Transcript Network Structure

Analog to Digital
(digital telephony)
• Given an analog function (voice?) we
wish to represent it as a sequence of
digital values
Pulse Amplitude Modulation (PAM)
Pulse Code Modulation (PCM)
• Pulse Code Modulation is a variation of PAM
Measurements are changed to a set of integral values
• How often should the analog signal be sampled?
• A result of Nyquist’s Theorem says
For a signal with frequency x
Maximum sampling rate = 2 * x
•
For a voice channel
Max frequency of voice channel = 4000 Hz
Maximum sampling rate needed = 2 * 4000
= 8000 samples per second
Pulse Code Modulation
• Establish a set of integral values
Plus/minus 7 bits
Total of 8 bits per sample
Total of 256 distinct values
• This is how voice is encoded as a digital signal
• Sample 2 * 4000 = 8000 samples per second
• One sample every 125 microseconds
Pulse Code Modulation
VOICE
64 Kbps
Codec
8000
samples/sec
These samples are pooled into groups of 24
1
2
Frame
Codec
24
8 bits 8 bits 8 bits 8 bits
8 bits F
8 * 24 = 192 bits
1 frame bit
Pulse Code Modulation (PCM)
• 24 channels
• Each frame is 193 bits
• Sampled 8000 times per second
193 bits * 8000 samples
= 1,544,000 bits per second
• This is called a T1 facility
• The circuit is also called a Digital Service-1 (DS-1)
• This is the fundamental digital circuit from the telephone
company
• A single channel within the DS-1 is often called a DS-0
• There is a European standard that groups 30 channels
instead of 24 – Called an E1 circuit
Digital Service
• Uses Alternate Mark Inversion or AMI
encoding
• Requires all one bits to be alternating
positive and negative voltages
• Zero bits have 0 voltage
0
1
1
1
0
0
0
1
Multiplexing
• To more fully utilize channel capacity more than
one transmission can ‘share’ a single channel
• The is called multiplexing
• Multiplexing can take many forms
Frequency Division
The total bandwidth is divided a number into a number
of frequency ranges
Each transmission utilizes one of these ranges
Cable Television
• A single coax cable system can have a total capacity of
350 Mhz and to over 650 Mhz
• Each TV channel occupies 6 Mhz
Channel
Number
Band
Mhz
2
3
4
……
20
…….
40
…….
61
54-60
60-66
66-72
……….
156-162
……….
318-324
………..
444-450
Wave Division Multiplexing
• Using multiple ‘colors’ over fiber optical
cable is a form of frequency division
• Multiple transmission can exist
concurrently over the same fiber cable
• Each uses a different wave length,
typically called a ‘lamda’
Time Division Multiplexing
• Total bandwidth is divided into a series of
‘n’ time slots
• Each transmission gets one of the time
slots in a round robin fashion
T1
T1
T2
T2
T4
T3
T2
T1
T4
T3
T2
T1
T3
T3
Communications Channel
T4
T4
Statistical Multiplexing
• Like time division except when a station
does not have data to send, slot is passed
on to next transmission
• Could have variable length data for each
transmission
• Any one link can actually get use of full
link of other are idle
• Better link utilization
• More overhead
Statistical Multiplexing
T1
T1
T2
T2
T1dd
T3dd T1d T4ddd T3dd T1d
T3
T4
T3
Communications Channel
T4
Inverse Multiplexing
• A number of slower speed lines are
grouped together to form a higher speed
circuit
M
U
X
M
U
X
Multiplexing
M
U
X
M
U
X
Inverse multiplexing
Recall PCM
• Establish a set of integral values
Plus/minus 7 bits
Total of 8 bits per sample
Total of 256 distinct values
• This is how voice is encoded as a digital signal
• Sample 2 * 4000 = 8000 samples per second
• One sample every 125 microseconds
Pulse Code Modulation
VOICE
64 Kbps
Codec
8000
samples/sec
These samples are pooled into groups of 24
1
2
Frame
Codec
24
8 bits 8 bits 8 bits 8 bits
8 bits F
8 * 24 = 192 bits
1 frame bit
Multiplexing Voice Conversations
Digital Hierarchy
Phone Company pools T1s into larger circuits
1.544 mbps
DS-2
24 DS-0
6.312 Mbps
64 kbps
DS – 3
44.736 Mbps
4 DS-1
DS-4
274.176
7 DS-2
6 DS-3
Synchronous Optical Network
SONET
• Standard for digital transmission over fiber optics
• Also called Synchronous Digital Hierarchy (SDH)
by the ITU
• Provides for interoperability between carriers
• Standardizes US and European hierarchy
• Extends Digital Hierarchy beyond existing DS-3
and DS-4
• Makes provision for Operations, Administration,
and Maintenance
SONET
• Synchronous
• Basic frame
810 bytes
8,000 frames per second
6480 bits per frame or 51.84 Mbps
• This forms a basic SONET channel
6480
6480
6480
Synchronous Transport Signal -1
STS-1
6480
SONET Signal Hierarchy
Integrated Services Digital Network
ISDN
• Designed to provide digital services to end users
(total telephone redesign)
• Approved in 1984 (ISO, ITU)
• All digital services all the way to the home
• Uses same twisted pair cabling
• Initially slow implementation
• Usually priced as a measured service, even for
local calling
• Very limited user acceptance
ISDN Services
• Basic Rate Interface (BRI)
2 B (basic rate) channels
each 64 Kbps
used for voice, concurrent data
1 D channel
16 Kbps, used for signaling
• Primary Rate Interface (PRI)
23 B channels
1 D channel (at 64 Kbps)
Digital Subscriber Line (DSL)
• Effort by telephone company to bring
lower cost, high speed service to home
• Provides concurrent voice/data
• Uses regular phone twisted pair
• Intended to compete with cable service
• Multitude of variations
• Speeds from several hundred Kbps to
several Mbps
DSL Variants
• Asymmetric DSL




Frequency division multiplexing
0 – 25 Khz
Voice
25 – 200 Khz
Upstream data
250 – 1000 Khz Downstream data
• Rates up to 1 Mbps upstream
8 Mbps Downstream
are theoretically possible
Asymmetric DSL
Digital Subscriber Lines
Operation of ADSL using discrete multitone
modulation (DMT).
Upstream: Usually 512 Kbps or less
Downstream: Up to 8 Mbps
Digital Subscriber Lines
Bandwidth versus distanced over category 3
UTP for DSL.
Other DSL Variants
• Symmetric DSL (SDSL)
 Upstream and downstream speeds the same
 Single copper pair
• High Data Rate DSL (HDSL)
 Symmetric services
 Requires two copper pairs
 Cost effective way to deliver T1 equivalent service
• Very High Data Rate (VDSL)
 Asymmetric
 Up to 50 Mbps downstream possible
DSL Modem
Digital Subscriber Line Access Multiplexor
Cable Modems
• Why cable?
 Infrastructure exists
 Coax cable better characteristics than twisted pairs
• TV channels are 6 Mhz
 Allows for high speed data
 What is maximum possible data rate?
• Shared channel – many users on a segment
• Now a standard
Data Over Cable System Interface Standard
DOCSIS
Cable Spectrum Allocation
Frequency allocation in a typical
cable TV system used for data
Cable modems
• Upstream: 5-42 Mhz
 Uses an encoding scheme of 2 bits per baud
 1 baud per Hz
 12 Mbps per channel possible per 6 Mhz
channel
• Downstream: 550-750 Mhz
 Uses 64-QAM
 6 bits/baud, 1 bit is used for error correction
 Theoretical 30 Mbps per 6 Mhz channel
Wireless Broadband Alternatives
• Current efforts are to provide 802.11 wireless in
large areas through meshed access points
• Wireless broadband to the home standard being
developed
• IEEE 802.16 - WiMAX
 Potential of upto155 Mbps
 Over wide areas (miles)
 Considered a replacement for last mile
http://grouper.ieee.org/groups/802/16/index.html