Voice over Wireless LAN
Outline
Introduction to VoWLAN
Wireless LAN Technology
Why VoWLAN?
VoWLAN Requirement
VoWLAN Challenge
Summary
Introduction to VoWLAN
VoWLAN or Voice over Wireless Local Area Network expands the capability of WLANs or Wireless LANs
VoWLAN is a natural extension of VolP
VoWLAN is the added feature that will enable you to make phone calls using this mobile Internet access
VoWLAN Technology
VoIP + Wireless LAN
VoIP
SIP, RTP, H.323
Wireless LAN
WiFi : 802.11a/b/g
WiMAX : 802.16
802.11 Overview
Infrastructure mode
Access Point
Access Point Wired Network
End Device
End Device End Device
End Device
802.11 Overview
adhoc mode
End Device
End Device
End Device
End Device
802.11 Overview
DCF : Distributed Coordination Function
Contention-based MAC function
802.11 Overview
PCF : Point Coordination Function
Contention-free MAC function
802.11 Overview
11/3 11/3
12/8 11/3
Channels/non- overlapping
PBCC OFDM
OFDM DSSS/CCK
Modulation Encoding
83.5MHz 83.5MHz
300MHz 83.5MHz
Available Spectrum
2.4GHz 2.4GHz
5GHz 2.4GHz
Frequency
6 Mbps 20~25Mbp
27Mbps s 4~5Mbps
Average Actual throughput
22/44Mbp s
54Mbps 54Mbps
11Mbps Raw Data Rates
802.11b 802.11g +
802.11a 802.11b
WiFi Phone protocol stack
VoIP Application
Vocoder MMI
Application
Control Plane Data Plane Management Plane
SIP/SDP RTP/RTC
P RADIUS/DIAMETER
UDP/TCP IP
802.11 MAC
802.11 a/b/g/ RF/BB EAP/802.1x/…
802.11e/f/h/i/k …
Why VoWLAN
Low cost
Free Charge of ISM Band
ISM band : free (2.4-2.4835 GHz)
3G band : NTD 10 Billion
Inexpensive network deployment
Reuse of existing network, easy to setup
Low cost of Access Point VS. High cost of Base Station
Why VoWLAN
Low complexity
Centralized architecture in cellular network
PBX contains most intelligence of the network
Typically hard to maintain the proprietary system
Decentralized architecture in VoIP network
Intelligence are implemented in User Agent
Easy for maintenance
Why VoWLAN
Low transmission power
Small coverage of the AP, small transmission power needed
GSM: 500mW ~ 2W
WLAN: < 100mW
Easy for providing value-added service
Voice and data service is integrated into VoIP
Flexibility of SIP protocol
Why VoWLAN
Market trend
VoWLAN market will reach $507 million (end user revenue) by 2007 (In Stat/MDR)
VoWLAN handset will grow by more than 89 percent annually until 2007 when there will be more than 653,000 (On world)
VoWLAN Requirement
Performance
Voice quality must be as good as wired network
Delay >100 ms is typically sensible by human
Low latency : <50 ms latency is recommended
Reliable transmission over wireless channel
Low packet lost rate
User mobility management
Support roaming between wireless network
VoWLAN Requirement
Capacity management
Heavy traffic load increase packet lost rate and latency
Number of Users must be controlled
Channel assignment
11 channels in 802.11b
Manage operating channel among adjacent Access Point
VoWLAN Requirement
Security
Data ciphering
Wireless channel is insecure
Data over wireless should be protected
AAA
Authentication : legal user identification
Authorization : service level differentiation
Accounting : statistics for billing
Location Tracking
VoWLAN Challenge
Due to the requirements of VoWLAN, several issue should be solved
User Mobility Issue
Power Consumption Issue
Security Issue
QoS Issue
Capacity Issue
Other Related Issue
User Mobility Issue
Supporting user mobility is an important feature of VoWLAN
Typically concern about two factors
Handoff latency
Packet lost rate
Seamless handoff
Fast handover : focus on reducing handoff latency
Smooth handover : focus on reducing packet loss during handoff
Handoff Approach
Layer 2 approach
Access Point
End Device
Access Point
End Device 1
2 3
4
1 : reassociation
2 : auth (802.1x, EAP)
3 : auth (RADIUS/DIAMETER) 4 : packet send/recv
AAA Server
Handoff Approach
MIP approach
Access Point Access Point
HA FA
CN
MN
Handoff Approach
Access Point
MN
Access Point
SIP Proxy AAA Server
MN REINVITE
SIP Mobility approach
Handoff Approach
Intra ESS
L2 approach with/without authentication
Inter ESS
DHCP + MIP
DHCP + SIP Mobility
Inter Domain
Same as Inter ESS, but business policy should be concerned
Power Consumption Issue
Always be a problem since only limited battery power available at mobile
device
System
CPU, Memory, LCD, DSP/Codec
WLAN
Physical Layer: RF
MAC Layer: 802.11a/b/g
Network Layer: TCP/IP
802.11 Power Saving Mode
Reduce power consumption of
transceiver when mobile device is idle
AP buffers data packet for the mobile device which is in PSM, and inform it to receive by sending beacon
Mobile device in PSM periodically wake up to receive data packet buffered in AP
802.11 Power Saving Mode
PS data
AP
Client 1 1
1
1 1
1
Beacon
Interval Wait
Interval
beacon beacon with data PS poll
2
Client 2 2
2 2
2 sleep
sleep
802.11h
Supplementary to 802.11a (5GHz)
TPC (Transmission Power Control)
Keeps signal strength efficient, using only enough power to reach active users rather than using a uniform power output
DFS (Dynamic Frequency Selection )
Selects the radio channel at the access point to minimize interference with other systems
Security Issue
Data ciphering
WEP, 802.11i
AAA (Authentication, Authorization, Accounting)
802.1x, RADIUS, DIAMETER
WEP
WEP use RC4 to encrypt data which is dependent on the IV (Initialization
Vector) and Shared Key
Data ICV
IV Shared Key
CipherStream RC4
CipherText IV
XOR
802.11i
Data transfer protection
TKIP : based on RC4
CCMP : import AES algorithm with better security
Authentication
802.1x, EAP
EAP 802.1x
CCMP TKIP
Authentication
Data ciphering
TKIP
CCMP
802.1x
General-purpose, port-based network access control mechanism for any 802 technology
Enables mutual authentication of devices
Provides service for exchange of 802.11 session keys
Leverages existing AAA infrastructure
Extensible protocol to support future authentication methods (RFC 2284)
End Device
802.1x – EAP Authentication
Access Point Auth Server Request/Identity
Response/Identity Radius-Access-Request Radius-Access-Challenge EAP-Request
EAP-Response Radius-Access-Request Radius-Access-Challenge EAP-Success
802.1X RADIUS
QoS Issue
Typically, voice quality is depend on the delay and loss rate of packets
No QoS guarantee in legacy 802.11 DCF, since each mobile device contends for
the channel by using CSMA/CA
There are some proprietary QoS
schemes proposed, but QoS is still an open issue
802.11e
Promise to bring QoS capabilities WLAN system need for streaming applications
Introduce HCF (Hybrid Coordination
Function) to provide some QoS facilities
EDCA : Enhanced Distributed Cannel Access
HCCA : Hybrid coordination function Controlled Channel Access
EDCA
Contention-based channel access
Four backoff entities within one station
Each backoff entities represents one
Access Category (AC) and has different contention window size
AC_VO (voice), AC_VI (video) , AC_BE (best effort) , AC_BK (background)
EDCA
Mapping to Access Category
AC_VO Queue
AC_BK Queue AC_BE
Queue AC_VI
Queue
MSDU
Backoff Mechanism
Transmission Backoff
Mechanism
Backoff Mechanism
Backoff Mechanism
HCCA
Controlled channel access
HC can allocate TXOP (Transmission Opportunity) during CFP or CP by
transmitting QoS CF-Poll frame
During CFP, this mechanism is the same as legacy 802.11
During CP, it will allocate the medium after detecting the channel being idle for PIFS
HCCA
Contention Free Period Contention Period
Polled TXOP TXOP Polled TXOP
802.11e Superframe
Capacity Issue
Voice quality is a key component of voice service (real-time, high throughput)
CSMA/CA mechanism limits the max # of subscribers under the AP
A VoIP streams typically requires less than 10Kbps
Ideally, the number of simultaneously VoWLAN sessions is
11M / (10K * 2) = 550
However, the maximum number of VoIP sessions is about 12 if GSM 6.10 (13.2Kbps) is used
Capacity Issue
An analysis result from “W. Wang et al, Solution to Performance Problems in
VoIP over 802.11 Wireless LAN”
11.4 G.729
10.8 G.726-32
17.2 G.732.1
10.2 G.711
11.2 GSM 6.10
Max # of user Codec
Multiplex-Multicast Scheme
Multiplex : Combine several downlink data into one
Multicast : Multicast the packet to all destination
De-Multiplex : Retrieving the corresponding payload
DeviceEnd
Access
Point Voice
Gateway Access
Point Voice
Gateway
DeviceEnd
DeviceEnd End
Device Internet
MUX MUX
DEMUX DEMUX
Other Related Issue
Codec Compression
The ability to maximize the wireless bandwidth for voice, intelligent use of compression codec is
important.
Often require hardware assist, the target device is hardware dependent and needs to be specially
designed
PBX Integration
Provide the PSTN access, often a gateway solution
SIP ENUM
SIP ENUM
Other Related Issue
Combine WLAN and Cellular
WLAN
High bandwidth, Low Cost, Multimedia Service, Video Phone
Cellular
Large Coverage, High Mobility, Mature Billing System, Popularity
ServerAAA
WLAN + Cellular
Access Point Multimedia
Center
Access Point
HLR
PhoneWiFi WiFi Phone
ServerWeb
PhoneWiFi WiFi
Phone WiFi Phone PhoneWiFi
Internet
Cellular Sim-based AAA
Voice Session Multimedia Access
Summary
The existing VoWLAN solutions may not be robust and reliable enough to support
deployment for a large base of users
QoS of VoWLAN is always an open issue
it may or may not have a good solution
Security and Capabilities for fast handoff between APs still needs some improvement
Summary
RF management and MAC layer mechanism affect power consumption of mobile device
Voice over WLAN is a trend
Many cellular operators have devoted to the development of VoWLAN
Government promotes the combination of WLAN and Cellular network
VoWLAN may just be the next big thing in mobile telephony
