VoIP over Wireless Networks 12 September 2006 Yeonsik Jeong ysjeong@ece.gatech.edu

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

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 (http://www.skype.com) MSN messenger (http://www.msn.com) Vonage (http://www.vonage.com) Hardware phones Cisco 7940 Linksys CIT200 Signaling protocol Locating partners agreeing on port numbers of RTP/RTCP, codec, and media type

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

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

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

Overview of VoIP SIP Examples General scenario: using proxy server

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

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

Overview of IEEE 802.11 Networks Physical Layer 802.11 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

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)

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

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

VoIP over IEEE 802.11 Networks Call Capacity of VoIP over IEEE 802.11b 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

VoIP over IEEE 802.11 Networks Testbed Setup

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

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)

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

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

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

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

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

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)

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

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

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

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

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

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

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

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