Transcript Chapter 5

CSCI 4550/8556
Computer Networks
Comer, Chapter 12:
Long Distance Digital Connection Technologies
Introduction
Previous technologies cover “short” distances (e.g.
campus, building, a few city blocks)
They can be extended over short distances.
We also need to cover longer distances - e.g., San
Francisco to Boston
The name for long distance technology is WAN Wide Area Network
There are two categories:
Long distance between other networks
“Local loop”
Digital Telephony
The telephone system spans long distances.
The use of digital telephony improves long distance
service:
Better quality than analog technology
More logical connections in the same wire than if analog
technology is used.
Digitizing Voice
Problem: how to encode an analog audio signal as
digital data
Solution:
Sample the audio signal at periodic intervals
Convert the samples to digital using an A-to-D converter
Send the digital data samples over wire (or fiber, or …)
Regenerate the audio using a D-to-A converter
Example
The continuous line shows one possible analog signal (e.g.
voice).
The vertical lines show possible digital equivalents resulting
from periodic sampling of the analog signal.
Sampling Parameters
We want to carry signals up to 4000Hz (recall the
telephone system bandwidth)
To do this, we select a sampling rate of 8000Hz
(twice the maximum frequency in the source)
Each sample is in the range 0-255 (so we can use
8 bits per sample).
The standard for this is called Pulse Code
Modulation (PCM).
Synchronous Communication
Converting these samples back to audio requires
that the data be available “on time.”
Digital telephony systems use clocking for
synchronous data delivery.
The samples must not be delayed as network
traffic increases.
Using Digital Telephony for Data Delivery
So, digital telephony can handle synchronous data
delivery.
Can we use that for arbitrary data delivery?
An Ethernet frame is not 8-bit PCM synchronous
data.
Thus, to send Ethernet frames we need to convert
data formats...
Conversion for Digital Circuits
To use digital telephony for data delivery:
Lease a point-to-point digital circuit between sites; and
Convert between local and PCM formats at each end.
The conversion uses a Data Service Unit / Channel
Service Unit (DSU/CSU) at each end of the line.
CSU - manages control functions
DSU - converts data
Using a DSU / CSU
Telephone Standards
Several standards exist for data transmission rates
in telephone systems.
These are called the T-series standards, and are
similar throughout the world.
Name Bit Rate
Voice Circuits
-
0.064 Mbps
1
T1
1.544 Mbps
24
T2
6.312 Mbps
96
T3 44.736 Mbps
672
Intermediate Capacity
The price for a leased line does not go up linearly with
speed.
For example, the cost for a T3 line is less than the cost for
28 T1 lines.
However, if all you need is 9 Mbps, the cost for a T3 line is
greater than the cost for 6 T1 lines.
Solution: combine multiple T1 lines with an inverse
multiplexor.
Higher Capacity Circuits
Standard name
Optical name
Bit rate
Voice circuits
STS-1
OC-1
51.840 Mbps
810
STS-3
OC-3
155.520 Mbps
2,430
STS-12
OC-12
622.080 Mbps
9,720
STS-24
OC-24
1,244.160 Mbps
19,440
STS-48
OC-48
2,488.320 Mbps
38,880
About the Terminology
T-standards define the underlying bit rate; Digital Signal
Level standards (DS standards) define:
how to multiplex calls
The effective bit rates
A T1 line transmits data at DS-1 rate
Synchronous Transport Signal (STS) standards define high
speed connections over copper, Optical Carrier (OC)
standards are for fiber
The C suffix indicates concatenated:
OC-3 == three OC-1 circuits at 51.84 Mbps
OC-3C == one 155.52 Mbps circuit
SONET
Synchronous Optical Network (SONET) defines how to use
high-speed connections
Framing: STS-1 uses 810 bytes per frame
Encoding: Each sample travels as one octet in payload
Payload changes with data rate
STS-1 transmits 6,480 bits in 125 microseconds (== 810 octets)
STS-3 transmits 19,440 bits in 125 microseconds (==2,430 octets)
Getting To Your Home
The term local loop describes the connection from
a telephone end office to your home.
This is sometimes called POTS (Plain Old
Telephone Service).
The legacy infrastructure is copper wires.
Other available connections to your home include
cable TV, wireless, and electric power.
ISDN (Integrated Services Digital Network)
Provides digital service (like T-series) on existing local loop
copper wiring.
Three separate circuits, or channels, in a typical home
connection:
Two B channels, 64 Kbps each; equivalent to two voice circuits
One D channel, 16 Kbps; used for control functions
Often written as 2B+D; called Basic Rate Interface (BRI)
ISDN has been slow to catch on, because…
It is/was expensive.
It is charged by time used.
It is now (almost) equaled by analog modems.
DSL (Digital Subscriber Line)
DSL is a family of technologies.
It is sometimes called xDSL.
It provides high-speed digital service over the existing
local loop.
One common form is ADSL (Asymmetric DSL).
It has a higher speed into a home than out of it.
More bits flow in (“downstream”) than out (“upstream”).
ADSL maximum speeds:
6.144 Mbps downstream
640 Kbps upstream
ADSL Technology
Uses existing local loop copper
Takes advantage of higher frequencies on most local loops
Can be used simultaneously for POTS
Adaptive Transmission
Individual local loops have different transmission
characteristics
Different maximum frequencies
Different interference frequencies
ADSL uses FDM
286 frequencies
• 255 downstream
• 31 upstream
• 2 control
Each frequency carries data independently
All frequencies are outside the audio range
The bit rate adapts separately to the quality in each frequency
Other DSL Technologies
SDSL (Symmetric DSL) provides divides
frequencies evenly.
HDSL (High-rate DSL) provides DS1 bit rate in both
directions
Useful over short distances
Requires a four wire circuit (POTS is two wires)
VDSL (Very high bit rate DSL) provides up to 52
Mbps
Very short distance
Requires an Optical Network Unit (ONU) as a relay
Cable Modem Technologies
Cable TV already brings high bandwidth coaxial
cable into your home.
Cable modems encode and decode data from cable
TV coaxial cable:
One modem in the cable TV center connects to the
network
One modem in your home connects to your computer
Features of Cable Modems
The available bandwidth is multiplexed among all
users.
It is a multiple access medium (like Ethernet): your
neighbor can see your data!
Not all cable TV coaxial cable plants are
bidirectional; thus not cable TV systems can
support networking as well as television.
Upstream Communication
Cable TV requires sending in one direction only.
The signal is broadcast from a central location.
Amplifiers boost the signal as it travels through the
network
Amplifiers are unidirectional, so how can data
travel successfully “opposite” to television signals?
Solutions:
Retrofit the system with bi-directional amplifiers.
Use an alternate upstream path - e.g., dialup.
Alternatives
In addition to POTS, ISDN, xDSL and cable TV
coaxial cable, there are other alternatives for
connecting residences to networks.
Satellite systems, either per subscriber or per
“neighborhood”
“Fiber to the curb”
Fiber to each subscribers home is much too expensive
Fiber to the neighborhood is a potential, then use shortdistance copper local loops to provide connectivity to
fiber cable.
Summary
WAN links between sites use digital telephony
Based on digitized voice service
Several standard rates
Requires conversion via DSU/CSU
Local loop technologies
ISDN
xDSL
Cable modem
Satellite
Fiber to the curb