1. Copyright 2004 F. W. Bowen

2. If I have DSL service, should I do Voice communications over the Internet, or VoIP (Voice Over IP) ? Should I switch from conventional phone to cellular? What about Cable? What are the technical differences between a conventional telephone, cellular, and Internet voice? Understanding the service differences and technical issues can aid in making service choices.

3. There’s an abundance of technologies and choices Cellular CDMA – Code Division Multiple Access Verizon, Sprint TDMA – Time Division Multiple Access (GSM) Cingular, AT&T, T-Mobile Wireless - Wi-Fi (802.11a,b,g), Wi-Max (802.16) Wireline - Analog, ISDN DSL, ADSL, Cable TV

4. Many of the choices are migrating toward Internet Protocol (IP) near the User and Application Layers But can use different underlying transport technologies: ISDN (Wireline) Sonet (Wireline) TDM (Time Division Multiplexing) (wireline) Frame Relay Packet Protocol Ethernet – Gigabit, 100BaseT, 10BaseT Sonet, Proprietary (Cable)

5. Intro Internet Basics Wireline Basics Cellular Basics Cable Basics Service Criteria Summary

6. The Internet uses a packet protocol that defines the basic information packet IP – “Internet Protocol”

7. Basic IP Packet Layout

8. Typical Packet Network (Internet) packet flows

9. Variable packet length Each packet includes its own source address and destination address Variable packet routing for a “session” Multiple “sessions” share the same resources Asynchronous – no network clocking of packets “Connectionless” Key IP/Internet Characteristics

10. Packets are processed by routers as they arrive and contend with other packets for router processing Packet traffic is notoriously bursty and hard to predict No fixed path, guaranteed delivery order and delay Multiple “sessions” using the same paths/circuits can cause congestion and delay when arriving packets destined for an output exceed capacity

11. Packet Delay Packet delay - the time it takes for an originating packet to arrive at the destination. Example: packet leaves origination at 12:30:00 and arrives at destination at 12:30:03. Delay is 3 seconds

12. Packet delay should typically represent the server and router processing times plus transmission time without congestion delays

13. Packet Jitter

14. Jitter typically reflects network delays due to congestion, traffic load differences, packet order processing differences and routing differences Interruptions in packet forwarding can make jitter very significant Jitter complicates application processing at the destination for streaming sessions (audio or video) and requires “buildout” buffering to prevent gaps in stream presentation or buffer overrun This buffering and processing can increase total delay significantly Variable server packet processing times can also generate significant jitter Jitter

15. Pulse Code Modulation: DS-0 Digitizer samples voice every 125 microseconds converting amplitude measurement into an 8-bit signal level octet Generates a “Digital Signal Level 0 (DS-0)” digital output Digital Voice Networks – Digitizing Voice DS-0

16. Digital Voice Network DS-0 Characteristics DS-0 constitutes basic 64 kbps digital “channel” – 8,000 8-bit octets per second One octet every 125 microseconds Digital “voice circuit” consists of two DS-0 signals, one in each direction Voice frequency range: 0-8,000 khz

17. Digital Voice Network – DS-1 Multiplexer 24 DS-0 channels are combined into one DS-1 (Digital Signal Level 1) digital stream

18. DS-1 Characteristics 24 DS-0 Channels plus one framing bit constitute 1 DS-1 Frame Generates 1.544 megabit per second data rate Commonly referred to as a “T-1” (Transmission Facility Level 1) DS-1 constitutes basic network transmission rate Higher rates (T-3, OC-12, …) consist of multiples of T-1

19. Digital Voice Network – Typical Call

20. Key Digital Voice Network Characteristics Fixed length octet “packets” At call origination a dedicated “circuit/channel” is established/reserved between the calling and called parties “Connection-oriented” Only octets for one call are sent on this circuit, and only on this circuit Synchronous network – all switches and facilities are synced to a master network clock (“atomic clock” accuracy) and consecutive octets are clocked through the network in lock-step every 125 microseconds

21. Maximum end-to-end delay typically much less than 250 milliseconds – the comfort limit Standard requirement is that thru-switch delay be no more than 1,250 microseconds Jitter effectively zero (compared to packet)

23. DSL Data Network Telephone Analog Line Digital Trunk Local Class 5 Office Packet/ATM Switch Office Internet DSLAM Data rates vary Down - 224 kbps up to about 6mbps Up - 224 kbps up tp 1.5 mbps

24. Typical Cellular (CDMA) Network CV- Compressed Voice Packets PCM – Pulse Code Modulation Octets

25. Compressed Voice Compressed voice packets consist of 24-byte packets that each represent a digitized 20 millisecond voice sample Compressed voice packets typically occupy a 9,600 bps radio channel During congestion or radio difficulties, available channel may only be 4,800 or 2,400 bps, requiring different voice compression and dramatically reducing voice quality Compressing voice adds delay for the 20 millisecond sample period and compression processing

26. Cellular Voice Compression Methods (CDMA) EVRC – Enhanced Variable Rate Voice Coder SMV – Selectable Mode Voice Coder

27. Key Cellular Network Characteristics Cellular systems are extensions of the voice network that use voice compression packets to fully utilize the available radio spectrum For a cell phone call, compressed voice is converted to/from PCM When most calls were mobile to wireline cellular systems worked acceptably well For mobile-to-mobile calls, compressed voice is converted to PCM and then from PCM to compressed voice For these calls, the delay is frequently unacceptable > 250 ms This is a strong motivation to convert cellular systems to IP networks

28. Cellular Broadband Data Plans (CDMA) Limited data access available now, short message service also 1XRTT – 144 kbps – Verizon trialing now in some markets 1XEV-DO – 2.4 mbps peak, average 620 kbps 1XEV-DV – 5 mbps peak, better suited for VoIP Similar GSM/TDMA data with GPRS, EDGE (lower peak rates?)

29. Wi-Fi Wireless Wi-Fi wireless provides IP service over shorter distances in “unlicensed” frequency spectrum Exploding popularity for “Hot Spot” Internet access, meetings, inter-PC and peripheral data communication (printer, keyboard, mouse) Extensions of the original Ethernet 802.x series IEEE LAN standards 802.11a – 54 mbps max, 27 mbps effective, 5 ghz frequency range 802.11b – 11 mbps max 4-5 mbps effective, 2.4 ghz frequency range 802.11g – 54 mbps max, 20-25 mbps effective, both frequency ranges

30. Cable is “digital radio in a wire” Coax headend - fiber backend Carries MPEG and IP Data-Over-Cable Service Interface Specification DOCSIS 2.0 (latest version) Symmetric Service Adapts cellular technology - Advanced TDMA (A-TDMA), Synchronous CDMA (S-CDMA) 30 MBPS shared capacity VoIP in plans Cable Basics

31. Quality of Service (QoS) addresses user needs: Effective Bandwidth (data rate) Delay Jitter For voice it can include: Echo – signal returned to sender from receiver Sidetone – human factors aid Voice Fidelity – effective frequency range More detailed fidelity assurances require a “Transmission Plan”. Quality of Service

32. Different services have different QoS requirements: Voice – Delay (250 ms max), Jitter (0 to minimal), Sidetone, Echo (inaudible), Voice fidelity -comparable to traditional analog Streaming video and audio (MPEG, MP3) – Needs bandwidth, adequately low jitter (Less than delay) Depends on build-out buffering Delays in the seconds can be tolerated Web Surfing – delay measured in seconds generally considered acceptable

33. Sidetone Sidetone is the amplitude reduced “Mouth” signal added to the “Ear” signal to mimic normal talking situations where the speaker “hears” what she is saying. It’s been a human factors element of voice networks for many years.

34. Echo Every voice circuit has some signal reflected back from the other end: Echo, Frequently recognized as a singing or “tinny” sound. The longer the circuit, the greater the delay, the worse the echo.. Voice networks manage Echo by using Echo Cancellers: Echo cancellers are DSPs (Digital Signal Processors) that remove echo. Echo cancellers add to the delay.

35. Is the DSL, Ethernet equipment (phone/modem) powered by the DSL/Ethernet cable? What provisions are there for E911? What are the reliability statistics for the system? How many minutes a year is the system down? (Some digital voice switches significantly outperform the requirement of 2 hours in 40 years, or 3 minutes a year) Does the system have to regularly go down for cold reboots? Other criteria to consider

36. Technologies and Services are converging to Broadband IP and wireless IP QoS, especially for voice, is frequently unacceptable today. Service providers make no QoS guarantees: “Can you hear me now?” Ask anyway. Eventually network capacities may be such that VoIP becomes more acceptable. Wire, cellular, cable? Choose your poison, but understand the tradeoffs. Copyright 2004 F. W. Bowen

37. Cellular All-IP Architecture