LTS Voice Over IP: Architectures, Applications and Challenges Tom Chapuran Telcordia Technologies [email protected] 973 829-4186 April 15, 2002 An SAIC Company . Slide 1 What Is VoIP? Initially, PC to PC voice calls over the Internet Public Switched Telephone Network Gateways allow PCs to also reach phones
PSTN (NY) Gateway Multimedia PC Gateway IP Network Multimedia PC PSTN (DC) or phones to reach phones LTS Telcordia Technologies, Inc. Slide 2 Outline Why Voice over IP?
Packet voice transport Signaling and control architectures Network applications Outlook and challenges LTS Telcordia Technologies, Inc. Slide 3 Origins of VoIP Lessons learned Internet Telephony software for multimedia PCs (1995) Users frustrated by poor QoS, difficulty of use, lack of interoperability Standards are critical for success Coding/decoding (codec) between analog voice and digital packets Locating the party you want to call Signaling to set up, modify, tear down the voice call Access to vertical services (call forwarding, 3-way calling, ) Gateways to PSTN Media routing, quality of service (QoS) left to other IP mechanisms (not VoIP-specific) LTS Telcordia Technologies, Inc.
Slide 4 Circuit-Switched Telephony Traditional PSTN Approach SCP SS7 Signaling Network Signaling Class 5 Switch Class 4 Switch Circuit-based Trunks Most service logic in local switches, rest in SCPs Class 5 Switch
64 kb/s digital voice Typically analog loop, conversion to digital at local switch Media stream Data travels over a parallel (but separate) network LTS Telcordia Technologies, Inc. Slide 5 VoIP Goals and Potential Benefits Consolidation of voice, data on a single network Simplify infrastructure, operations; provide bundled services Support for intelligent terminals as well as phones Increased flexibility Multiple bit rates, multiple media types, richer signaling Distinguish calls from connections (add/modify streams during call) Separation of service control from switching/routing Accelerate new service development, increase end-user control,
evolve from VoIP towards advanced services Expansion of competition LTS Telcordia Technologies, Inc. Slide 6 Packet Voice Transport Key targets for voice call service quality: Average packet loss: < 2% Consecutive packet loss: < 200 ms burst End-to-end (lip-to-ear) delay: < 150 ms for comfortable conversation Packet loss cannot be corrected by retransmission (TCP), because the packets arrive too late to be useful Use RTP (Real-time Transport Protocol) over UDP (User Datagram Protocol) for voice or video transport Payload ID, sequence numbers, timestamps, monitoring via RTCP Packet and buffer lengths limited by constraint on end-to-end delay Typical codecs: G.711 (64 kb/s), G.729 (8 kb/s) G.723 (~ 6 kb/s) Transmitted bit rates depend on overheads, optional silence suppression LTS
Telcordia Technologies, Inc. Slide 7 H.323 Architecture ITU-T H.323 Gatekeeper 3 stages of signaling: RAS to Gatekeeper H.225 call signaling H.245 media stream control (can be simplified for VoIP) PSTN H.323 Terminal H.323 Gateway H.323 Zone H.323 Multipoint Control Unit
Telco-centric multimedia,multiparty conferencing (initially for LANs) Gatekeeper for network control, heavy-weight protocols Widely deployed in first wave of VoIP standardization LTS Telcordia Technologies, Inc. Slide 8 SIP (Session Initiation Protocol) IETF Multimedia Architecture Internet-centric alternative, initially for large multicast conferences SIP for call signaling, SDP (Session Description Protocol) for media Initially very simple, light-weight, loosely-coupled sessions; oriented towards direct signaling between endpoints Network servers for additional capabilities: Registrar for terminal registration, aliases Redirect returns contact address directly to end user Proxy forwards signaling (requests, responses) Evolution towards greater use of proxy/registrar for locating users, vertical services, call tracking, network control Strong, rapidly growing support (e.g., Microsoft XP, 3GPP)
LTS Telcordia Technologies, Inc. Slide 9 SIP Call Setup Simplified View lts.ncsc.mil telcordia.com DNS Proxy INVITE [email protected] Location server INVITE Ringing 200 OK Proxy ACK
INVITE Ringing 200 OK Media Streams Linda Peter INVITE SDP proposes media type(s), IP & ports to send to 200 OK SDP accepts/rejects media, gives IP & ports to send to LTS Telcordia Technologies, Inc. Slide 10 Where Do Services Live? Some implemented at the endpoints Last-number redial, call hold... Others may be better supported from the network Avoid need for PC or IP phone to be turned on (call forwarding) More complex services, such as conferencing Integration with web-based services (unified messaging)
Example: SIP Proxy runs a script for each incoming call for Peter Parallel forking: forward INVITE to multiple endpoints simultaneously Sequential forking: try his office PC first, then lab, then cell phone, LTS Telcordia Technologies, Inc. Slide 11 SIMPLE (SIP for IM and Presence) Simplified Example lts.ncsc.mil telcordia.com Presence server Proxy SUBSCRIBE NOTIFY NOTIFY SUBSCRIBE [email protected]
Proxy Update Presence Linda Peter Linda subscribes to notifications of changes in Peters status: Off-line, on-line, busy, away, available, ... LTS Telcordia Technologies, Inc. Slide 12 NGN Architecture Next-Generation Network Oriented towards application of VoIP (or VoATM) to large-scale public networks Focus on scalability, network control, support for traditional phones, sophisticated gateway (GW) to the PSTN and its services Media GW interfaces voice stream to PSTN trunk or phone line Signaling GW allows signaling directly to SS7 network
Softswitch controls Media GWs and does call processing Allows smaller, cheaper Media GWs (e.g., for individual homes) Control via MGCP (Media Gateway Control Protocol) or H.248 LTS Telcordia Technologies, Inc. Slide 13 NGN Example NGN Voice over DSL or Cable Modem PSTN Softswitch SCP /SI 3 2 H3 P
CP G M Customer Gateway DSL or PacketCable Access I SU P, CP MG IP Phones, PCs TC AP SS7 Gateway
Core Packet Network Voice Streams SS7 Signaling Network Trunk Gateway Class 5 Switch Can also use to interconnect PSTN clouds (long-distance), or PSTN switches (interoffice backbone) LTS Telcordia Technologies, Inc. Slide 14 International Voice Market Calls Terminated on PSTN Call Volumes (B min/year) 160
International VoIP 140 3.7 120 1.7 0.15 100 80 6.2 International PSTN 0.008 60 40 20 0 1997
1998 1999 Year 2000 2001 Source: Telegeography 2001 (2001 figures were projections) LTS Telcordia Technologies, Inc. Slide 15 Carrier Applications of VoIP First major inroads for VoIP have been in long-distance Avoid regulation, high international PSTN tariffs VoIP invisible to end user, doesnt rely on him to do anything Installed base dominantly H.323, movement now towards NGN Local-carrier interest for interoffice connections Consolidate voice and data networks (typically ATM) Use NGN, or packet-enable existing switches
Many trials of VoIP to residences, but deployments few Cable TV has laid groundwork for NGN approach (DOCSIS 1.1) Decline of CLECs likely to slow multi-line VoDSL LTS Telcordia Technologies, Inc. Slide 16 Enterprise VoIP Location B Location A PSTN Centrex or PBX Core IP Network GW GW IP PBX Softswitch
IP phone IP phone Many possible combinations of VoIP and circuit-switched telephony LTS Telcordia Technologies, Inc. Slide 17 Enterprise Applications of VoIP Leverage spare data-network capacity, minimize phone bills, create platform for multimedia conferencing H.323 and SIP both being deployed, softswitches and IP-PBX options emerging, unclear which will prevail Examples: Telcordia/SAIC (H.323), Telia (SIP) Carrier-managed VPN networks last year from AT&T (H.323) and Worldcom (SIP) VoIP adoption slower than expected, partly due to plunging PSTN long-distance prices, QoS concerns LTS Telcordia Technologies, Inc.
Slide 18 Peer-to-Peer VoIP PC-to-PC Internet Telephony revisited, often facilitated by software or network servers from new types of voice service providers Microsoft, Net2Phone, Dialpad, AOL, Yahoo! Mass market alternative to telcos, requiring limited network infrastructure, capital costs, operating expenses Whats the business case for free VoIP? Sell advertising, software, or enhanced services Charge for PC-to-phone, phone-to-phone Give away as a competitive differentiator Mostly H.323 today, likely to move towards SIP Could be key industry driver, even if penetration were limited LTS Telcordia Technologies, Inc. Slide 19 Outlook for VoIP
Current Status and Trends VoIP is not monolithic many applications, with different drivers, will maintain a heterogeneous mix of technologies H.323 is most widely implemented today, but trends are towards SIP for intelligent terminals, NGN for most carrier networks Most success thus far in long-distance networks, perhaps with local carrier backbones to follow in next few years Footholds made in enterprise and access markets, but VoIP has not taken off as fast as initially expected Adoption being slowed by economic conditions, plummeting long distance rates, declining advertising market (peer-to-peer) LTS Telcordia Technologies, Inc. Slide 20 Continuing Challenges Quality of Service Diffserv, MPLS, traffic engineering, bandwidth brokers, call admission What is really needed for consolidated voice and data networks? Security, reliability Extending SIP to provide conference control Operations (configuration of IP phones, version control and
upgrading of highly distributed software, accounting/billing,) Packet-level interconnection of VoIP islands which use competing architectures and protocols Controlling feature interactions in a distributed-services environment Traversal of NATs and firewalls Support for services beyond voice LTS Telcordia Technologies, Inc. Slide 21 NAT Traversal Network Address Translators (NATs) map a private IP address space to externally visible (public) IP addresses Conserve scarce public IP addresses Shield internal hosts from outside world Useful for enterprises, cable modem networks, broadband access routers, internet cafes NATs interfere with peer-to-peer protocols such as SIP SIP clients must identify the IP address and ports they will use to receive media streams (in payload of their signaling messages) But they dont know their externally visible addresses One of the SIP communitys biggest problems
LTS Telcordia Technologies, Inc. Slide 22 STUN Simple Traversal of UDP Through NATs draft-rosenberg-midcom-stun-01.txt STUN Server Internet Private Network A Private Network B STUN Request/Response NAT STUN Client SIP Client NAT SIP Signaling
STUN Client SIP Client SIP Proxy/Registrar Source: P. Thermos, Telcordia STUN client contacts STUN server, discovers NAT, address translation SIP client uses external address in signaling for setup of media streams This approach being implemented and tested at Columbia and LTS LTS Telcordia Technologies, Inc. Slide 23 Advanced Services VoIP: natural platform for evolution to advanced services Supports intelligent terminals and rich signaling Separates calls from connections Multimedia capabilities already in the protocols (SIP/H.323) Removes bottleneck by separating call control from switching Thus far, focus is almost entirely on voice For many players (but not all), voice is the killer app Solve the simpler problem first
This simplifies many network control issues, because of predictability of voice bandwidth, traffic patterns But current solutions are likely to require significant extensions to accommodate more flexible advanced services LTS Telcordia Technologies, Inc. Slide 24 Moving Beyond Two-Party Voice Whats Different About Advanced Services? Flexibility in media streams, participants, ownership; service not pre-defined at call setup Multiple media per call, differing (and very wide range of) bandwidths Dynamic reconfigurability during call Potential for multicast conferencing, streaming Implications Call admission control becomes more complex Much less aggregation, localization of flows than with NGN voice Usage, traffic patterns may be highly variable and hard to predict New approaches to traffic engineering, resource allocation and network control will be needed to address even a modest penetration of these new services
LTS Telcordia Technologies, Inc. Slide 25 Acknowledgements Ron Menendez Stu Wagner Tim Feustel Peter Thermos Dave Gorman Nigel Dewdney Gary Hayward LTS Telcordia Technologies, Inc. Slide 26
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