VOIP over Wireless Network

VOIP over Wireless Network Prof. Anirudha Sahoo KReSIT IIT Bombay

Outline • Primer on Voice over IP System • QoS in VOIP • Primer on Wireless LAN (802.11) • Different approaches to VOIP over wireless network • Mobility Issues • Summary

Voice Over IP (VOIP) • Transmission of digitized voice in packet network (e.g. IP, ATM, Frame Relay) • Enables telephone conversation to be carried over IP network (in part or end-to-end) • Provides a toll bypass path for telephone calls • Enables Telephony providers to provide cheaper service

PSTN Network IP Network PSTN gateway PSTN gateway gatekeeper VOIP System PBX PBX (A typical PSTN system) (A typical VOIP system)

VOIP System (cont.) IP Network CPE router CPE router LAN LAN PSTN Gateway SIP proxy PSTN Soft phone IP phone IP phone (Another VOIP system)

QoS in VOIP • VOIP applications (e.g. telephone call) are real time in nature • So they require QoS from the underlying system • Many factors determine voice quality • Choice of codec • Delay • Jitter • Packet loss

Delay • VOIP packet can experience delay at various point on its path • Encoding delay in the codec (algorithmic + processing) (~17ms) (for G729 codec) • Packetization/Depacketization delay (~20ms) • Access (up) link transmission delay • Delay in the backbone network • Access (down) link transmission delay • Jitter buffer delay (10 – 60ms) • Decoder delay in codec (at the receiver) (2ms) • Playout delay (0.5ms)

Delay (cont.) • ITU-T G.114 recommends the following one-way delay time limits • 0 – 150 ms : acceptable for most user apps • 150 – 400 ms : acceptable for international connections • > 400ms : unacceptable • Thus packet delay is a very important QoS parameter in VOIP system for an acceptable telephone conversation

Delay (cont.) • From the breakdown of end-to-end delay it is clear that some delays are unavoidable • Delay in the network is the component that can be controlled • Network QoS

Network QoS • Can be provided by few approaches • Engineering the network • IntServ • DiffServ • MPLS-based

Network QoS : Engineering the network • Set aside separate resources for voice flows • Priority queuing at the routers for voice packets • Weighted Fair Queueing with high weight for voice • Policing traffic so that some percentage of bw is reserved for voice traffic.

VOIP QoS : Intserv • RSVP is the protocol of choice for providing QoS under IntServ architecture • Uses a separate reservation phase to allocate resources for voice calls • Guaranteed service model used in RSVP can provide delay guarantee to voice call • Has scalability problem and large overhead • Hence only suitable for an enterprise network (e.g. intranet)

VOIP QoS : Diffserv • Diffserv was developed to circumvent some of the problems in Intserv • Achieves scalability by providing differentiated service to aggregate traffic • Packets carry the PHB (Per Hop Behavior) info. in the header (DS field) • Resources are provisioned for particular Class of Service by the ISP • Policing and Shaping is done at the edge of the network to check for conformance (with SLA) • Thus appropriately classifying voice packets will provide QoS to voice calls

VOIP QoS : MPLS • Use MPLS to achieve traffic engineering • Use RSVP-TE to reserve resources as well as provide explicit routing • CR-LDP can also be used to engineer traffic by providing explicit route • DiffServ can also be combined with MPLS to map DiffServ Behavior Aggregates (BA) to LSPs.

VOIP QoS : Summary • So there are architectures and mechanisms available to provide QoS for VOIP applications in a wired network so that the delay constraint of such applications can be met

VOIP in Wired Network RSVP/Diffserv/MPLS/ Engineered Network IP Network PSTN gateway PSTN gateway gatekeeper PBX PBX (Delay bounded VOIP system)

Wireless Network • Wireless networks are better than wired networks with regards to ease of installation and flexibility • But they suffer from lower bandwidth, higher delays and higher bit error • Thus running VOIP application over such a network is quite challenging and requires additional measures

IEEE 802.11 network • Most widely used WLAN • Uses a shared medium • Low medium utilization • Risk of collision • No service differentiation between types of traffic • Has two access methods (MAC) • Distributed Coordinator Function (DCF) • Point Coordinator Function (PCF)

DCF • Uses a CSMA/CA algorithm in MAC • Before a data frame is sent, the station senses the medium • If it is idle for at least DCF interframe (DIFS) amount of time, the frame is transmitted • Otherwise a backoff time B (measured in time slots) is chosen randomly in the interval [0, CW)

DCF (cont.) • After medium is detected idle for at least DIFS, the backoff timer is decremented and frame is transmitted when it reaches zero • If medium becomes busy during count down, backoff timer is paused and restarted when medium is idle for DIFS period • If there is a collision, CW is doubled according to

DCF (cont.) Where i = number of retransmissions k= constant defining minimum CW • A new backoff time is then chosen and the backoff process starts over.

DCF Timing diagram DIFS Src Data SIFS Dest Ack DIFS Contention Window Others Next MPDU Backoff after Defer Defer Access

B1 = 25 B1 = 5 wait data data wait B2 = 10 B2 = 20 B2 = 15 B1 and B2 are backoff intervals at nodes 1 and 2 cw = 31 DCF Example

PCF(Point Coordination Function) • Contention-free frame transfer • Single 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 the DCF (PIFS < DIFS) • During PCF mode, • PC polls each station for data • After a transmission of a MPDU, move on to the next station

VOIP over Wireless (VoW) • Since VOIP requires bounded delay it is obvious that DCF is not suitable for VOIP traffic (since it is contention based, it cannot provide any deterministic delay bound) • PCF, being polling based, can provide delay bound, hence is a good candidate for VOIP • But most 802.11 products do not have PCF implementation • Delay can be large when too many stations have data to send in CFP

VOIP over Wireless (cont.) IP Network CPE router CPE router PSTN Gateway SIP proxy PSTN Soft phone Mobile IP phone Mobile IP phone (A VOIP over Wireless System)

VOIP over Wireless (cont.) • Various mechanisms can be used to provide delay bounds for VOIP communication • Enhanced DCF (EDCF) • Distributed Fair Scheduling • Wireless Token ring • Blackburst

Enhanced DCF • Provides service differentiation • Traffic can be classified into 8 different classes • Each station has 4 access categories to provide service differentiation

Access Category (AC) • Access category (AC) as a virtual DCF • 4 ACs implemented within a QSTA to support 8 user priorities • Multiple ACs contend independently • The winning AC transmits frames AC0 AC1 AC2 AC3 A A A A B B B B I I I I B B B B a a a a F F F F c c c c O O O O S S S S [ [ [ [ k k k k [ [ [ [ o o o o 0 1 2 3 0 1 2 3 f f f f ] ] ] ] ] ] ] ] f f f f Virtual Collision Handler Transmission Attempt

Differentiated Channel Access • Each AC contends with • AIFS[AC] (instead of DIFS) and CWmin[AC], CWmax[AC] (instead of CWmin, CWmax)

Priority to AC Mapping

Distributed Fair Scheduling (DFS) • Based on SCFQ • Uses a distributed approach for determining the smallest finish tag using backoff interval mechanism of 802.11 • Backoff interval is chosen such that it is proportional to the finish tag of packet to be transmitted • So packets with smaller finish tag will be assigned smaller backoff interval

Distributed Fair Scheduling (cont.) • Backoff interval is inversely proportional to weight assigned to a node. Thus node with higher weight is given a higher priority (because of smaller backoff interval) • VOIP application can use the scheme to achieve better QoS by availing priority over data traffic

Wireless Token Ring Protocol • Wireless Token Ring Protocol (WTRP) can support QoS in terms of bounded latency and reserved bandwidth • Efficient, since it reduces the number of retransmissions • Fair in the sense that every station takes a turn to transmit and gives up its right to transmit (by releasing the token) until the next round • Can be implemented on top of 802.11

WTRP (cont.) • Successor and predecessor fields of each node in the ring define the ring and the transmission order • Station receives token from predecessor, transmits data and passes the token to the successor. • Sequence number is used to detect any nodes that are part of the ring, but not in the range of a node

B B B A C C F E D Transmission range of E WTRP (cont.) Connectivity table of E

WTRP (cont.) • Implicit acknowledgement is used to monitor successful transmission of token • Timer is used to guard against loss of token (successor might have moved out of range) • Using connectivity table, the ring can be reformed when a node moves out of range • By controlling the token holding time and token rotation time delay of packets can be bounded. • Hence WTRP can be used for VOIP applications

Blackburst • Devised with a view to minimizing delay for real-time traffic • Stations are assigned priority • When a high priority station wants to send a frame • Senses the medium to see if it is idle for PIFS time period and then sends its frame • If medium is busy, station waits until channel has been idle for a PIFS and then enters a black burst contention period • The station sends a black burst by jamming the channel for a period of time

Blackburst • The length of the black burst is proportional to the amount of time the station has been waiting to access the medium (calculated as a number of black slots) • After transmitting black burst, the station listens to the medium for a short period of time (less than a black slot) to see if some other station is sending a longer black burst (hence has waited longer) • If the medium is idle, then station sends its frame • Otherwise it waits until the medium becomes idle again and enters another black burst contention

Blackburst • After successful transmission of a frame, the station schedules the next access instant tschseconds in the future. • This has the nice feature that real-time flows will synchronize and share the medium in a TDM fashion • Unless there is a transmission by low priority station when a high priority station accesses the medium, very little blackbursting needs to be done once stations have synchronized • Low priority stations use ordinary DCF access mechanism

VoW RSVP/Diffserv/MPLS/ Engineered network IP Network CPE router CPE router EDCF/DFS/ WTRP EDCF/DFS/ WTRP PSTN Gateway SIP proxy PSTN Mobile IP phone Soft phone Mobile IP phone (Delay bounded VoW system)

VoW (cont.) • Since end-to-end delay of a VOIP call is important, in the VoW system it is necessary to budget the delay appropriately across the various components (e.g. wired network, wireless LAN) in the path of the call • Calls have to be admitted carefully so that end-to-end delay is within acceptable limit

Mobility • Mobility adds complexity to VOIP connections • Need to have fast and smooth handoff • Can be of two types: • Micro mobility • Mobile station (MS) moves within a domain, usually within an enterprise • Can quickly connect to the new AP (~300ms) (link layer handoff) • Macro mobility • MS moves into a different domain (e.g. moves from one hotspot to another and the two hotspots are managed by different ISPs)

Mobility Internet Hot Spot B Hot Spot A AP AP AP AP Micro mobility Macro mobility Micro mobility

Mobility • Two approaches available: • Mobile IP • handoff at network layer • SIP • handoff at the application layer

Handoff using Mobile IP • 3 Parts of Mobile IP • Advertising Care-of Addresses • Registration • Tunneling

VOIP over Wireless Network