively easy for the handoff from one AP to another to occur

• If the BSS’ are connected by a hub, there’s no problem – the host disassociates from one AP and associates with another



Mobility within subnet

• If the BSS’ are connected by a switch, the self-learning features of switches is too slow to keep up well– The new AP has to send a broadcast Ethernet

message to update the switch with the new association

• An 802.11f standards group was working on this issue – standard was withdrawn 2/06



Advanced 802.11 Features

• 802.11 hints at supporting added features– Adapt transmission rate, depending on the

SNR (signal to noise ratio) and other channel characteristics (e.g. lost frames)

– Power management, by limiting the time various functions are on; done by putting itself to sleep

• It can tell its access point it’s asleep, so frames aren’t sent to it until it wakes up!



802.15 WPAN

• The 802.11 standards are designed for wireless communication up to 100 meters

• The 802.15 wireless personal area network (WPAN) is for ad hoc wireless networking with a range of about 10 meters

• Based on Bluetooth, it’s designed to handle up to eight ‘active’ local devices near a host in a piconet, controlled by a master node



802.15 WPAN

• The master node decides which devices are active or parked– Can have up to 255 parked devices

• Operates at 2.4 GHz using TDM with slot of 625 s, and 79 channels

• Hops randomly across channels (frequency-hopping spread spectrum, or FHSS)

• Data rates up to 4 Mbps



WiMAX

• WiMAX is World Interoperability for Microwave Access, IEEE 802.16

• It uses a base station to coordinate sending and receiving packets, similar to 802.11 infrastructure mode, using TDM

• Each frame defines the physical layer properties for later packets; hence the transmission approach can change to get the best reception possible



WiMAX

• The transmission time allocated to each subscriber can be controlled

• WiMAX uses a connection identifier in the packet to allow quality of service (QoS) to be customized– MAC addresses are mapped to the

connection identifiers

• WiMAX is a complex beast, and is changing rapidly



Cellular Internet Access

• Since Wi-Fi is limited to about 100 meters, how do we connect to the Internet when far from an access point?– Use your cell phone!

• Key concerns are:– Is it fast?– Is it reliable?– Is it going to be better than a long distance

wireless LAN?48INFO 331 chapter 6


GSM Cellular Architecture

• Cellular architecture is broken into … cells

• Each cell is a geographic area served by a cell tower, which routes through a mobile switching center (MSC)– Acts like a switching center or central office

• The center is connected to the Internet directly, and/or the phone system (Public Switched Telephone Network)



Cellular Architecture

Mobile Switching

Center

Public telephonenetwork, andI nternet

Mobile Switching

Center

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

MSC

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

cell

wired network

Mobile Switching

Center


Mobile Switching

Center

Mobile Switching

Center

Mobile Switching

Center


Mobile Switching

Center

Mobile Switching

Center

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

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

MSCMSC

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

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

cellcell

wired networkwired network



Sharing Frequencies

• Each cell tower handles many calls simultaneously, so multiple access protocols are needed– Combined FDM and TDM– CDMA (code division, not carrier sense)



Cell Technology Generations

• The standards used for communication between cell phones and cell towers are grouped by the generation of technology involved

• First Generation (G1) was the analog FDMA phone, now nearly dead in the US

• Second Generation (G2) was the start of digital phone service



Second Generation

• Second generation cell phones used– IS-136, a combined FDM/TDM derived from

FDMA– GSM, a European-initiated FDM/TDM, now

widely used in North America– IS-95, a CDMA-based approach from

Qualcomm

• To bridge the gap to third generation, generation 2.5 was developed



Generation 2.5

• Generation 2.5 includes– GPRS, an upgrade from GSM which uses

circuit switching (slow and inefficient for Internet); max data rate only 9.6 kbps

– EDGE, was to replace GSM/GPRS and crank data rate up to 384 kbps

– CDMA2000, an upgrade of IS-95 to get up to 144.4 kbps, also called 1xRTT



3G

• 3G cell technology claims at least 2 Mbps indoors, and 384 kbps outdoors

• Is really UMTS/HSDPA, but that’s too long!

• Runs on multiple frequencies: 850, 1900, and 2100 MHz*

* http://hspa.gsmworld.com/ and http://www.apple.com/iphone/specs.html



Generations 3 and 4

• Third generation cellular technology includes– UMTS, a GSM upgrade by Cingular and T-mobile to

get realistic speeds of 300-400 kbps– More CDMA2000 variations, such as EV-DO and EV-

DV, aiming for peak speeds of 2.4 Mbps

• Generation 4 might see WiMax take over the cell phone wars, possibly in conjunction with the 3G cell standards

• Next slide is c*net’s view of cell technology



Cellular Internet Technologies

From cnet.

Services and speeds

Generation Technology Speeds Features

1G AMPS n/a Analog (voice only)

2G GSM CDMA iDen

Less than 20Kbps

Voice; SMS; conference calls; caller ID; push to talk

2.5G GPRS 1xRTT EDGE

30Kbps to 90Kbps

MMS; images; Web browsing; short audio/video clips; games, applications, and ring tone downloads

3G UMTS 1xEV-DO

144Kbps to 2Mbps

Full-motion video; streaming music; 3D gaming; faster Web browsing

3.5G HSDPA (upgrade for UMTS) 1xEV-DV

384Kbps to 14.4Mbps

On-demand video; videoconferencing

4G and beyond WiMax* 100Mbps to 1Gbps

High-quality streaming video; high-quality videoconferencing; Voice-over-IP telephony

*WiMax has been mentioned as a possible 4G technology, but no standards have been set.



4G and Beyond

• We’d like to see cell and wireless IP technologies merge so we can – take the best connection speed available, – keep a TCP connection when we move

around, – support real time voice and video over IP, – and be available anywhere

• Oddly enough, it isn’t that far away…



Mobility Management

• That concludes addressing the wireless aspect of networking

• Now, how do we handle a host moving from one part of the network to another?– From the network layer, a laptop that moves

around in one subnet isn’t mobile– From the link layer, if they stay keep using

one access point, they aren’t mobile



What is mobile?

• Does a user connect separately at different parts of the network, or need to maintain a connection while moving?

• Does their IP address need to be the same?

• What wired infrastructure is available?



Mobility Terms

• Your home network is the network you started in– Your first hop router is a home agent

• While moving, you are in a foreign or visited network– Your first hop router is a foreign agent

• You want to communicate with a correspondent



Mobility Terms

Home agent in home network

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)

f oreign agent: entity in visited network that performs mobility f unctions on behalf of mobile.

correspondent: wants to communicate with mobile

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)

f oreign agent: entity in visited network that performs mobility f unctions on behalf of mobile.

correspondent: wants to communicate with mobile



Addressing

• As hinted in the previous slide, addressing is a key concern

• How does the visited network indicate the home host is there?– Could update routing tables to indicate that

particular address is in the visited network– But what about when 1000’s of users are

mobile? Routing tables would get huge & hard to maintain



Addressing

• Instead, push mobility concerns to the edge of the network – the edge routers– Let the home agent keep track of the permanent

(home) address, and the foreign address– A care-of-address (COA) is the address of the foreign

agent of the host– The COA is used to re-route datagrams to the foreign

agent, who then passes them to the host

• Use this via indirect or direct routing



Indirect Routing

• We could blindly forward datagrams to the home agent– Let it change the address to the COA/foreign agent – The foreign agent sends them to the host

• It works, but it’ll take a while • The home agent needs to encapsulate the

datagram to get to the COA, who unwraps it– This is like tunneling for IPv6



Indirect Routing

wide area network

homenetwork

3

2

41

correspondent addresses packets using home address of mobile

home agent intercepts packets, f orwards to f oreign agent

foreign agent receives packets, f orwards to mobile

mobile replies directly to correspondent

wide area network

homenetwork

333

222

444111











Indirect Routing

• So for indirect routing, we need– A mobile node to foreign agent protocol – A foreign agent to home agent protocol – A home agent encapsulation protocol– A foreign agent de-encapsulation protocol

• Every time the node moves to a new foreign agent, it has to register its presence (association) and update its home agent

• Is used in the mobile IP standard (RFC 3344)



Direct Routing

• Direct routing avoids the inefficiency inherent in indirect routing– The correspondent goes through a

corresponding agent (router), who learns the COA of the node

– Then the corresponding agent sends data directly to the COA

• Need a mobile-user location protocol, to get the COA from the home agent



Direct Routing

wide area network

homenetwork 4

2

41correspondent requests, receives f oreign address of mobile

correspondent f orwards to f oreign agent



3

wide area network

homenetwork 44

22

4411correspondent requests, receives f oreign address of mobile

correspondent f orwards to f oreign agent





33



Direct Routing

• But how update the corresponding agent if the node’s COA changes during a session?– Use an anchor foreign agent (the first foreign

agent used) to keep track of the current COA– Then if the node is out of the anchor’s

network, encapsulate it and forward to the current foreign agent

• A little tedious, but probably more efficient than indirect routing



Mobile IP

• How mobile IP addresses can be handled is a huge topic

• RFC 3344, hinted earlier, defines many allowable approaches– With or without foreign agents– How agents and nodes can discover each

other – Single or multiple COAs– Many forms of encapsulation



Mobile IP

• The three key functions of mobile IP are– Discovery - how agents and nodes advertise

their presence to each other– Registration – how nodes and agents

register and deregister COAs with one’s home agent

– Indirect routing – how home agents can reroute datagrams, with forwarding rules, error handling, and different forms of encapsulation



Agent Discovery

• A node arriving at a new network needs to identify the network– This is called agent discovery

• Two ways to do this are agent advertisement or agent solicitation

• Agent advertisement is when the agent broadcasts its services over ICMP (type 9, router discovery)



Agent Advertisement

• The broadcast gives the IP address of the router (agent) and:– Whether the agent is willing to act as a home

and/or foreign agent (H or F bits)– If registration is needed before you can get a

COA in a foreign network (R bit)– If other forms of encapsulation is needed

(M or G bits)– COA data (one or more COA addresses)



Agent Advertisement

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 care-of-addresses

0 8 16 24



Agent Solicitation

• Agent solicitation is used when a node wants to find agents without waiting for advertisements– Solicitations are ICMP messages with

type = 10

• When an agent gets a solicitation, it responds directly to the node, and registration proceeds normally from there



Registration with home agent

• When a mobile node gets a COA, that address must be registered with its home agent (router)

• This could be done by the foreign agent, or by the node

• In the former case, there are four steps




1. Node sends registration message to foreign agent (over UDP, port 434)

2. Foreign agent gets message, records node’s permanent IP address, and sends registration message (UDP/434) to home agent

3. Home agent verifies the message, and connects node’s permanent IP to the COA

4. Foreign agent gets registration reply, and forwards it to the mobile node




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 ….

time




• When registration is complete, the node can get data sent to its permanent address via the new COA– The actual registration lifetime granted (in

seconds) is less than that requested– The identification number acts like a

sequence number, to match reply with its request

• Deregistering a COA isn’t needed, since it will be overwritten by a new COA



Managing Cellular Mobility

• For contrast to IP networks, let’s peek at how cellular networks manage handing off a connection

• Look at the GSM architecture, since it’s a mature example– It follows an indirect approach– The home network is officially called the home

public land mobile network (PLMN)– The foreign network is here a visited network




• The home network maintains a home location register (HLR) with your cell phone number subscriber information, and current location information

• A switch in the home network, the gateway mobile services switching center (GMSC), is contacted when an outside call is placed to the cell phone– Here call this switch the home MSC




• The visited network maintains the visitor location register (VLR), with an entry for each mobile user currently in the network– The VLR and the MSC are generally

colocated

• So a given cellular network is the home network for its subscribers, and a visited network for phones from other providers



Routing Calls to Cellular User

• For a call to get to a cellular user:– A correspondent places the call– The call is routed to the MSC in the home

network– The home MSC checks the HLR to see where

the user is located• It might return the mobile station roaming number

(MSRN, here just roaming number), a fake phone number which points to the user when in the network




• Or it will return the VLR of the visited network; the MSC will ask the VLR for the roaming number

– Given the roaming number, the MSC can now route the call to the VLR and get to the user

• For this to work, the user must exchange signaling messages with the VLR, who then passes that information to the HLR




Public switched telephonenetwork

mobileuser

homeMobile

Switching Center

HLR home network

visitednetwork

correspondent

Mobile Switching

Center

VLR

1 call routed to home network

2

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

3

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

4

MSC in visited network completescall through base station to mobile

Public switched telephonenetwork

mobileuser

homeMobile

Switching Center

homeMobile

Switching Center

HLRHLR home network

visitednetwork

correspondent

Mobile Switching

Center

Mobile Switching

Center

VLRVLR



2


22


3


33


4


444




Handoffs in GSM

• Handoff is when a user changes association during a call– Here from the old base station to the new

base station

• If both base stations share the same MSC, life is easier– Might need to handoff due to weak signal, or

high traffic load on the old base station



Handoffs in GSM

• The handoff process includes – Old base station (BS) informs MSC that

handoff is needed– MSC sets up path for new BS and opens

channel– New BS allocates resources and new channel– New BS tells MSC and old BS that user

should be told what’s going on



Handoffs in GSM

– Mobile user is told it should handoff– Mobile and new BS exchange messages to

activate new channel– Mobile user sends handoff complete message

to new BS– Old BS de-allocates resources

• So how does this process change when a different MSC is involved?



Handoffs in GSM

• For handoff between MSCs, the first one is the anchor MSC

• The anchor MSC stays the same regardless of where the user goes

• The current user location is the visited MSC• Hence the home MSC, anchor MSC, and visited

MSC are tracked throughout the call– IS-41 networks maintain chains of MSCs



GSM versus IP networks

Care-of-address

Routable address for telephone call segment between home MSC and visited MSC, visible to neither the mobile nor the correspondent.

Mobile Station Roaming Number (MSRN), or “roaming number”

Foreign agentVisited MSC: responsible for setting up calls to/from mobile nodes in cells associated with MSC. VLR: temporary database entry in visited system, containing subscription information for each visiting mobile user

Visited Mobile services Switching Center.Visitor Location Record (VLR)

Visited networkNetwork other than home system where mobile user is currently residing

Visited System

Home agentHome MSC: point of contact to obtain routable address of mobile user. HLR: database in home system containing permanent phone number, profile information, current location of mobile user, subscription information

Gateway Mobile Switching Center, or “home MSC”. Home Location Register (HLR)

Home networkNetwork to which the mobile user’s permanent phone number belongs

Home system

Mobile IP elementComment on GSM element GSM element

Care-of-address

Routable address for telephone call segment between home MSC and visited MSC, visible to neither the mobile nor the correspondent.

Mobile Station Roaming Number (MSRN), or “roaming number”

Foreign agentVisited MSC: responsible for setting up calls to/from mobile nodes in cells associated with MSC. VLR: temporary database entry in visited system, containing subscription information for each visiting mobile user

Visited Mobile services Switching Center.Visitor Location Record (VLR)

Visited networkNetwork other than home system where mobile user is currently residing

Visited System

Home agentHome MSC: point of contact to obtain routable address of mobile user. HLR: database in home system containing permanent phone number, profile information, current location of mobile user, subscription information

Gateway Mobile Switching Center, or “home MSC”. Home Location Register (HLR)

Home networkNetwork to which the mobile user’s permanent phone number belongs

Home system

Mobile IP elementComment on GSM element GSM element



Mobile effect on higher layers

• Mobile protocols clearly affect the physical, link, and often the network layers

• Are transport and application layers affected?– Mostly performance is affected– Since TCP retransmits lost segments, much worse

performance can be seen under wireless• The congestion window size (CongWin) is reduced

frequently, reducing efficiency, even though there may be little actual congestion



Mobile effect on higher layers

• Ways around this have been proposed– Use ARQ methods to detect and repair bit errors– Split TCP into two segments; one wired and one

wireless– TCP-aware link protocols – Change TCP so it handles wireless losses differently

than wired losses

• Applications need to consider low bandwidth, e.g. from 3G phone, and small image sizes


Chapter 6

Documents

wireless devices

wireless client

wireless relay

wireless connectivity

wireless links infrastructure

lan protocols info

range info

local wireless networks