©2008, Sami Alshuwair
VoIP via IEEE 802.11
“VoWLAN”
EETS 8313: Internet Telephony
Sami Alshuwair
Agenda
Evolution of Enterprise Telephony
Why Deploy Vice over Wireless LAN
VoWLAN Market Trends
VoIP Challenges and Solution
VoWLAN Challenges and Solution
WLAN System Architectures Considerations
VoWLAN Alternatives
Recommendations
6 April 2008
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©2008, Sami Alshuwair
Evolution of Enterprise Telephony
From circuit-switched to packet-switched
Traditionally private branch exchange (PBX)
–Analog/digital
g g
pphones, proprietary
p p
y
Internet Protocol PBX began a new era
–Converge voice/data, IP-based enterprise network
Session Initiation Protocol (SIP) emerges
g to include WLANs
VoIP extends coverage
–SIP-based, VoWLAN phones provide comm.
Fixed mobile convergence solution emerging
–Seamless handoff between mobile cellular and Wi-Fi
networks
6 April 2008
3
Why Deploy VoWLAN?
Clearly understand your needs
Reduce mobile cellular minutes
– VoWLAN conversations are “free” calls
Improve in-building voice coverage
– Deploy more access points to improve coverage
Integrate mobile & enterprise telephony systems
phones are not an option
p
When cellular p
– VoWLAN signals use 2.4 GHz or 5 GHz bands
Provide wireless foundation for unified comm.
– Voice, video, IM, all over common IP network
6 April 2008
EETS 8313: VoIP over IEEE 802.11
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©2008, Sami Alshuwair
VoWLAN Market Trends
Growing becoming pervasive
WLAN phone market reach
$3.7B
$
.7 by 2009
9
Integrate enterprise
Telephony + mobility
Source: Infonetics Research
Vendors integrate their voice
solutions
Vendor consolidation
IEEE 802.11 tech. improving
Mobile cellular tech. improving
Source: Gartner (November 2007)
[1] [2]
6 April 2008
5
How VoWLAN Solutions Can Reduce Cost
and Improve Productivity
Dual-mode “Smart Phones”
TOC Comparison of Cell Phones and Dual-Modes
Enterprise:
–Market forces
employees to be more
productive
Healthcare:
–Critical to reach the
right person immediate
[3]
Warehouse/retail:
–Effective customer
services
6 April 2008
EETS 8313: VoIP over IEEE 802.11
Source: Cisco 2006
Cisco ROI Model Assumption:
Si
Six-year
period
i d
1000 users; 40% mobile users
US tariff rates
Deploy Cisco Unifies wireless Solution
Cisco Dual Mode net price $290
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©2008, Sami Alshuwair
VoIP Protocol Stack Header Overhead
Vocoder Frame
PHY
15 – 24
802.11b
RTP
Vocoder Frame
UDP
RTP
Vocoder Frame
RTP
Vocoder Frame
IP
UDP
MAC
IP
UDP
RTP
Vocoder Frame
MAC
IP
UDP
RTP
Vocoder Frame
20
IPv4
8
12
Variable based on
coder used
34
802.11
Codec Comparison
Codec & Bit
Rate (Kbps)
G.711 (64 )
G.729 (8)
G.723.1 (6.3)
Codec Information
Bandwidth Calculations
Codec
Codec
Mean
Voice
Voice
Packets Per
Bandwidth Bandwidth
Bandwidth MP
Sample Size
Sample
Opinion
Payload Size Payload Size
Second
w/cRTP MP or Ethernet
or FRF.12 (Kbps)
(Bytes) Interval (ms) Score (MOS)
(Bytes) “Pctz D” (ms)
(PPS)
FRF.12 (Kbps) (Kbps)
80 Bytes
10 ms
4.1
160 Bytes
20 ms
50
82.8 Kbps
67.6 Kbps
87.2 Kbps
10 Bytes
10 ms
3.92
20 Bytes
20 ms
50
26.8 Kbps
11.6 Kbps
31.2 Kbps
24 Bytes
30 ms
3.9
24 Bytes
30 ms
34
18.9 Kbps
8.8 Kbps
21.9 Kbps
G.723.1 (5.3)
20 Bytes
30 ms
3.8
20 Bytes
30 ms
34
17.9 Kbps
7.7 Kbps
20.8 Kbps
G.726 (32)
G.726 (24)
G.728 (16)
20 Bytes
15 Bytes
10 Bytes
5 ms
5 ms
5 ms
3.85
80 Bytes
60 Bytes
60 Bytes
20 ms
20 ms
30 ms
50
50
34
50.8 Kbps
42.8 Kbps
28.5 Kbps
35.6 Kbps
27.6 Kbps
18.4 Kbps
55.2 Kbps
47.2 Kbps
31.5 Kbps
3.61
http://www.cisco.com/warp/public/788/pkt-voice-general/bwidth_consume.html
•Compressed Real-Time Protocol (cRTP) reduces the IP/UDP/RTP headers to 2 or 4bytes (cRTP is not available over Ethernet).
•6 bytes for Multilink Point-to-Point Protocol (MP) or Frame Relay Forum (FRF).12 Layer 2 (L2) header.
•G.711 (64) = 80 bytes * 8 bit * 10ms = 64kbps
[4]
6 April 2008
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VoIP Bandwidth Calculation per Call
For example, the required bandwidth for a G.729 call
(8 Kbps codec bit rate) with cRTP, MP and the default 20
bytes of voice payload is:
–Codec
Codec bit
b ratee = codec sample
s p e size
s e / codec sample
s pe
interval
–Total packet size (bytes) = (MP header of 6 bytes) +
(compressed IP/UDP/RTP header of 2 bytes) +
(voice payload of 20 bytes) = 28 bytes
–Total packet size (bits) = (28 bytes) * 8 bits per byte =
224 bits
–Voice payload size = 20 bytes (default voice payload) *
8 bits per byte = 160 bits
–PPS = (8 Kbps codec bit rate) / (160 bits) = 50 pps
–Bandwidth per call = voice packet size (224 bits)
* 50 pps = 11.2 Kbps
[4]
6 April 2008
EETS 8313: VoIP over IEEE 802.11
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©2008, Sami Alshuwair
VoIP Challenges and Solutions
Bandwidth Management:
–Challenge: loaded network/SW with adv. encoding
–Solution: load balancing, call admiss. control
Packet Loss:
–Challenge: RTP: packet loss < 3%
–Solution: Packet loss concealment & redund. algor.
Latency:
–Challenge:
g One wayy latency
y “G.113” <= 150 mSec
–Solution: 802.11e/WMM + wired QoS mechanism
Jitter:
–Challenge: Packet arrival time <= 30 mSec
–Solution: Jitter butter, 802.11e/WMM + wired QoS
6 April 2008
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802.11 WLAN “de facto” Standards
Standard
IEEE 802.11a
IEEE 802.11b
IEEE 802.11g
IEEE 802.11n
(Draft 2.0)
Op.
Frequency
PHY
MAC
Throughput
(Typ)
Max Raw
Rate
5 GHz
OFDM
2.4 GHz
DSSS,
CCK
Service
Interference
CSMA/CA
23 Mbit/s
54 Mbps
CSMA/CA
4.3 Mbit/s
11 Mbps
-MW Oven
-Bluetooth Device
-Amateur Radio
-Cordless phone
-Amateur Radio
-Aeronautical Radio
-Navigation
2.4 GHz
OFDM
, CCK
CSMA/CA
19 Mbit/s
54 Mbps
-Same as 802.11b
2.4 GHz &
5 GHz
OFDM
, CCK
CSMA/CA
74 Mbit/s
248 Mbps
-Same as
802.11b/11a
[5] [6]
6 April 2008
EETS 8313: VoIP over IEEE 802.11
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©2008, Sami Alshuwair
802.11 Medium Access
MAC Frame Types:
–Data
–Control: RTS, CTS & ACK
–Management: Beacon
Distrib. Coord. Func. (DCF)
–Uses a CSMA/CA algorithm
–Senses the medium first
–If
If it is idle for DIFS amount of
time, the frame transmitted
–Otherwise a backoff time B is
randomly in the interval [0, CW]
–Has no priory “best effort”
DCF Method Signal Flow
SIFS = Short Interframe Space. 16us for .11a and
10us for .11b/g
DIFS = DCF Interframe Space = SFIS + 2 x Slot Time
PIFS = PCF Interframe Space = SIFS + Slot Time
Slot Time = 20us for .11b & 9us for .11a , .11g is
9us when all STA high-speed otherwise 20us
Backoff = Random * Slot Time
[7]
6 April 2008
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802.11 Medium Access Cont.
Point Coordination Function (PCF)
–Contention-free frame transfer
–Single
g Point Coordinator (PC)
( ) controls access to the
medium. AP acts as PC.
–PC transmits beacon packet when medium is free for
PIFS time period. PCF has higher priority than DCF
(PIFS < DIFS)
–PCF not used b/c not ideal for real-time traffic:
U hi d performance
Unachieved
f
when
h traffic
ffi load
l d increas.
i
Polling schemes prolong the delay in WLAN
PCF Method Signal Flow
[7]
6 April 2008
EETS 8313: VoIP over IEEE 802.11
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©2008, Sami Alshuwair
IEEE 802.11e “QoS” Overview
Legacy 802.11
Hybrid Coordination
Function (HCF) combines
polled and CSMA/CA
channel access
AC0
1
2
0
3
4
5
6
7
Low Priority
AC1
AC2
AC3
BK
BK
BK
BK
BK
BK
BK
BE
BE
BE
BE
BE
BE
BE
BE
BK
Video
Video
Video
Video
Video
Video
Video
Video
Voice
Voice
Voice
Voice
Voice
Voice
Voice
Voice
Backoff
AIFS[3]
BO[3]
Backoff
AIFS[2]
BO[2]
Backoff
AIFS[1]
BO[1]
6 April 2008
High Priority
Single Queue
Backoff
AIFS[0]
BO[0]
[8] [9]
802.11e WLAN Station
Backoff
SIFS
Enhanced DCF Channel
Access (EDCA)
–Contention based
–DiffServ-Service like
–Traffic can be classified
into 8 different classes
–Each station has 4 Access
Categories (AC)
802.11e EDCA
Legacy
Station
Scheduler
Virtual Collision Handler
Transmission attempts
p
AIFS: Arbitration
Interframe
Spacing
AIFS (2)
AIFS (1)
VoWLAN
DIFS: Distributed
Interfame Spacing
AIFS (0) = DIFS
Contention Window
PIFS
DIFS/AIFS
SIFS
Busy
Medium
Defer Access
PIFS: Point
Interframe
Spacing
SIFS: Short
Interfame Spacing
Backoff Slots
Next Frame
13
IEEE 802.11e “QoS” Overview Cont.
• HCF Controlled Channel Access (HCCA)
–“InServ-service like”
–Specify
Specify time interval for STA “TDM-service
TDM service like
like”
–Can start polling period at any time
Automatic Power Save Delivery (APSD)
–Reduces power consumption 50% to 80% in active
mode.
[10] [11]
6 April 2008
EETS 8313: VoIP over IEEE 802.11
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©2008, Sami Alshuwair
VoWLAN Security
Wired Equivalent Privacy (WEP)
–40 bit – 128 bits key
–Authentication easy
y to break
802.11i:
–802.1x: “authentication”
Varies; vendors preference
–TKIP known as WPA
–CCMP known as WPA2
Increased overhead
More processing time/delay
–Hard to implement in interdomain handover
L2 & L3 Handover
Procedure for VoWLAN
TKIP = Temporal Key Integrity Protocol
WPA = WiFi Protected Access
CCMP = Counter Mode with Cipher Block Chaining
Message Authentication Code Protocol
PMK = Pairwise Master Key
PTK= Pairwise Transient Key
[7] [8] [10]
6 April 2008
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VoWLAN Security Cont. VLANs:
802.1Q Wired
Network w/VLANs
VLAN Prevent:
-Toll fraud
-Denial of Service (DoF) attacks
-Eavesdropping & Interception
AP Channel:
SSID “Data”
Data = VLAN 1
SSID “Voice” = VLAN 2
SSID “Visitor” = VLAN 3
SSID: Data
Security:
PEAP + AES
SSID: Voice
Security:
LEAP + WPA
SSID: Visitor
Security:
Open
SSID = Service Set ID, PEAP = Protected Extensible Authentication Protocol, AES= Advanced Encryption Standard,
LEAP= Lightweight Extensible Authentication Protocol
[13] [14]
6 April 2008
EETS 8313: VoIP over IEEE 802.11
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©2008, Sami Alshuwair
VoWLAN Handover/Roaming
WLAN AP covers 50m2 to 300m2 Æ freq. handover
< 50ms recommended, but >200ms packet lost
L
Layer
2 “V” handover:
h d
from
f
AP to
t AP w/same
/
IP
Layer 3 “H” handover: from AP to AP w/new IP
– SIP or MIP used to resume voice session
With 802.1x & .11i association takes 300-500ms
–Include
Include scan
scan. joining new ch
ch, authen
authen. & associat
associat.
–Passive scan during active VoIP to reduce scan time
–Establish secure key introd. delay “four handshake”
802.11r fast handover: cache context “neighbor graph”
[7] [8] [12]
6 April 2008
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VoWLAN Fast Secure Roaming
Layer 2 “Vertical”- Same IP Subnet
Central
Authentication
Server
Si
WAN Link
*WDS
[13] [14]
*WDS = Wireless Domain Server (Local Authentication Server + Access Point)
6 April 2008
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©2008, Sami Alshuwair
VoWALN Fast Secure Roaming
Layer 3 “Vertical”-Different IP Subnet
1.
Subnet A
3
4 5
GRE Tunnel
Client associates with AP and
receives an IP address,
optionally using WPA (802.1x)
or VPN for securityy
2. Control “B” create client
database: MAC & IP, security
context & association, QoS, 168.1.1.1
etc.
3. Client roams to new subnet or
roams out of radio coverage and
returns
4. Client associate with new AP
& controller. Client database is
copied to new controller
5. New controller recognizes 168.1.1.1
roaming event and provides
client with the same initial IP
address
1
Subnet B
Corporate
Network
2
Radius, LDAP,
Active Directory,
NT Domain Server
Corporate
Servers
GRE = Generic Routing Encapsulation is a tunneling protocol
[13] [14]
6 April 2008
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VoWALN Capacity
What is the max. number of voice call AP can support?
–Ask your vendor
Transmission rate “throughput”
Voice packet payload length depended on encoding
Capacity enhancement solutions:
–Enhanced air access MAC “HCF”
–Virtual
i
l cells
ll “Meru Networkk propriety”
i
–Header compression
–Frame aggregation
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©2008, Sami Alshuwair
VoWALN Capacity Cont.
Throughput Effect:
802.11b WLAN UDP-based Throughput
measurement by Arranz et al
80.11b is around 6Mbps &
802.11a/g around 26/17Mbps
VoIP capacity can not be
estimated based on raw data
throughput:
–Assuming 10Kb/s voice source, theoretical capacity
of 802.11b is 11Mbps/10Kbps =1100 → 550 twoway VoIP sessions.
–In practice only a few VoIP users can be supported in
802.11b!
–Low payload to overhead ratio for short VoIP
packets and inherent inefficiency in 802.11 MAC
[7]
6 April 2008
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VoWALN Capacity Cont.
Max Number of VoIP Connection using
802.11b and Different Packet Intervals
Packet Interval &
Vocoder Effect:
M
Many
vendor
d select
l 30ms
30
Voice call increase
w/packet interval increase
Max Number of VoIP Connection using
802.11a and Different Packet Intervals
VoWLAN 802.11a 5x
than VoWLAN 802.11b
Larger packatization
interval Æ more end-toend delay
[7]
6 April 2008
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©2008, Sami Alshuwair
VoWALN Capacity Enhancement
No over-the-air QoS
“CSMA/CA”
Over-the-air QoS
“TDMA-like HCF”
Channel Access with Today’s 802.11 AP
Channel Access with Meru AP for QoS
12
12
10
8
6
4
AP
10
8
6
4
AP
5.36 5.38 5.4 5.42 5.445.46 5.48 5.5 5.52 5.54 5.56
Time (Sec)
5.36 5.38 5.4 5.42 5.445.46 5.48 5.5 5.52 5.54 5.56
Time (Sec)
HCF “802.11e”doesn’t reduce overhead rather
improves air traffic control “QoS”; contention loss is
reduced and jitter is predictable/low
Reduce AP spacing: data
Further AP spacing: voice
802.11n/abg virtual cells
[12] [15] [16]
6 April 2008
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VoWALN Capacity Enhancement
Timing Overhead of single Voice Frame over 802.11b
Frame Aggregation into RTP Frame by Voice Source
Frame
'1'
Frame
'2 '
Frame
'n'
Aggregated
gg g
Vocoder Frame
Bulk overhead lies on L2 & L1
Header compression for IP layer and
above would be insignificant
Inter-Call Frame Aggregation: Multicast. Multiplexing “M-M”
• Capacity 80%-90% higher than VoIP
• Security & Delay (collision & PSM)
[17] [18]
RTP
Aggregated Vocoder Frame
Packet delay increases
Frame Aggregation “Intra-Call”
Requires changes in the MAC layer
implement. of the interface queue
PSM = Power Saving Mode
6 April 2008
EETS 8313: VoIP over IEEE 802.11
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©2008, Sami Alshuwair
VoWLAN Challenges and Solutions
Location Tracking
–Challenge: meet E911 and E112 requirements
–Solution: WLAN appliance, GSM/CDMA tracking
Handset Battery Life
–Challenge: improve talk/standby time to 4+/100+
–Solution: U-ASPD, propriety
Coverage
Cove
age & Capacity
Capac y
–Challenge: Pervasive coverage, no dropped call, etc.
–Solution: HCF, propriety, frame aggregation
6 April 2008
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VoWLAN Challenges and Solutions Cont.
Roaming
–Challenge: Maintain connection/quality <50 ms
–Solution: 802.11r, propriety, 802.11k/.11v (future)
Security
–Challenge: avoid eavesdropping, maintain access
control
–Solution: WAP2 (wireless) + VLAN (wired), 802.1x
6 April 2008
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©2008, Sami Alshuwair
WLAN System Architectures Consideration
Centralized versus distributed
Definition
–Centralized
Centralized (C) – controller-based data forwarding
–Distributed (D) –AP-based forwarding
Impact on wired network
– C – may switch packets twice
– D – switch packets only once
Impact on WLAN controller
– C – controller touches packets from every AP
– D – AP – touches packets from associated stations
[19]
6 April 2008
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WLAN System Architectures Consideration
cont.
Impact on VoWLAN jitter/latency
– C – controller processes every VoWLAN call/packet
– D – AP processes packets for associated stations
Other factors
–Emergence of 802.11n
802.11n will increase traffic by 5x
6 April 2008
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©2008, Sami Alshuwair
VoWLAN Alternatives; Pros and Cons
Mobile Cellular Phones
–Example: Nokia E60, iPhone, etc.
–Pro: Widely available, good outdoor coverage
–Con: Costly, limited in building coverage, not
integrated with enterprise telephony system (FMC)
Cordless PBX Handsets
–Example:
Example: Avaya TransTalk 9040,
9040 Philips 5111
–Pro: Excellent range, good voice quality, no WLAN
security issues
–Con: Requires an additional wireless network, no
high-speed data, limited capacity
6 April 2008
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Recommendations
Determine the need for VoWLAN
g is sufficient
Make sure that WLAN coverage
Verify WLAN is ready for VoWLAN
Ensure VoWLAN compatibility with emprise
WLAN
Gain eexperience
perience with
ith single
single-mode
mode phones
before introducing dual-mode “smart phones”
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©2008, Sami Alshuwair
References
[1] http://www.gizmag.com/go/7258/
[2] http://mediaproducts.gartner.com/reprints/merunetworks/153883.html
[3] How Cisco Mobility Solutions Can Reduce Costs and Improve Productivity for Today's Enterprise, Cisco, 2006
[4] Voice Over IP - Per Call Bandwidth Consumption, Cisco, 2006
[5] Fundamentals of Wireless LANs, Ciscopress, 2004
[6] IEEE 802.11n, http://en.wikipedia.org/wiki/802.11n
[7] Trond Ulseth and Paal Engelstad, “Voice over WLAN (VoWLAN)- A Wireless voice alternative?,”Telenor R&D, 2006
[8] Jean-Michel Lauriol, Luc Brignol, Philippe Laine, Philippe Dauchy and Alberto Conte “WLAN, a Converged Data and Voice
Mobility Solution for Enterprise,” Alcatel-Lucent, 2004
[9] Lin Cai1,Yang Xiao2, Xuemin (Sherman) Shen, Lin Cai3,and Jon W. Mark, “VoIP over WLAN Voice capacity, admission
control, QoS, and MAC,” International Journal of Communication System, 2006
[10] Shiao-Li Tsao,“Research Challenges and Perspectives of Voice over Wireless LAN,” Dept. of Computer Science and
Information Engineering, National Chiao Tung University, Hsinchu, Taiwan, R.O.C., 2005
[11] WMM Power Save for Mobile and Portable Wi-Fi Certifid Devices, Wi-Fi Alliance, December 2005
[12] Is It the Network? Solving VoIP Problems on a Wireless LAN, Global Knowledge Training LLC, 2007
[13] Is Your WLAN Ready for Voice, Cisco, 2007
[[14]] Design
g Principles
p for Voice over WLAN,, Cisco,, 2007
[15] The enterprise is ready for wireless VoIP. Is wireless VoIP ready for the enterprise, Meru Networks, 2005
[16] Fulfilling the promise of 802.11n in the Enterprise without compromise, Meru Networks, 2007
[17] Wei Wang and Soung Chang Liew,” Solutions to Performance Problems in VoIP Over a 802.11 Wireless LAN,” IEEE Journal,
2005
[18] Sangki Yun, Hyogon Kim, Heejo Lee, and Inhye Kang,“100+ VoIP Calls on 802.11b The Power of Combining Voice Frame
Aggregation and Uplink-Downlink Bandwidth Control in Wireless LANs,” IEEE Journal, 2007
[19] Adding Voice to the Wireless LAN - Developing an Implementation Strategy, TechTarget, 2007
6 April 2008
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Frame Aggregation “Intra-Call”
Legacy Burst
FCS
MPDU
Payload
MPDU
Header
Preamble
PLCP
header
PSDU3
FCS
MPDU
Payload
Preamble
PLCP
header
FCS
MPDU
Payload
MPDU
Header
Preamble
PLCP
header
Preamble
+ PLCP
Header
Perform
aggregation
MPDU
Header
PSDU2
PSDU1
SIFS
SIFS
A-PSDU
Preamble + PLCP headers + SIFS will be saved
Some overhead will be induced to identify each MPDU
6 April 2008
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16