Transcript Slide 1

Copyright 2004 F. W. Bowen
•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.
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 • Cable TV
Analog, ISDN DSL, ADSL,
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)
• Intro
• Internet Basics
• Wireline Basics
• Cellular Basics
• Cable Basics
• Service Criteria
•Summary
IP – “Internet Protocol”
The Internet uses a packet protocol that
defines the basic information packet
Header
Data
Basic IP Packet Layout
Vers Hlen Svc Type
ID
TTL
Total Length
FLG
Protocol
Fragment Offset
Header Checksum
Source IP Address
Destination IP Address
IP Options
Data
Padding
Server
Router
Router
Computer
Server
Computer
Server
Router
Router
Typical Packet Network (Internet) packet flows
Key IP/Internet Characteristics
• 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”
• 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
Packet Delay
Server
Computer
Server
Router
Router
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
Packet delay should typically represent the server
and router processing times plus transmission time
without congestion delays
Packet Jitter
Ideal packet spacing - no differences in packet delay
Jitter
Jitter reflects differences in packet delay
Jitter
• 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
Digital Voice Networks – Digitizing Voice
Pulse Code Modulation: DS-0
Analog Voice Signal
Digitizer
10010111
DS-0
Digital Voice
•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 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
Digital Voice Network – DS-1 Multiplexer
10010111
10010111
10010111
t
e
xt
tet
x
te
x
t
Multiplexer
10011...001
193-bit DS-1 "Frame"
10010111
24 DS-0 channels are combined into one DS-1 (Digital Signal Level 1)
digital stream
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
Digital Voice Network – Typical Call
Toll/Tandem
Office
Digital Trunk
Analog Line
Digital Trunk
Tolll/Tandem
Office
Digital Trunk
Analog Line
Telephone
Telephone
Local Class 5
Office
Local Class 5
Office
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
• 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)
DSL Data Network
Data rates vary
Down - 224 kbps up to about
6mbps
Up - 224 kbps up tp 1.5 mbps
Packet/ATM
Switch Office
Digital Trunk
Internet
Digital Trunk
Analog Line
Telephone
DSLAM
Local Class 5
Office
Typical Cellular (CDMA) Network
CV
CV
PCM
Digital Voice
Network (PCM)
Cell Site
Radio Tower
Mobile Switching
Office
CV- Compressed Voice Packets
PCM – Pulse Code Modulation Octets
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
Cellular Voice Compression Methods (CDMA)
• EVRC – Enhanced Variable Rate Voice Coder
• SMV – Selectable Mode Voice Coder
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
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?)
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
Cable Basics
•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
Quality of Service
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”.
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
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.
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.
Other criteria to consider
• 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?
• 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
Cellular All-IP Architecture
Network
EIR
Subscription
Profile
DSI
Policy
Rules
Service
Application
Advertising
Agent
7
...
Databases
2 3 4
1
5
8
6
Subscription
Quality of
Service
Manager
10
11
9
Position
Server
18
AAA
15
19
20
21
16
22
12
17
25
Core
Quality of
Service
Manager
28
cdma2000 Access Network
BSC/RNC
+ PCF
27
31
Access
Gateway
32
35
36
MM
FA /
Attendant
Other Access Networks
41
43
TIA/EIA-41
GSM
Trunk
Signaling
Gateway
24
23
MAP
14
13
Session
Control
Manager
Position
Determining
Entity
BTS
Roaming
Signaling
Gateway
Network
Capability
Gateway
26
Media
Gateway
Control
Function
29
Media
Resource
Function
30
Media
Gateway
33
37
Mobile IP
Home
Agent
38
34
39
PSTN
40
42
Border
Router
44
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47
Mobile
Station
Legacy MS
Domain
48
Support
IP
Network