Existing MACs for Wireless nets

1
Existing MACs for Wireless nets
Note in last class I said that the noise floor
in 802.11b was -110 dBm) That was wrong, it was
100 dBm. Thermal 173 dBm/Hz Bandwidth
10log10(20e6) 10log10(2) 10log10(1e7) 73
dB Giving 100 dBm
2
Basic 802.11 media access control - 802.11 DCF

• DCF distribution coordination function
• Basic medium access protocol
• All implementations must support DCF

3
Wireless MAC

• Aloha inefficient
• CSMA listen before sending
• Hidden node problem
• Expose node problem

Hidden node
exposed node
4
Medium Access Collision Avoidance (MACA/MACAW )

• RTS-CTS-Data-ACK
• Exposed nodes

The media supports both B and C transmitting
simultaneously. But the MAC does not
The media supports both A and D transmitting
simultaneously. But the MAC does not
The above arguments are not true if we require
ACKs. In wireless networks, it is very difficult
to not have ACKs. And we will see that in some
cases, higher data rates are permitted if acks
are used. In this case, all transmission are
two-way, and so there is no exposed node.
5
Basic Problems of MACA

• Overhead (well see this later)
• If the CTS is not decoded, it does not mean that
  the node will not interfere.
• Example
• Suppose that the noise floor is -100 dBm and the
  receiver needs 7 dB or more SNR or SNIR. And
  suppose 40 loss in the first meter
• HA,B -64 dB
• A transmits at 15 dBm
• So B receives As transmissions at -89 dBm (well
  above the SNR limit, we could even use a higher
  data rate)
• HC,B-70 dB
• B transmits CTS at 15 dBm
• C receives CTS at -95 dBm and is unable to decode
• C transmits and B receives interference at -95
  dBm.
• The SNIR at B is 6 dB, not enough for 1Mbps (much
  less the higher data rate that was selected).

data
A
B
C
6
Interference with RTS/CTS

• More details
• C will transmit if it does not hear Bs CTS. The
  received signal strength at B is the same that C
  could not receive, so it cannot be a very strong
  signal
• How strong can it be?
• Suppose that C is listening, there is
  interference from many transmitters so it cannot
  hear anything Bs CTS even though the signal is
  very strong. Then C could cause very high
  interference.
• Solution clear channel assessment (one type) if
  C hears a strong signal, even if it cannot decode
  it, it cannot transmit for a short period, to
  allow B to receive (even if B is not receiving)
• The interference at B is the same as the signal
  strength received by C from B (but unable to
  decode).
• Option 1
• If C hears even a weak signal, then dont
  transmit
• Drawback this will cause exposed node
• Option 2
• If C hears a stronger signal, then dont transmit
• Drawback B might receive strong interference
• 802.11 defines a strong signal as around -75 dBm.
  Which is quite strong, and implies that B can
  expect interference of nearly -75 dBm! It is
  difficult to imagine a setting where this is
  useful for interference from 802.11 (it could be
  from some other source). In 802.11, if the
  received signal strength is -75 dBm, it should
  likely be decodable (unless sent at high data
  rate, but CTS are always sent at low data rate)
• In conclusion, C will receive Bs CTS when the
  SNIR is high enough. If the interference at C is
  strong, then C could cause strong interference.

7
Overhead in 802.11 (ignoring backoff)
DIFS
RTS
SIFS
SIFS
Data
SIFS
CTS
ACK
Check that the channel is idle
Packet arrives from network layer
RTS arrives at receiver
CTS arrives at sender
data arrives at receiver
done
NAV
8
Frame sizes (and contents)
MAC frames are called MPDU (MAC protocol Data
Unit)

• Frame formats
• RTS (20B)
• Frame control (2B)
• Protocols version (2bits) (802.11 00, so if not
  00, then the following means something different)
• Type (2 bits)- management, control,data, reserved
• subtype (4bits), e.g., controlRTS,
  managementbeacon,
• To DS (DP) (1 bit)
• From DS (1bit)
• More frag (1b)
• This is a Retry (1b)
• Pwr mgt (1b) (1 means the STA will go to power
  save (i.e., periodic sleep) after the frame is
  received)
• More data (1b) if a STA is in power save mode,
  then this is sued to signify that more data is
  waiting, so maybe the STA should wake up and get
  the data!)
• Protected frame (1b) (WEP, etc.)
• Order (1b)
• Duration / ID (2B) (but MSB is zero, so 15 bits
  for the duration) in microsec (rounded up) (ID is
  used in power save mode)
• RA receiver address (6B)
• TA transmitter address (6B)
• FCS CRC-32 (4B)
• CTS (14B)

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Physical Protocol Data Unit (802.11b)

• PLCP (phy layer convergence protocol phy with
  MAC frame inside)
• PLCP preamble (144 bits)
• SYNC (128b) to let the physical layer acquire
  sync (transmitter actually begins transmitting
  before this. This period is called ramp up where
  the transmitter goes from 0 zero to full/desired
  power. This typically takes 1us)
• SFD (16b) like a radio ID, IEEE 802.11 has
  SFD-F3A0hex
• PLCP header (48 bits)
• Signal (8b) specifies the modulation
• Service (8b) reserved
• Length (16b) in micro seconds
• CRC (16b)
• PLCP SYNC and header always take 192 us
• MPDU (MAC protocol data unit) or PSDU (PLCP
  service Data unit)

10
Short PLCP (optional)

• Preamble 72 bits at 1 Mbps
• PLCP header 48 bits at 2Mbps
• The preamble and header at 96 us
• MPDU / PSDU at 2, 5.5, Mbps
• Since the PLCP is sent at 2 Mbps, the channel
  must be able to support 2 or more Mbps
• In practice the short header does not work well
  unless the channel is very good.

11
802.11 g

• PLCP preamble and header are similar to 802.11b
• PSDU
• Long Training sequence
• 1.6 us guard interval
• Long training symbol (3.2 us)
• Long training symbol (3.2us) (again)
• Total 8 us
• OFDM signal
• Guard interval 0.8 us (why why why is it reduced
  to 8 us?)
• Signal 3.2 us
• Total 4 us
• Transmitted at 6 Mbps OFDM modulation which is
  the worst performing modulation it has the same
  SNR-BER relationship at 9 and 12 Mbps.
• Data
• 6 us of quiet time (time to decode) to SIFS is 16
  us

12
802.11 overhead

• Four 192b from PLCP
• At 1Mbps 192 microsec so 768 microsec
• 82B from RTS, CTS, ACK, data
• 656 mic sec at 1Mbps
• 328 micro sec at 2Mbps
• 131 at 5Mbps
• 60 at 11Mbps
• 12 at 54Mbps
• 1 DIFS, 3 SIFS, 4 propagation delays
• 30 microsec 320 micro 42 microsec 98
  microsec
• Total overhead time
• 1552 mic sec at 1Mbps
• 1194 micro sec at 2Mbps
• 997 at 5Mbps
• 925 at 11Mbps
• 878 at 54Mbps
• Data duration
• 40B packet
• 320 mic at 1Mbps gt efficiency data duration over
  all duration 17

13
802.11 overhead w/o RTS/CTS

• two 192b from PLCP
• At 1Mbps 192 microsec so 384 microsec
• 14B from RTS, CTS, ACK, data
• 112 mic sec at 1Mbps
• 56 micro sec at 2Mbps
• 22 at 5Mbps
• 10 at 11Mbps
• 2 at 54Mbps
• 1 DIFS, 1 SIFS, 1 propagation delays
• 30 microsec 20 micro 2 microsec 52 microsec
• Total overhead time
• 548 mic sec at 1Mbps
• 492 micro sec at 2Mbps
• 458 at 5Mbps
• 446 at 11Mbps
• 438 at 54Mbps
• Data duration
• 40B packet
• 320 mic at 1Mbps gt efficiency data duration over
  all duration 36

If the packet is smaller than RTSThreshold, then
RTS/CTS are not used
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802.11 without RTS/CTS

• See write-up

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Contention

• When a collision does occur, the transmitters
  must back-off
• Also, if a node desires to transmit (i.e., a
  frame is received from the upper layer), and it
  finds the channel busy, it should no transmit as
  soon as the channel is free.
• Random backoff
• Backoff time Random()SlotTime
• Random uniform on 0,CW
• CWMin(7) lt CW lt CWMax (1023) depending on the
  previous contention experienced during this
  transmission
• SlotTime 20 us (CW1023 -gt 10 ms of idle time
  between transmission attempts. At 1Mbps, this is
  a bit more than one 1500B packet.)
• The backoff is a timer, for ever slot time that
  is DIFS after the channel is not busy, the timer
  decrements.
• When the time reaches 0, the node attempts to
  transmit.
• If the transmission fails, CW is doubled (but is
  never greater than CWMax)
• After a successful transmission or the so many
  attempts were tried that the packet was dropped,
  CW CWMin

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Max number of transmissions

• If an RTS fails to generate a CTS, then the Short
  Retry count is incremented, until is reaches the
  short_retry_limit
• If the pkt size is below RTSThreshold (so no RTS
  is transmitted), and fails to generate an ACK,
  then the Short retry count is incremented
• When small data pkt is successful, the short
  retry count is reset to zero
• If pkt size is above RTSThreshold and fails to
  result in an ACK, then the long retry is
  incremented.
• If the pkt is successfully transmitted, then long
  retry count is reset to 0
• If a pkt is dropped due to too many failed
  attempts, the retry counters are set to zero

17
Using ACKs to increase data rate

• Envelop analysis
• Suppose that the pkt error prob for 10 Mbps is 0
  and the pkt error prob for 20 Mbps is 0.25.
• The effective data rate at 10 Mbps is 10 Mbps,
  and at 20, is 0.7520 gt 10, so 20 is better, even
  though there will be many losses and
  retransmissions.
• When a transmission fails, it must wait DIFS and
  then back off
• Suppose an empty channel
• DIFS, Data, SIFS, ACK
• If Fails, then wait, 0,7SlotTime and repeat
  the above
• If Fails again, then wait, 0,13SlotTime and
  repeat the above

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PCF point coordination function

• Provide contention free (CFP) access
• Also, allows contention (CP)
• AP is master it polls STAs
• The beacon is used to set STAs NAV
• The AP does not have to use PCF. But if it does,
  the STAs must obey. But they will anyway, since
  it obeys DCF
• When a STA joins, it announces whether it can be
  polled.
• The AP may select to only transmit data during
  the CF and force STAs to use the CP to transmit
• All transmission must be ACKed.
• If no ACK is received, the AP can retransmit the
  next time the AID comes up (AID association ID)
• The AP can retransmit after waiting at least PIFS

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PCF centrally controlled access

• When the AP wants to transmit, it waits for the
  current transmission to complete and then waits
  for PIFS and begins to transmit (the PCF may have
  to wait a long time to begin the CFP). Since the
  PIFS is smaller than the DIFS, the AP will always
  get the channel over the mobile hosts.
• When the AP has control of the channel it is
  called the contention-free period (CFP).
• Then the AP is not in a CFP, then hosts can use
  the DCF to transmit.
• The CFP begins with the AP sending a beacon.
• During the CFP the AP polls the mobile hosts
• Beacon
• Timestamp (64b), beacon interval (units are
  1024microsec) (16b) and capability info
• CFP max duration every mobile host saves this
  info into its NAV.
• Service set ID (SSID), supported rates, phy
  parameters, CF parameters, IBSS parameters,
  traffic indication map
• 802.11d
• Country info and hopping pattern parameters
• 801.22e
• QBSS and EDCA parameters
• 802.11g
• ERP info
• 802.11h
• Power constraint, supported channels, channel
  switch announcement and quiet info
• 802.11i
• RSN info

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CFP
All nodes update their NAV to 32000 us
beacon
DataCF-poll
DataCF-ACK from station1
DataCF-ACKCF-poll
DataCF-ACK from station1
DataCF-ACKCF-poll
PIFS
PIFS
PIFS
SIFS
SIFS
PIFS
Trans error
DataCF-ACKCF-poll
DataCF-ACK from station1
DataCF-poll
PIFS
No ack
SIFS
No data sent, but data was received
DataCF-ACKCF-poll
ACKCF from station
PIFS
SIFS
Station had no data to send, so AP regains
control after PIFS
CF-ACKCF-poll
DataCF-poll
PIFS
One problem is that the mobile station may send
very large packets at slow rates and hence use
the channel for a long time
PIFS
CF-ACKCF-end
End of CFP
All nodes update their NAV
PIFS
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Tim and DTim

• A CFP will occur after a beacon.
• But it does not occur after every beacon
• Every few beacons is a DTIM and every few DTIMs
  is a CFP
• In the TIM is a bit string with a 1 if the STA
  with the AID corresponding to the bit number has
  a data packet ready to delivery from the AP.
• This string is 2008 bits long.
• Node not in the list can sleep
• Since beacons are transmitted after listening,
  the CFP might be a bit late
• If the channel found to be busy, then the AP
  backoff between 0 and CWMin. This is to reduce
  collisions with other APS

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Transmit Power control (TPC) in 802.11 802.11h

• 802.11a is at 5GHz, so it might interfere with
  other users. 802.11a is not the primary user of
  5GHz.
• To control the impact of 802.11a on these other
  spectrum users, 802.11a must attempt to reduce
  the power and the AP must monitor the total power
  over all mobile nodes.
• Note that all 802.11a cards must have the
  capability to control transmission power.
• Power control is communicated via a new frame
  called the action frame
• The mobile maintains a variables local power
  constraint and country RF power constraint
  channel TX power. The later depends on the
  country and the channel. The former is received
  from the AP. The mobiles transmit power is
  country RF power constraint channel TX power -
  local power constraint .
• The mobile will report to the AP
• Its min and max possible transmission power
• The current transmit power
• Current average link margin
• The AP will also demand that the mobiles are
  quiet so radars can be detected
• 802.11h supports changing to a different channel
  if there is not enough power allowance on the
  current channel. Also, a mobile might be rejected
  from joining a BSS if there are power problems
  (but this is not specified in the spec)

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802.11 types

• 802.11a,b,g
• 802.11h 802.11a with power/spectrum control
• 802.11d international operation
• 802.11f mobility, inter-AP communication
• 802.11e QoS
• 802.11i security enhancements
• 802.11s mesh

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Features in 802.11e

• 8 priority classes
• Hybrid coordination function
• Contention free period (CFP) and contention
  period
• During the contention period, the AP may still
  gain control and poll nodes.
• During CP the AP may poll all nodes but with
  specific priority with a single poll. In this way
  the nodes with high priority can compete among
  each other and not with low priority traffic.
• During the controlled contention, nodes can only
  send requests for transmission ops (TXOPs)
• The requests are replied to with acks from the ap
• The AP polls nodes and issues TXOPs (transmit
  opportunities)
• The CFP ends with at the time specified in the
  initial beacon or when a CFP end packet is sent
• Within packet header is a place for nodes queue
  size as well as a place to request a TXOP.
• Thus, the AP knows all queue sizes (but it only
  know the aggregate of each nodes queue size, not
  each priority class queue.

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Features of 802.11e

• Enhanced DCF
• Each nodes has 4 or 8 queues one for each
  priority class
• The queues compete for the channel internally
• If the queue collide, then they increase their
  CW. But high priority queues increase there CW
  more slowly than low priority queues
• Block ACKs
• Instead of ACKing every packet, it is possible to
  ack several packets.
• The block ack contains the list on packets ACKed.
  
• The block ACK can be requested
• Or it can automatically occur at the end of the
  transfer, in which case the block ack is acked.
• No ACK

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BSS

• A BSS is the set of nodes associated with an AP
• An IBSS is an ad hoc BSS
• A DS (distribution system) connects the BSS to
  other BSS and forms a EBSS
• But usually the BSS do not directly communicate
  and so the BSSs are independent, and dont form a
  EBSS.
• But this might not be the case in future networks
• To support mobility
• To support QoS/hi-capacity

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Associated

• First a client must be associated
• Then it become authenticated

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802.16 / WiMax

• The original WiMAX standard, IEEE 802.16,
  specifies WiMAX in the 10 to 66 GHz range.
• 802.16a added support for the 2 to 11 GHz.
• IEEE 802.16 provides up to 50 km (31 miles)
• data rates up to 70 Mbit/s
• "last mile" connectivity

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Frame Control Header (FCH), specifies the burst
profile and the length of one or more DL bursts
The DL-MAP, UL-MAP, DL Channel Descriptor (DCD),
UL Channel Descriptor (UCD), and other broadcast
messages that describe the content of the frame
are sent at the beginning of these first
bursts. The remainder of the DL subframe is made
up of data bursts to individual SSs Each data
burst assigned a burst profile that specifies the
code algorithm, code rate, and modulation level
that are used for those data transmitted within
the burst
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(No Transcript)