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

Wiped out frequencies

frequency hopped spread spectrum
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
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.
802 11 phy layers
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
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
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.
exposed terminals
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.
the maca scheme
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.
  • 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.
  • 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
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
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
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
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
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.
the internet
The Internet
  • A Network of Networks

A Logical interconnection of physical networks.

the internet protocol
The Internet Protocol
  • Architecturally above the Link layer.
  • Ties together various link layer possibilities.
service model
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
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)
more about the datagram
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
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
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
An Example
  • Maximum Ethernet size = 1500, Maximum FDDI size = 4500 and maximum PPP size = 532.
  • IP header -- 20 bytes.
to note
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.
flag and offset fields
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
Next in Chapter 4...
  • Addressing with IP
  • Routing.
  • Achieving scalability -- Global Internet.
  • Sections -- 4.1 4.2 and 4.3