HIGH SPEED WIRELESS LANS
Principle of Network Design
1
University of Tehran
Dept. of Electrical and Computer Engineering
By: Dr. Nasser Yazdani
Lecturer: Peyman Teymoori
TOPICS

IEEE 802.11
Network
 MAC Format

IEEE 802.11e
 Paper Review

Performance Analysis and Enhancement for the
Current and Future IEEE 802.11 MAC Protocols
 Aggregation with Fragment Retransmission for Very
High-Speed WLANs


IEEE 802.11n
2
IEEE 802.11 TOPOLOGY




Independent basic service set (IBSS) networks (Ad-hoc)
Basic service set (BSS), associated node with an AP
Extended service set (ESS) BSS networks
Distribution system (DS) as an element that interconnects BSSs
within the ESS via APs.
3
IEEE 802.11 TOPOLOGY
4
MEDIUM ACCESS IN WLANS
IEEE 802.11
 MAC frame format
 CSMA/CA
 RTS/CTS
 IEEE 802.11e

5
IEEE 802.11 REFERENCE MODEL
6
MAC FRAME FORMAT
7
FRAME CONTROL FIELD (1)










Protocol Version (2 bits) –current version of the
standard
Type (2 bits) –differentiates among a management
frame (00), control frame (01), or data frame (10)
Subtype (4 bits) –further defines the type of frame
Type 00, subtype 0000 –association request
Type 00, subtype 0001 –association response
Type 01, subtype 1011 –RTS
Type 01, subtype 1100 –CTS
Type 01, subtype 1101 –ACK
Type 10, subtype 0000 –data
Many others…
8
FRAME CONTROL FIELD (2)







To/from DS (1 bit each) –flags set when the frame is
sent to/from the distribution system
More Fragment (1 bit) –flag set when more
fragments belonging to the same frame are to follow
Retry (1 bit) –indicates that this frame is a
retransmission
Power Management (1 bit) –indicates power
management mode (active, power saving)
More data (1 bit) –more frames buffered by station
for the same destination
WEP (1 bit) –payload encrypted with WEP
Order (1 bit) –strictly-ordered service
9
OTHER FIELDS






Duration ID (2 bytes) –for data frames, it contains
the duration of the frame
Sequence control (2 bytes) –sequence #
Frame body (0 to 2312 bytes)
FCS (4 bytes) –Frame Check Sequence (32 bit CRC)
Address fields (6 bytes each) –may contain BSSID,
source/destination address, transmitting/receiving
station address
Interpretation depends on values of
ToDS/FromDSbits
10
ADDRESS FIELDS
11
INDIRECTION BY DISTRIBUTION SYSTEM
12
PHY




MAC Protocol Data Unit (MPDU) is encapsulated by
PLCP
Format of PLCP PDU different for IEEE 802.11
(DSSS, FHSS, IR), IEEE 802.11b (long preamble/short
preamble), IEEE 802.11a
PLCP PDU for IEEE 802.11b with long preamble
compatible with PLCP PDU for IEEE 802.11 DHSS
In this lecture, we will focus on IEEE 802.11b PLCP
PDU
13
802.11B LONG PREAMBLE PLCP PDU




Compatible with legacy IEEE 802.11 systems
Preamble (SYNC + Start of Frame Delimiter) allows receiver
to acquire the signal and synchronize itself with the
transmitter
Signal identifies the modulation scheme, transmission rate
Length specifies the length of the MPDU (expressed in time to
transmit it)
14
802.11B SHORT PREAMBLE PLCP PDU

Not compatible with legacy IEEE 802.11 systems
15
802.11 MEDIUM ACCESS

Distributed Coordination Function (DCF)



Stations contend for the medium and transmit when the
medium becomes idle
Mandatory in 802.11 standard
Point Coordination Function (PCF)



Works in conjunction with DCF
Optional
Access point polls stations during contention free periods
and grants access to individual station
16
WHY NOT USE CSMA/CD?





In IEEE 802.3 (Ethernet), nodes sense the medium,
transmit if the medium is idle, and listen for
collisions
If a collision is detected, after a back-off period, the
node retransmits the frame
Collision detection is not feasible in WLANs
Node cannot know whether the signal was corrupted
due to channel impairments in the vicinity of the
receiving node
IEEE 802.11 uses Carrier Sense Multiple Access
(CSMA), but adopts collision avoidance, rather than
collision detection
17
CSMA







Station waits a random amount of time before
transmitting, while still monitoring the medium
Avoids collisions due to multiple stations
transmitting immediately after they sense the
medium as idle
Loss of throughput due to the waiting period is
compensated by fewer retransmissions
No explicit collision detection
Retransmissions are triggered if ACK is not received
Exponential backoff similar to IEEE 802.3
Optionally, transmitting and receiving nodes can
exchange control frames to “reserve” the channel
18
NETWORK ALLOCATION VECTOR (NAV)



Counter maintained by each station with amount of
time that must elapse until the medium will become
free again
Contains the time that the station that currently has
the medium will require to transmit its frame
Station cannot transmit until NAV is zero


Each station calculates how long it will take to transmit its
frame (based on data rate and frame length); this
information is included in the Duration field of the frame
header
This information is used by all other stations to set
their NAV
19
TIMELINE
20
TIMELINE DISCUSSED

DCF = Distributed Coordinated Function


DIFS = DCF Inter Frame Space


Stations must listen to an idle medium for at least that
amount of time before transmitting
SIFS = Short Inter Frame Space


Basic access method for 802.11 (uses CSMA/CA)
Period between reception of the data frame and
transmission of the ACK
SIFS < DIFS

What happens if another station starts listening to the
medium exactly during the idle period between data
transmission and acknowledgment?
21
SIFS/DIFS
SIFS makes transmission atomic
Example: Slot Time = 1, CW = 5, DIFS=3, PIFS=2, SIFS=1,
22
HIDDEN NODE PROBLEM

Node A is not aware that node B is currently busy
receiving from node C, and therefore may start its own
transmission, causing a collision
23
EXPOSED NODE PROBLEM

Node B wants to transmit to node C but mistakenly thinks
that this will interfere with A’s transmission to D, so B
refrains from transmitting (loss in efficiency)
24
RTS/CTS
1.
2.
3.
4.
5.
Sender transmits a Request to Send (RTS)
indicating how long it wants to hold the medium
Receiver replies with Clear to Send (CTS) echoing
expected duration of transmission
Any node that hears the CTS knows it is near the
receiver and should refrain from transmitting for
that amount of time
Nodes that hear the RTS but not the CTS are free
to transmit
Receiver sends ACK to sender after successfully
receiving a frame. All nodes must wait for the
receiver to ACK before attempting to transmit
25
TIMELINE WITH RTS/CTS
26
SPECIAL FRAMES: ACK, RTS, CTS

Acknowledgement
bytes
2
Frame
Control
ACK

6
4
Receiver
Duration
CRC
Address
Request To Send
bytes
RTS

2
2
Frame
Control
2
6
6
4
Receiver Transmitter
Duration
CRC
Address Address
Clear To Send
bytes
CTS
2
Frame
Control
2
6
4
Receiver
Duration
CRC
Address
27
AP VS. AD-HOC
28
IEEE 802.11E





MAC enhancements to support quality of service
(QoS) in IEEE 802.11a/b/g
Defines different categories of traffic
Each QoS-enabled station marks its traffic according
to its performance requirements
Stations still contend for the medium, but different
traffic types are associated with different inter
frame spacing and contention window
Qualitative, comparative QoS(no “guarantees”)
29
802.11 STA VS. 802.11E STA
30
SERVICE DIFFERENTIATION
31
EDCA REVIEW

TXOP (Transmission Opportunity)


TID (Traffic identifier)



An interval of time when a particular STA has the right to access
the wireless medium.
TID value is specified in the QoS Control field of the 802.11e
QoS data’s frame MAC header.
There are 16 possible TID values , where the value from 0-7
specify the user priority value of a frame, and the value from 815 specify the traffic stream which the frame belongs to.
Block Ack (BA)

During a TXOP, a STA (or AP) can transmit a number of frames
without receiving any Ack. After frame transmissions completed,
transmitter sends a control frame (Block Ack request, BAR) .
Then the receiver respond with BA.
32
802.11E TXOP AND BLOCK ACK
33
WIRELESS NETWORKING

PROTOCOLS
The 802.11 family of radio protocols are commonly referred to as
WiFi
• 802.11a supports up to 54 Mbps using the 5 GHz ISM and UNII bands.
• 802.11b supports up to 11 Mbps using the 2.4 GHz ISM band.
• 802.11g supports up to 54 Mbps using the 2.4 GHz ISM band.
• 802.11n supports up to 300 Mbps using the 2.4 GHz and 5 GHz ISM
and UNII bands.
• 802.16 (WiMAX) is not 802.11 WiFi! It is a much more complex
technology that uses a variety of licensed and unlicensed frequencies.
34
WLAN VS. OTHER SOLUTIONS
Mobility
WAN
WLAN
Outdoor
Vehicle
Walk
UMTS
Walk
Fixed/
Desktop
802.11n
802.11b
Wideband
Cellular
802.11a/g
Fixed
Indoor
High performance WLAN
Wired LAN
Bluetooth
0.1
1
10
100
Mbps (Tx Rate)
35
PAPER REVIEW
Performance Analysis and Enhancement for
the Current and Future IEEE 802.11 MAC
Protocols
Yang Xiao, Jon Rosdahl
36
HIGH DATA RATES
The industry is seeking Higher Data Rates
(HDR's) over 100Mbps (in 2002)
 More data rate intensive applications exist such as







Multimedia conferencing,
MPEG video streaming,
Consumer applications,
Network storage, and
File transfer;
Finally, there is a great demand for higher capacity WLAN
networks in the market such as




Hotspots,
Service providers,
Wireless back haul, and
An increasing number of users per access point
37
HIGH DATA RATES
We explore the overhead of HDR's to find out
whether the MAC is good enough
 We prove that a theoretical throughput upper
limit and a theoretical delay lower limit exist for
IEEE 802.11 protocols
 In order to reduce overhead, we propose a burst
transmission and acknowledgement ( BTA )
mechanism

38
PPDU FRAME FORMAT OF IEEE 802.11A
39
IEEE 802.11A

Data rates for IEEE 802.11a :


6, 9, 12, 18, 24, 36, 48, and 54 Mbps
Some IEEE 802.11a parameters








Tslot
Tsifs
Tp
CW0
Tsim
Tdifs
Tphy
τ
= 9µs
(Slot time),
= 16µs (SIFS time),
= 16µs (Physical layer's preamble),
= CWmin = 16,
= 4µs
(Symbol time),
= 34µs (DIFS time),
= 4µs
(PHY header time), and
= 1µs
(Propagation delay).
40
IEEE 802.11A BEST-CASE PERFORMANCE




Ldata: length of the payload
Tdata and Tack: transmission times of a data frame and an ACK,
respectively.
MT: Maximum throughput
MD: Minimum delay
41
IEEE 802.11A BEST-CASE PERFORMANCE



BE: bandwidth efficiency
TUL: theoretical throughput upper limit
DLL: theoretical delay lower limit
42
IEEE 802.11A BEST-CASE PERFORMANCE
43
BURST TRANSMISSION AND
ACKNOWLEDGEMENT


A BTA sequence
MAC frame format (FC: Frame Control; DU: Duration; A:
Address; QoS: QoS Control; FB: Frame Body) (Size is in bytes)
44
BURST TRANSMISSION AND
ACKNOWLEDGEMENT


BurstAckReq frame format (FC: Frame Control; DU: Duration;
RA: Receiver Address; TA: Transmitter Address; BAR: BAR
Control; R: Reserved) (Size is in bytes)
BurstAck frame format (FC: Frame Control; DU: Duration; RA:
Receiver Address; TA: Transmitter Address; R: Reserved; W:
Wait; SC: Sequence Control; BM: Ack Bitmap) (Size is in bytes)
45
BURST TRANSMISSION AND
ACKNOWLEDGEMENT




Tr : time required to transmit the burst acknowledgement
request frame,
Ta : time required to transmit the burst acknowledgement frame
Tpo : time required to transmit the CF-Poll frame
Nb : number of burst
46
BURST TRANSMISSION AND
ACKNOWLEDGEMENT
47
BURST TRANSMISSION AND
ACKNOWLEDGEMENT
48
BURST TRANSMISSION AND
ACKNOWLEDGEMENT
49
PAPER REVIEW
Aggregation with Fragment Retransmission for
Very High-Speed WLANs
Tianji Li, Qiang Ni, David Malone, Douglas Leith,
Yang Xiao, Thierry Turletti,
50
OUTLINE
Goal:
To design a new MAC with high efficiency for
very high-speed next-generation WLAN (e.g.
802.11n)
Difficulty:
Overhead at MAC and PHY
Solution:
aggregation at MAC
51
GOAL
Now:
802.11b: PHY rate= 11Mbps, MAC throughput
= 70%*11 = 7 Mbps
802.11a: PHY rate = 54Mbps, MAC
throughput = 50%*54 = 27 Mbps
Future:
PHY rate >= 216 Mbps (up to 648 Mbps),
MAC throughput = ???
52
DCF: THE CURRENT MAC
backoff
DIFS
PHYhdr
frame
SIFS
PHYhdr ACK
the real thing
Overhead: DIFS, backoff, SIFS, PHY headers,
and ACKs.
53
WHAT IF USING DCF IN VERY HIGH-SPEED ?
MAC throughput < 50 Mbps for ever !
54
WHY DCF SO SLOW?
Tframe

Tdifs  Tbackoff  2 * Tphy , hdr  Tsifs  Tack
Tframe = frame size / R, it scales with 1/R.
Tack = ack size / R, it scales with 1/R.
But, other items in denominator are
constant, which leads to
  0 while R  
Solution: We need to make all in
denominator scale also with 1/R.
55
PRIOR WORK (1/2)
backoff
BurstACK
DIFS
PHYhdr Frame SIFS ACK SIFS
SIFS PHYhdr Frame SIFS ACK
backoff
BlockACK
DIFS
PHYhdr Frame SIFS
PHYhdr Frame SIFS
BlockAck
Request
Burst ACK: proposed in early versions of
802.11e
Tdifs and Tbackoff scale with 1/R.
Block ACK: in the current 802.11e
Tdifs , Tbackoff and TACK scale with 1/R.
SIFS
Block
Ack
56
PRIOR WORK (2/2)
backoff
Mobicom
2004
DIFS
PHYhdr Frame
Sub
Frame
PHYhdr
Sub
Frame
PHYhdr
SIFS
ACK
MD Frame
SIFS
ACK
backoff
WoWMoM
2005 (802.11n)
DIFS
PHYhdr MD
Frame MD Frame
Aggregation from [Ji et. al.]
Tdifs , Tbackoff , Tack and Tsifs scale with 1/R.
Aggregation from [Kim et. al.]
All in denominator scale with 1/R, then why I
am here…
57
WHAT ARE STILL MISSING?
How to have very large frames?
Wait or not if no enough information?
How much time to wait for?
Is there a limit for the frame size? What is the
best size?
What is the best size for retransmission?
What the delay will look like?
58
OUR SAMPLE SCHEME: AFR
The Aggregation with Fragment Retransmission (AFR)
T CP/IP Layer
Packet 1
Packet 2
Packet n
LLCLayer
Fragment 1.0
Fragment
1.1
Fragment n.0 Fragment n.1
MAC Layer
Fragment n.1
FCS
Fragment n.0
FCS
Fragment
2.0
FCS
Fragment
1.1
FCS
Fragment 1.0
FCS
fragment
headers
FCS
FCS
MAC
header
frame
PHYlayer
59
ZERO-WAITING
Question:
how much time should we wait for enough information to aggregate?
Answer:
Zero-waiting: transmit immediately
Why:
In heavily loaded networks, aggregation happens automatically
In slightly loaded networks, AFR degenerates to the legacy DCF
Zero-waiting is proven to be stable where feasible
60
MAXIMUM FRAME SIZE
Constant throughput is possible with increasing frame
sizes
Maximum frame size: 65536 bytes
61
FRAGMENT SIZES (1/2)
Fragmentation is necessary with large frame in bad channels
62
FRAGMENT SIZES (2/2)
A single fragment size can be found for near-optimal efficiency
63
MAC DELAY
CSMA/CA delay for a ‘frame’ is worse than in DCF
64
MAC + QUEUE DELAY
Total delay is much better due to ‘pipeline-like’ ability
65
AFR VS DCF
66
HDTV (SIMULATION)
67
802.11N
The latest approach toward High-Speed WLANs
 What we review:

Some New MAC Concepts
 802.11n Features
 Performance Evaluation

68
MAC DEFINITIONS



MPDU stands for MAC Protocol data unit. MPDUs are messages
(Protocol data units) exchanged between MAC entities in a
communication system based on the layered OSI model.
In systems where the MPDU may be larger than the MSDUs, then the
MPDU may include multiple MSDUs as a result of Packet aggregation.
In systems where the MPDU is smaller than the MSDU, then one MSDU
may generate multiple MPDUs as a result of Packet segmentation.
69
MAC DEFINITIONS

Packet aggregation is the process of joining multiple packets
together into a single transmission unit, in order to reduce the
overhead associated with each transmission


A-MPDU
A-MSDU
70
A-MSDU AGGREGATION FRAME
STRUCTURE
A structure containing multiple MSDUs, transported within a single
(unfragmented) data MPDU
71
A-MPDU AGGREGATION FRAME
STRUCTURE
A structure containing multiple MPDUs, transported as a single PSDU by
the PHY
72
IEEE 802.11N FEATURES
MIMO-OFDM physical layer
 Aggregation
 Block ACK
 Reverse direction

73
MIMO-OFDM




The most commonly used method is to increase the raw data rate
in the PHY layer
MIMO can effectively enhance spectral efficiency with
simultaneously multiple data stream transmissions
Orthogonal frequency division multiplexing (OFDM) transmission
scheme has been used to increase PHY layer transmission rate
With this enhancement in the PHY layer, the peak PHY rate can
be boosted up to 600 Mbps
TX
RX
TX
RX
MIMO
Channel
MIMO
Processor
Input
Output
74
AGGREGATION


The key feature to improve the 802.11 MAC transmission
efficiency
designed as two-level aggregation scheme
A-MSDU
 A-MPDU





The maximum length of an A-MSDU, 3839 or 7935
These MSDUs must be in the same traffic flow (same TID)
with the same destination and source
The TID of each MPDU in the same AMPDU might be
different.
The maximum size limit of A-MPDU is 65535 bytes
75
TWO-LEVEL AGGREGATION IN IEEE
802.11N
76
BLOCK ACK





Problem: frame error rate is higher as the size of the frame increases!
Large frames in high bit-error-rate (BER) wireless environment have
a higher error probability and may need more retransmission
To overcome this drawback in aggregation, the block ACK mechanism
in 802.11n is modified to support multiple MPDUs in an A-MPDU.
When an A-MPDU from one station is received and errors are found in
some of the aggregated MPDUs, the receiving node sends a block ACK
only acknowledging those correct MPDUs. The sender only needs to
retransmit those non-acknowledged MPDUs.
Note, block ACK mechanism only applies to AMPDU, but not AMSDU!
The maximum number of MPDUs in an A-MPDU is limited to 64 as
one block ACK bitmap can only acknowledge at most 64
77
BLOCK ACK WITH AGGREGATION
78
REVERSE DIRECTION
Reverse direction mechanism allows the holder of
TXOP to allocate the unused TXOP time to its
receivers to enhance the channel utilization and
performance of reverse direction traffic flows
 The major enhancement in reverse direction
mechanism is the delay time reduction in reverse
link traffic
 This feature can benefit a delay-sensitive service
like VoIP

79
REVERSE DIRECTION
80
802.11N MAC FRAME FORMAT

Data Frame

HT Control field
81
802.11N MAC FRAME FORMAT

BlockAckReq frame

BA Information field (BlockAck)
82
802.11N MAC FRAME FORMAT

A-MSDU structure

A-MSDU subframe structure
83
802.11N MAC FRAME FORMAT

A-MPDU format

A-MPDU subframe format
84
BLOCK ACK PERFORMANCE
85
Thanks for you attention
Any question?
86