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 18 September 2018

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 18 September 2018

VOIP System (A typical PSTN system) (A typical VOIP system) IP Network PSTN gateway IP Network PSTN gateway PSTN Network gatekeeper PBX PBX (A typical PSTN system) (A typical VOIP system) 18 September 2018

VOIP System (cont.) (Another VOIP system) IP Network CPE router LAN LAN PSTN Gateway SIP proxy PSTN Soft phone IP phone IP phone (Another VOIP system) 18 September 2018

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 18 September 2018

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 18 September 2018

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) 18 September 2018

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 18 September 2018

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 18 September 2018

Network QoS Can be provided by few approaches Engineering the network IntServ DiffServ MPLS-based 18 September 2018

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. 18 September 2018

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) 18 September 2018

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 18 September 2018

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. 18 September 2018

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 18 September 2018

VOIP in Wired Network (Delay bounded VOIP system) RSVP/Diffserv/MPLS/ Engineered Network IP Network PSTN gateway PSTN gateway gatekeeper PBX PBX (Delay bounded VOIP system) 18 September 2018

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 18 September 2018

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 18 September 2018

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) 18 September 2018

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) 18 September 2018

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 18 September 2018

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. 18 September 2018

DCF Timing diagram Src Dest Others 18 September 2018 Data Ack DIFS Src Data SIFS Dest Ack DIFS Contention Window Others Next MPDU Defer Access Backoff after Defer 18 September 2018

DCF Example cw = 31 data wait B1 = 5 B2 = 15 B1 = 25 B2 = 20 B2 = 10 B1 and B2 are backoff intervals at nodes 1 and 2 cw = 31 B2 = 10 18 September 2018

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 18 September 2018

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 18 September 2018

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) 18 September 2018

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 18 September 2018

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 18 September 2018

Enhanced DCF Provides service differentiation Traffic can be classified into 8 different classes Each station has 4 access categories to provide service differentiation 18 September 2018

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 B A B A B A B B I a B I a B I B I F F F a F a O c S O c O c O c [ S S S [ k [ [ k [ [ k [ [ k o 1 o 2 o 3 o ] ] f f ] 1 f 2 3 ] f ] ] f f f ] ] f Virtual Collision Handler Transmission Attempt 18 September 2018

Differentiated Channel Access Each AC contends with AIFS[AC] (instead of DIFS) and CWmin[AC], CWmax[AC] (instead of CWmin, CWmax) 18 September 2018

Priority to AC Mapping Priority Access Category (AC) Designation (Informative) Best Effort 1 2 3 Video Probe 4 Video 5 6 Voice 7 18 September 2018

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 18 September 2018

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 18 September 2018

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 18 September 2018

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 18 September 2018

WTRP (cont.) F seq=2 A Seq=3 unknown seq=4 seq=5 D seq = 1 A B C D E F Transmission range of E seq = 1 F seq=2 A Seq=3 unknown seq=4 seq=5 D Connectivity table of E 18 September 2018

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 18 September 2018

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 18 September 2018

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 18 September 2018

Blackburst After successful transmission of a frame, the station schedules the next access instant tsch seconds 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 18 September 2018

VoW (Delay bounded VoW system) 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) 18 September 2018

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 18 September 2018

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 18 September 2018

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) 18 September 2018

Mobility Internet Hot Spot B Hot Spot A AP AP AP AP Micro mobility Macro mobility Micro mobility 18 September 2018

Mobility Two approaches available: Mobile IP SIP handoff at network layer SIP handoff at the application layer 18 September 2018

Handoff using Mobile IP 3 Parts of Mobile IP Advertising Care-of Addresses Registration Tunneling 18 September 2018

Mobile IP A mobility agent is either a foreign agent or a home agent or both Mobility agents broadcast agent advertisements (periodically) Mobile hosts can solicit for an advertisement Advertisements contain: mobility agent address care-of addresses lifetime 18 September 2018

Registration 18 September 2018

Tunneling 18 September 2018

Handoff using SIP Two scenarios Pre-call mobility Mid-call mobility 18 September 2018

Pre-call mobility (2)INVITE Correspondent node Home Network (3) 302 moved temporarily SIP server (5) 200 OK (4) INVITE (1) Registration of New contact with registrar Visited network Mobile node 18 September 2018

Mid-call mobility Correspondent Home Network node SIP server (2) 200 OK (1) re-INVITE Visited network Mobile node 18 September 2018

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 18 September 2018

Summary VOIP applications require QoS Delay is the most important QoS parameter Wired networks have mechanisms available to provide QoS (RSVP, Diffserv, MPLS) Wireless LAN such as 802.11 does not have implementation that can support VOIP communication adequately EDCF (802.11e), DFS, WTRP and blackburst are few mechanisms that can be used to facilitate VOIP communication in wireless LANs 18 September 2018

Summary (cont.) Handoff can be handled By Mobile IP By SIP Delay has to be budgeted properly and calls have to be admitted carefully so that end-to-end delay bounds are within the acceptable limit 18 September 2018

References Goode B., “Voice over Internet Protocol” – Proc. of IEEE, vol. 90, no. 9, Septmember 2002. Schiller J., “Mobile Communications” - Addison Wesley, 2000. Benvensite M., et. al., “EDCF proposed draft text” – IEEE working document 802.11-01/131r1 (2001) Vaidya N.H., et. al., “Distributed Fair Scheduling in a wireless LAN” – Sixth International Conference on Mobile Computing and Networking, Boston 2000. Ergen M., et. al., “Wireless Token Ring Protocol” –Proceedings of 8th International Symposium on Computer and Communication 2003. Lindgren A., et. al., “Quality of Service Schemes for IEEE 802.11 Wireless LANs – An Evaluation” – Mobile Networks and Applications vol. 8, pp 223-235, Kluwer Academic Publishers, 2003. 18 September 2018

References Sobrinho J.L., Krishnakumar A.S., “Real-time Traffic over the IEEE802.11 Medium Access Control Layer” – Bell Labs Technical Journal (1996), pp. 172-187. Sobrinho J.L., Krishnakumar A.S., “Quality of Service in ad hoc carrier sense multiple access networks” – IEEE Journal on Selected Areas in Communications 17(8) (1999), pp. 1353-1368. Perkins C.E, “Mobile IP Tutorials”, http://www.computer.org/internet/v2n1/perkins.htm#r30 Schulzrinne H., Wedland E., “Application-layer mobility using SIP” – ACM SIGMOBILE Mobile Computing and Communications Review, vol. 4, no. 3, July 2000, pp. 47-57. 18 September 2018