Transcript Wireless LANs and Introduction to IP

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.
1
0
Data stream: 1010
1
0
Random sequence: 0100101101011001
1
0
XOR of the tw o: 1011101110101001
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, backoff when collision occurs ?
• Not really -- why ?
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.
A
B
C
D
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.
–
Tells receivers neighbors of intent to receive.
• Receiver sends a “Clear to send” or CTS to
sender.
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
Distribution system
AP-1
AP-3
F
AP-2
A
B
G
H
C
E
D
• 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
Netw ork 1 (Ethernet)
H1
A Logical
interconnection
of physical
networks.
H2
H7
H3
Netw ork 4
(point-to-point)
Netw ork 2 (Ethernet)
R1
R2
H4
Netw ork 3 (FDDI)
H5
R3
H6
H8
The Internet Protocol
H1
H8
TCP
R1
IP
ETH
R2
IP
ETH
R3
IP
FDDI
FDDI
IP
PPP
PPP
TCP
IP
ETH
• Architecturally above the Link layer.
• Ties together various link layer
possibilities.
ETH
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
• HLen --Header Length
0
4
Version
8
HLen
16
TOS
31
Length
Ident
TTL
19
Flags
Protocol
Offset
Checksum
SourceAddr
DestinationAddr
Options (variable)
Data
Pad
(variable)
• 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
0
4
Version
8
HLen
16
TOS
31
Length
Ident
TTL
19
Flags
Protocol
Offset
Checksum
SourceAddr
DestinationAddr
Options (variable)
Pad
(variable)
• Protocol -- Binds with
transport layer -TCP/UDP.
• Checksum -- Consider IP
datagram as a sequence of
16 bit words. Add words.
Take one’s complement.
Data
• 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 reassembled at end-host.
• If fragments are missing, entire
datagram discarded -- TCP/UDP cannot
handle fragmented segments.
An Example
H1
ETH IP (1400)
R1
R2
R3
R1
R2
R3
FDDI IP (1400)
H8
PPP IP (512)
ETH IP (512)
PPP IP (512)
ETH IP (512)
PPP IP (376)
ETH IP (376)
• 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 reencapsulated 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