1. VoIP over Wireless Networks
12 September 2006
Yeonsik Jeong

2. Contents Overview of VoIP Overview of IEEE 802.11 Networks
VoIP over IEEE Networks
Performance Analysis
Solution Derivation
Solutions & Simulation Results

3. Overview of VoIP VoIP (Voice over IP) Definition Example applications
Allows you to make telephone calls using a broadband Internet connection instead of a regular (or analog) phone line
Example applications
Skype (
MSN messenger (
Vonage (
Hardware phones
Cisco 7940
Linksys CIT200
Signaling protocol
Locating partners
agreeing on port numbers of RTP/RTCP, codec, and media type

4. Overview of VoIP Components of VoIP Signaling Protocols
To establish presence, locate users, set up/modify/tear down calls
H.323
SIP (Session Initiation Protocol)
Media transport protocols
To transmit packetized audio/video data
RTP/RTCP
Voice codec
To convert analog voice to digital data and compress it
G.711
G.723.1
G.729
Signaling protocol
Locating partners
agreeing on port numbers of RTP/RTCP, codec, and media type

5. Overview of VoIP SIP (RFC 3261) Overview Elements Methods
SIP UA (User Agent)
SIP Server
Used to locate SIP users or to forward messages
Proxy server or Redirect server
SIP Gateway
To PSTN for telephony interworking
Methods
REGISTER: binds a permanent address to current location
INVITE: initiates sessions
ACK: confirms session initiation and can only be used with INVITE
BYE: terminates a session
CANCEL: cancels a pending INVITE

6. Overview of VoIP SIP Examples Simple scenario: direct connection
1xx: Informational
2xx: Success
3xx: Redirection
4xx: Client Error
5xx: Server Error
6xx: Global Failure

7. Overview of VoIP
SIP Examples
General scenario: using proxy server

8. Contents Overview of VoIP Overview of IEEE 802.11 Networks
VoIP over IEEE Networks
Performance Analysis
Solution Derivation
Solutions & Simulation Results

9. Overview of IEEE 802.11 Networks
Wi-Fi
Technology of wireless local area networks (WLAN) based on the IEEE specifications
Two architectures

10. Overview of IEEE 802.11 Networks
Physical Layer
DSSS/FHSS: 1~2 Mbps / 2.4 GHz
802.11b HR-DSSS: 1~11 Mbps / 2.4 GHz
802.11g OFDM: 1~54 Mbps / 2.4 GHz
802.11a OFDM: 6~54 Mbps / 5 GHz
MAC Layer
DCF (Distributed Coordination Function)
Mandatory and distributed access method
Similar to CSMA/CA with some additional control features
PCF (Point Coordination Function)
Optional and centralized access method
can be implemented in an infrastructure mode, not in an ad-hoc mode

11. Overview of IEEE 802.11 Networks
DCF
Two access mechanisms
Basic mechanism: 2-way handshaking
RTS/CTS mechanism: 4-way handshaking
2-Way Handshake
(Basic Access)

12. Overview of IEEE 802.11 Networks
DCF
Basic (2-Way handshake) access mechanism
DIFS (Distributed Interframe Space)
SIFS (Short IFS)
Random backoff

13. Contents Overview of VoIP Overview of IEEE 802.11 Networks
VoIP over IEEE Networks
Performance Analysis
Solution Derivation
Solutions & Simulation Results

14. VoIP over IEEE 802.11 Networks
Call Capacity of VoIP over IEEE b Networks
Codec Parameters
Voice codec: G.711
Frame interval: 10 ms → Frame rate: 100
Frame size: 80 Bytes
Expected number of VoIP calls
85 calls
Actual number of VoIP calls in simulation
5 calls
Actual number of VoIP calls in testbed experiment

15. VoIP over IEEE 802.11 Networks
Testbed Setup

16. Contents Overview of VoIP Overview of IEEE 802.11 Networks
VoIP over IEEE Networks
Performance Analysis
Solution Derivation
Solutions & Simulation Results

17. Performance Analysis Terminology Maximum Frame Rate (MFR)
Captures the frame rate that can be expected at each layer L
Minimum Required Transmission Delay (mRTD)
Gives time taken to transmit the PDU at the corresponding layer
Maximum Number of VoIP Calls (MNVC)

18. Performance Analysis Application Layer Capacity Ideal throughput
The potential MNVC at the application layer is 85 calls

19. Performance Analysis Impact of Transport and Network Layers
Adds headers to the frames
The potential MNVC at the network layer, taking into account all the overheads of RTP, UDP, and IP headers, is 57 calls

20. Performance Analysis Impact of MAC Layer
Adds considerable overhead to the frame including MAC header, MAC backoff time, MAC ACK, and inter-transmission times (DIFS and SIFS)
The potential MNVC at the MAC layer, taking into account all the overheads of the higher layers, DIFS, SIFS, backoff delay, and ACK, is 6 calls

21. Performance Analysis Impact of Physical Layer
Adds long preamble known as PLCP header transmitted at the basic rate (1 Mbps)
The potential MNVC at the PHY layer, taking into account all the overheads of the higher layers and PLCP header, is 5 calls

22. Contents Overview of VoIP Overview of IEEE 802.11 Networks
VoIP over IEEE Networks
Performance Analysis
Solution Derivation
Solutions & Simulation Results

23. Solution Derivation Final Equation for MNVC
Five Schemes Possible to Improve MNVC
ACK Aggregation (AA): results in the reduction of TACK
Frame Aggregation (FA): decreases k
Link Adaptation (LA): can control R
Time Saving (TS): reduces TDIFS
Header Compression (HC): reduces ∑H(L)

24. Solution Derivation Impact of Each Scheme on Performance Improvement
Proposed Solutions
ACK Aggregation (AA)
Frame Aggregation (FA)
Link Adaptation (LA)

25. Contents Overview of VoIP Overview of IEEE 802.11 Networks
VoIP over IEEE Networks
Performance Analysis
Solution Derivation
Solutions & Simulation Results

26. Solutions & Simulation Results
ACK Aggregation (AA)
Algorithm summary
AA refers to sending a single ACK for a block of n frames
Adaptive AA algorithm uses variable block size based on the received block ACK information
Increases the block size upon receiving a block ACK with all successes
Reduces the block size on receiving a block ACK with even a single frame loss

27. Solutions & Simulation Results
ACK Aggregation (AA)
Simulation setup
Multiple wireless nodes and noisy channel
Simulation results

28. Solutions & Simulation Results
Frame Aggregation (FA)
Algorithm summary
FA refers to fusing multiple frames destined to the same end user into a single large frame
Enhanced piggybacking aggregates frames only when required
To improve the performance of upstream flow, a client maintains a variable which holds the number of aggregated frames in the previously received frame from the AP

29. Solutions & Simulation Results
Frame Aggregation (FA)
Simulation setup
Multiple wireless nodes and error-free channel
Simulation results

30. Solutions & Simulation Results
Link Adaptation (LA)
Algorithm summary
LA refers to changing the transmission rate for the data frames
SARF (Size-aware Auto Rate Fallback) algorithm is based on ARF, but it considers channel condition as well as the frame size
If a small frame is in error then there is a high probability of error for a large frame as well, and when a large frame is successful, there is a high probability of success for a small frame

31. Solutions & Simulation Results
Link Adaptation (LA)
Simulation setup
Single wireless node with background traffic and noisy channel
Simulation results