IP-Telephony Quality of Service (QoS)

IP-Telephony Quality of Service (QoS) Bernard Hammer Siemens AG, Munich Siemens AG 2001 1

Presentation Outline End-to-end OoS of VoIP services Quality of speech codecs Network-QoS IntServ RSVP DiffServ MPLS 2

Voice Quality in IP-Networks Voice quality has an end-to-end relationship Terminal A Network(s) Terminal B Voice quality is impacted by: Media coding method (coding delay, lossy compression) Delivery delay (packetization, IP-node buffering, terminal buffering) Delay jitter Packet loss the E-Model helps to predict QoS of voice services: Standards: ETSI ETR250; ITU-T G.107, G.108, G.108.01, G.109, G.113, G.114, P.833 3

Multimedia QoS Elements Voice Video Data Call Control Customer-Perceived QOS Subjective Measures Objective Measures Terminal-to-Terminal QOS Speed, Accuracy, Dependability Service Availability Voice Video Data Call Control Multimedia Terminal Network QOS Network QOS Network QOS Multimedia Terminal Control Network Control Network Control Network Bearer Network Bearer Network Bearer Network 4

Why is QoS needed? IP-networks are designed for non-realtime data HiPath 5500 Data Voice Data Signalling Data & Voice Queue PC IP PC optipoint 300 Delay, jitter and loss due to one queue for voice and data opticlient 3xx 5

Why is QoS needed? End to End Delay in Today s Networks Terminal Delay HiPath 5500 Terminal Delay Coding & Packetization 30..50ms Queuing 0..50ms Processing 20ms Processing 20ms Jitter Buffer 30..300ms Decoding 1ms Typical: 50..120ms Network Delay Typical: 50..300ms PC Queuing 0..50; ms IP Delay 1ms Typical: 1..50; ms PC optipoint 300 Delay, jitter and loss caused by queuing in the network opticlient 3xx 6

Terminal Delay IP terminal buffering delay (hardware and software) Packetization/ buffering delays. Packetization delay may be introduced while packets are being constructed. Buffering delay may be introduced when they are being disassembled. Codec delay: Processing delay before the output frame is generated; an algorithmic process called look-ahead in which some of the samples from the following frame are used to improve the performance of the compression process; and as a result of the rate at which the output frame is serially clocked out from the encoder output buffer, if this rate is chosen to provide a continuous bit stream without gaps a further frame delay is involved. 7

Network Delay Network delay is the time spent by packets to reach their destination during transmission through the network: the transmission delay, introduced by sending a packet over a link. (e.g. sending a 256 byte packet over a 64 kbit/s link takes 32 ms); the propagation delay, due to signal propagation over physical link. This delay is usually negligible if links are shorter than 1000 km; the node delay, due to router queuing and processing of packets; the protocol delay, due to packet retransmissions (if used, like for TCP) or network access (e.g. CSMA-CD for Ethernet); gateway delay, introduced by interfacing between networks (e.g. packet disassembly/assembly and speech coding/decoding). 8

End-to-End Delay in Next Generation Networks hiq Multimedia User H. 323 MGCP MGCP Switch SS7 STP hia IP hig PSTN / ISDN End-to-End Signal Delay < 150 ms ITU-T G.114 : (one-way-delay-recommendation) 0 to 150 ms: good quality 150 to 400 ms: acceptable quality above 400 ms: unacceptable 9

QoS - Objectives Distinguish traffic with strict timing requirements from those that can tolerate delay, jitter, and loss. QoS support does not create bandwidth, but manages it so it is used more effectively to meet a wide range of application requirements. The goal of QoS is to provide some level of predictability and control beyond the current IP best-effort service. * Quality of Service Forum: http://www.stardust.com/qosforum/ 10

Requirements for VoIP Speech Codecs Robustness of codecs: Error concealment needed (use tools like unequal error protection) Robust against Packet losses: Generally no time for requesting packet retransmission Graceful degradation of voice quality in case of loss (e.g. by generating supplement voice and background information) Measures for getting higher compression: IP Header compression Different type of packets for voice and background (incl. silence) noise (transmission of background noise pattern only when needed) These requirements are being taken care of in VoIP standardization 12

Speech Codecs für VoIP Speech Codecs (3.1 khz) ITU-T G.723.1 standard codec for PSTN, low bitrate but rel. high delay, unpleasant IPR situation ITU-T G.729 same as G.723.1, however due to smaller delay better suited to packet-based transmission ITU-T G.711 standard codec for narrow-band ISDN (64 kbit/s, low delay) - New annexes for VoIP applications, including packetizing and use of comfort noise. - Increasing use for wireline VoIP systems ETSI AMR specified as mandatory codec for circuit switched UMTS by 3GPP, can be used for a wide range of data rates without for transcoding Wide-band Speech Codecs(7kHz) ITU-T G.722.1 ETSI AMR excellent quality at 24 kbit/s, 16 Kbit/s version planned as above, compatible to narrow-band AMR codec Specification of System Parameters packetizing & delay-times: ITU-T G.114 round trip delay & jitter: ITU-T G.177, G.114 need 13

How to achieve QoS in IP-networks Network over-provisioning - may not economically viable Quality feedback (IETF RTCP) - application centered, no network impact QoS Services: Integrated Services: Application sets up path & per flow reservation of network resources acc. to QoS management policy. Specifications: IETF IntServ (RFC 1633, 2210, 2211, 2212) Differentiated Services: Priorization of traffic aggregates; packets are classified & marked at network ingress routers and treated acc. their.resp. service class Specifications: IETF DiffServ (RFC 2475), IEEE 802.1p - RSVP (RFC 2205) is used as a signalling protocol for path set up & resource reservation - MPLS, a packet forwarding scheme, is used to control traffic flow - in an end-to-end OoS architecture both IntServ and DiffServ are applied Policy management QoS policy framework & architecture (IETF POLICY/ RAP, QoS Forum) Authentification, Accounting (IETF AAA) 15

Integrated Services (IntServ) RFC 1633 (Integrated Services) defines a service framework Reserves resources in order to provide QoS for flows requires flow specific state information in routers does not scale very well 3 service classes (as of today): Guaranteed Service: bounded latency Predictive Service: reliable but not fixed latency Best Effort Service Consists of 4 components: packet scheduler, admission control, classifier, reservation setup protocol (usually RSVP) 16

Resource Reservation Protocol (RSVP) RSVP (RFC 2205) is a signaling protocol to configure QoS Applications can request End-to-End and per-flow QoS from the network indicate QoS requirements and capabilities to peer applications Not limited to per-flow-usage nor to IntServ semantics Information carried by RSVP classification information traffic description parameters (date rate etc.) service type required policy and traffic engineering information 17

How RSVP Works Sender Router PATH Router Router Receiver Router RESV Sender sends PATH message to receiver specifying the traffic characteristics. Every intermediate router forwards the PATH message to the next hop router determined by the routing algorithm. Receiver responds with RESV message to request resources for the flow. Intermediate router can accept or reject the request (based on available resources and/or policies). If the request is accepted, resources will be reserved for the flow and related flow state information is installed in the router, otherwise the process terminates. 18

RSVP + Policies Does the user have authorization for reserving the ressource? IF user IN ApprovedUsers AND service IN VideoServices AND source IN VideoSources AND time IN ApprovedTimePeriods AND NWstate IN OK_NWStates THEN provide JitterFreeMPEG2 Cops PDP LDAP Admin Receiver Sender PATH RESV PATH PATH 19

Differentiated Services (DiffServ) DiffServ defines the meaning of the DS field (6 Bit) in the IP header. Packets will be processed based on their DS field (i.e. DiffServ defines a relative priority scheme). This allows ISPs to develop and tailor the service offerings, i.e. offer different service classes - Service-Level-Agreements (SLAs) SLAs may contain any combination of attributes including: latency, availability, packet loss rates, jitter. Classification, shaping, marking, authentication, policing only necessary at ingress router. Interior routers implement per-hop-behavior. DIFFSERV is far simpler than RSVP however does not allow for resource reservation but only for some sort of priorisation. DIFFSERV can be used "stand alone" or in conjunction with an RSVP-based INTSERV network. 20

Intserv vs. Diffserv RSVP/Intserv Diffserv Emulation of SCN in PSN QoS per flow State machine Set up via RSVP signaling No control on routing Good for long duration transfer (slow setup (.3-1 s) Align with PSN concept QoS per hop Stateless Priority by DS code No control on routing Good for short duration transfer 21

QoS and Routing Problem description Within a connectionless routed net, the router looks at a packet's destination address each time the packet flows through a node. The packet's route is determined by the routing tables in each node. With this method, a packet's route can't be predicted, which makes it difficult to reserve resources or guarantee quality of service (QoS). Routing table lookups are also CPUintensive and time-consuming RSVP/Intserv and Diffserv are independent from routing Explicit routing enables establishing a QoS path. MPLS defines an underlying mechanism to provide an explicit routing Explicit routing enables Traffic Engineering 22

Multiprotocol Label Switching (MPLS) MPLS is a connection-oriented technology being integrated with connectionless IP networks MPLS lets a router or switch assign a tag to each of its routing table entries and communicate that tag to neighboring routers and switches. When one of those devices passes a packet to a neighbor, the router or switch adds to the packet a tag associated with the routing table entry. The tag lets the router or switch identify the next hop or hops in the path without looking up the address. The idea is to let tagged packets flow end to end without forcing other routers or switches to perform any address lookups. Features: Adding labels to packets and label swapping to routers as a means to aggregate forwarding information at OSI layer 2 The label provides forwarding along an explicit path The label is added at ingress points and removed by egress points by MPLS signaling protocols network (e.g. LDP) network resources for certain traffic can be reserved. RSVP and Diffserv can be mapped over MPLS 23

Concluding Remarks Lack of QoS is one of the barriers in VoIP deployment Speech quality: G.711 gets extended for VoIP usage, new G.722.1 & AMR are interesting alternatives for high quality speech RSVP related standards are mature and most routers support RSVP currently Diffserv may become a common QoS method Network QoS: is still in an infant state; work in progress at TIPHON, TIA, ITU-T, IETF Further Information: www.qosforum.com 24