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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 .

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Wireless lans l.jpg
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.

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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 l.jpg

  • 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.

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Spread Spectrum

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

Wiped out frequencies

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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).

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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.

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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.

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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 ?

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Hidden Terminals

  • 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.

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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.

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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.

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  • 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.

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  • 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.

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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.

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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.

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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.

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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.

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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.

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The Internet

  • A Network of Networks

A Logical interconnection of physical networks.

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The Internet Protocol

  • Architecturally above the Link layer.

  • Ties together various link layer possibilities.

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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.

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The IP Datagram

  • 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)

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More about the datagram

  • 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

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  • 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.

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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.

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An Example

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

  • IP header -- 20 bytes.

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To Note..

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

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

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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 ?

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Next in Chapter 4...

  • Addressing with IP

  • Routing.

  • Achieving scalability -- Global Internet.

  • Sections -- 4.1 4.2 and 4.3