Wireless LANs and Introduction to IP Slide Set 7.

Wireless LANs and Introduction to IP

Slide Set 7

Wireless LANs

• Wireless proliferating rapidly.• IEEE 802.11 --> link access standard

designed for use in a limited geographic setting.

• Various versions 802.11a, 802.11e, 802.11g, 802.11n.

• Physical layer evolution -- increased rates .

• As an example, 802.11n uses multiple antennas -- can provide very high data rates.

Physical Properties• Typically use 3 kinds of physical

media -- two based on spread-spectrum and one based on IR.

• IR : limited range. (not much in use)• Spread spectrum -- spread signal

over a higher frequency -- provides – reduced impact from external

interference.– more robustness to signal loss.

Fading• Signal travels and

reflects off objects.• Multiple copies converge

at receiver (Red copy and Green copy).

• Copies interfere -- may self destruct -- called multipath fading.

• Signal combination depends on frequency of transmission.

Spread Spectrum

• The use of larger bandwidth provides robustness to fading/interference.

Wiped out frequencies

Frequency hopped Spread Spectrum

• Transmit signal over a random sequence of frequencies (not really random but pseudo-random).

• Computed using a pseudo-random sequence generator.

• Receiver uses the same generator -- they can synchronize (same seed).

Direct Sequence Spread Spectrum

• Each bit translated into ‘N’ random symbols called chips.

• Random chips generated using the pseudo-random number generator.

• Transmitted sequence called a n-bit chipping code.• If receiver knows the chips, it can decode.• Others cannot, they see a higher frequency signal

-- can be filtered out as noise.

Random sequence: 0100101101011001

Data stream: 1010

XOR of the two: 1011101110101001

0

0

0

1

1

1

802.11 PHY layers

• One PHY layer uses frequency hopping over a 79.1 MHz range.

• A second version uses a 11 bit chipping sequence.

• Both run in the 2.4 GHz band.• Note: For other than the intended

receiver signal looks like noise.

Medium Access Control

• Can we use the same protocol as in the Ethernet ?

• Carrier Sensing -- Sense channel, transmit when channel is idle, back-off when collision occurs ?

• Not really -- why ?

Hidden Terminals

A B C D

• B can talk to A and C but not D.

• C can talk to B and D but not A.

• A sends to B -- C cannot make out (cannot sense), and it sends to D.

• Collision at B :(.

• A and C are hidden from each other -- hidden terminal problem.

Exposed Terminals

• On the other hand, if B is sending A, C will sense channel to be busy.

• Will not send to D.• Not good either!• C is “exposed” to

B’s transmission.

A B C D

The MACA scheme• 802.11 addresses these problems by

using an algorithm called MACA -- multiple access with collision avoidance.– Also referred to as “virtual carrier

sensing”.• Sender sends a “Request to Send” or

RTS to Receiver.– Tells sender’s neighbors of intent to send.

• Receiver sends a “Clear to send” or CTS to sender.– Tells receivers neighbors of intent to

receive.

Example• A sends to B.• A’s RTS tells everyone

in its neighborhood that it is sending.

• B’s CTS tells everyone in its neighborhood that it is receiving.– Now C knows that B is

receiving and does not initiate communications with D.

A B C D

Details

• RTS indicates the time for which the sender wishes to hold the channel.

• Receiver echoes this “duration” field to the sender.

• Every node knows -- how long the transmission is for.

Data transfer

• Upon a successful RTS/CTS exchange, nodes initiate data transfer.

• Receiver sends ACK after successfully receiving frame.– Exposed terminal issue left alone

• Random wait when CTS is not received – Back-off similar to what happens with

Ethernet.

Access Points• While 802.11 facilitates operations

in an “ad hoc” mode, typically, some of the wireless nodes connected to a wireline infrastructure.

• These are called access points (APs) -- some people also call them base-stations (more appropriate for cellular networks)

• Other mobile hosts connect to the Internet via these APs.

Distribution System

BH

A

F

G

D

AP-2

AP-3AP-1

C E

Distribution system

• APs connected via the distribution system -- could be Ethernet or FDDI based (or anything else).

• Distribution system runs at Layer 2 -- not Layer 3 (Network Layer) entity.

Selection of APs• Via a process called scanning.• When a node wants to select an AP, it sends a

probe message.• APs that get this, respond with a Probe-Response.• Node selects one of the APs (strongest

signal ?),and sends an Association Request.• Selected AP responds with an Association

Response.• Active scanning -- Probes sent actively when

mobile joins the network or moves around and out of coverage.

• Passive scanning -- APs send beacons -- mobiles hear and if they find a more attractive AP, they can switch.

Rest of Chapter 2

• Read about 802.11 Frame format.

• Section 2.9 about Network adaptors and Device Drivers -- self study.

• We skip Chapter 3 and move on to Chapter 4.

Chapter 4: Internetworking and IP

The Internet

• A Network of Networks

R2

R1

H4

H5

H3H2H1

Network 2 (Ethernet)

Network 1 (Ethernet)

H6

Network 4(point-to-point)

H7 R3 H8

Network 3 (FDDI)

A Logical interconnection of physical networks.

The Internet Protocol

R1 R2 R3

H1 H8

ETH FDDI

IP

ETH

TCP

FDDI PPP PPP ETH

IP

ETH

TCP

IP IP IP

• Architecturally above the Link layer.

• Ties together various link layer possibilities.

Service Model

• Best effort -- no delivery guarantees.• Fundamental unit is the IP datagram.

– Sent in a connectionless manner.– No advance set up.– Datagram contains enough info. to let

network forward it to correct destination.

– Unreliable.

The IP Datagram

Version HLen TOS Length

Ident Flags Offset

TTL Protocol Checksum

SourceAddr

DestinationAddr

Options (variable) Pad(variable)

0 4 8 16 19 31

Data

• HLen --Header Length

• TOS -- Type of Service -- can distinguish connections.

• Set priorities.

• Length -- Maximum size = 64 KB = 65,535 B

• TTL -- time to leave -- discard packets that have been going around in loops.

• In terms of hop count (was originally in seconds)

More about the datagram

Version HLen TOS Length

Ident Flags Offset

TTL Protocol Checksum

SourceAddr

DestinationAddr

Options (variable) Pad(variable)

0 4 8 16 19 31

Data

• Protocol -- Binds with transport layer --TCP/UDP.

• Checksum -- Consider IP datagram as a sequence of 16 bit words. Add words. Take one’s complement.

• Destination/ Source address -- 32 bits for IPv4.

• Flags and Offset - used in fragmentation/reassembly

Fragmentation/Reassembly

• Each underlying network has a max frame size -- Ethernet 1500 bytes/ FDDI -- 4500 bytes.

• MTU -- largest IP unit that the network can carry in a frame.

• IP datagram needs to fit into the link layer payload.

• If the MTU over a network is smaller, the “router” receiving the datagram will fragment the datagram.

Fragmentation/Reassembly (cont)

• All fragments of same datagram contain a unique identifier -- in the Ident field.

• Fragments of a datagram are re-assembled at end-host.

• If fragments are missing, entire datagram discarded -- TCP/UDP cannot handle fragmented segments.

An Example

H1 R1 R2 R3 H8

ETH FDDI

PPP IP (376)

PPP IP (512)

PPP IP (512) (512)

ETH IP

ETH IP

(512)ETH IP

(376)

IP (1400) IP (1400)

R1 R2 R3

• Maximum Ethernet size = 1500, Maximum FDDI size = 4500 and maximum PPP size = 532.

• IP header -- 20 bytes.

To Note..

1. Each IP Datagram is an independent datagram that is transmitted over a series of physical networks.

2. Each IP datagram is re-encapsulated for every physical network it travels across.

Flag and Offset fields• Flag has a bit called the M bit -- set to indicate that

further fragments on their way.– Not set for the final fragment.

• Offset -- Indicates offset from original datagram.– In the previous example, offset for first fragment on

PPP network = 0.– For the second fragment, offset = 512 and so on.

• A detail: Fragmentation to be done in 8 byte units of data -- Offset field counts only in units of 8 bytes.

• Assignment: Read code on Reassembly-- Implementation -- Important -- what are maps ? why are holes created ? how can they be filled ?

Next in Chapter 4...

• Addressing with IP• Routing.• Achieving scalability -- Global

Internet.

• Sections -- 4.1 4.2 and 4.3