Long-Distance Digital
Technologies
Lecture 7
Why Digital?
Digital communication allows us to
handle more amounts of data, as well
as allows us to send that data farther
distances.
Digital signals are encoded on some
medium, so they propagate farther than
non-encoded analog signals.
Digitization of a Signal
In general, a waveform is sampled, and
a digital approximation at those
sampled points becomes the new
signal.
An Example
When the signal
gets transformed
into a digital signal,
a sequence of
discrete values are
created
0,2,4,4,7,1,1,1,1,1,1,
1,2,4,4,4,4.
Sampling Frequency
Since our modern data networks were
modeled on the older analog networks,
we have adopted some of their
technology.
Nyquist’s Theory: To recreate a signal of
frequency x, you must take samples of
the original signal at a minimum
frequency of 2*x.
Voice Sampling in A/D
Systems
Voice reproduction requires a system of
4000Hz, so sampling must occur at
2*4000Hz = 8000Hz.
A/D conversion in a phone system
samples the signal every 1s/8000Hz =
0.125s.
This 0.125s sampling constant is everpresent in telephony.
Sampling Drawbacks
Very accurate sampling requires more bits.
Pick some range of analog values (say 11000V). If you wanted to represent these
voltages digitally, you would want a high
resolution, but that requires more bits.
Pulse Code Modulation (PCM) uses 0-255 to
represent a range of analog values.
Using PCM for 1-1000V would have a 4V
resolution. Your digital value would be (at
worst)  2V.
Synchronous Communication
Synchronous communications occur over long
distances with some form of clocking
(synchronizing).
Synchronous links can move data at a
constant rate, so even as traffic increases, the
rate stays the same.
When synchronous links are idle, they send
null information. Synchronous links must send
something at all times!
Why is Synchronous Good?
Consider making an long-distance
telephone call: You’re calling your
grandmother in Los Angeles. (I really
like this example, for some reason.)
Asynchronous: You start to talk at
normal speed, no delay. Suddenly a
huge delay is heard in LA. Then it
becomes normal. Annoying!
Synchronous (cont.)
Synchronous: You call your grandmother in
LA. You start talking normally. You may
experience some SMALL delay, but it’s always
constant. Bearable.
The synchronous link has clocking built in to
guarantee a constant stream of data from
one point to the next. Even on a multipoint
network!
I Want It – The CSU/DSU
Synchronization (clocking) is provided by
Channel Service Unit/Data Service Unit
(CSU/DSU).
Much of today’s modern equipment has
CSU/DSU capabilities built-in, but external
devices are still common.
The CSU (for short) also makes sure the
connection conforms to existing telephone
standards.
The CSU/DSU – DNS1500
DNS1500 Specifications
T1 - 1.544Mbps with B8ZS or AMI
encoding
DDS - 56/64Kbps with AMI encoding
DSX-1 – 1.544Mbps AMI or B8ZS/ESF
encoding
V.35 Interface
How CSU/DSUs Fit In…
A CSU/DSU is required at each end of
any digital point-to-point connection.
The CSU/DSU converts between the
digital standards of the telephone
system and those of the computer
(router) vendors.
Common Diagram
The CSU Part
The CSU has the following functions:



Line termination – deals with the electrical
signals on the leased line
Testing circuitry – loopback testing, linklevel testing
Prevents excessive 1 bits. (Original
thought: too many 1’s would lead to too
much current on the line.)
Excessive 1’s
Two main methods are used to prevent
too many 1’s:


Bit stuffing
0V for a logical 0, alternating +3V, -3V for
a logical 1.
The DSU Part
The DSU handles the data:

Translates the digital information from the
telephone circuit to the form which the CPE
desires. (V.35)
CPE – Customer Premise Equipment
Telephone Standards
The “T” in T1 refers to the “Telephone
Standards
The standards are not internationally
standard- Japan, Europe use their own
slightly different schemes.
North American Standards
Name
Bit Rate
0.064Mbps
Voice
Location
Circuits
1
International
T1
1.544Mbps
24
North America
T2
6.312Mbps
96
North America
T3
44.746Mbps
672
North America
European Standards
Name
Bit Rate
Voice
Circuits
Location
E1
2.048Mbps
30
Europe
E2
8.448Mbps
120
Europe
E3
34.368Mbps
480
Europe
Digital Signal Level Circuits
DS0
DS1
DS2
DS3
DS4
–
–
–
–
–
64kbps
24xDS0
4xDS1 = 96xDS0
28xDS1 = 672xDS0
168xDS1 = 4032xDS0
Fractional Circuits
The “fractional” circuit is getting a
smaller bandwidth on a given medium.
When you have a fractional T3 installed,
you have a pair of 75 ohm coaxial
cables. You get a fraction of the T3 by
using TDM.
Too Big, Too Small… Just
Right?
You can get multiple smaller circuits and
combine them in such a fashion that allows
you to use the combined bandwidth.
This is termed inverse multiplexing. Special
hardware is required in addition to CSU/DSUs
at both ends of the circuits.
Multi-homing is a form of inverse
multiplexing.
An Inverse Multiplexing
Example
Fat Pipes
Bigger circuits have their own
standards.
STSx – Synchronous Transport Signal
OCx – Optical Carrier
Note: The x refers to a different level.
Fat Pipes (cont.)
Standard Optical
Name
Name
Bit Rate
Voice
Circuits
STS-1
OC-1
51.840Mbps
810
STS-3
OC-3
155.520Mbps
2430
STS-12
OC-12
622.080Mbps
9720
Fat Pipes (cont.)
Standard Optical Bit Rate
Name
Name
Voice
Circuits
STS-24
OC-24
1,244.160Mbps
19440
STS-48
OC-48
2,488.320Mbps
38880
STS-192 OC-192 9,953.280Mbps
155520
STS-256 OC-256 13,271.040Mbps 207360
Fat Pipes (cont.)
OC3  OC3C.
An OC3 is really 3 separate OC1 circuits
running on the same physical medium.
The “C” stands for concatenated (or
clear-channel).
Remember: A T3 is NOT 3 T1s!
Framing Schemes
Three major types of framing schemes
exist:



AMI
B8ZS
ESF
AMI
Alternate Mark Inversion
AT&T definition: A line code that employs a
ternary signal to convey binary digits, in
which successive ones are represented by
digital elements that are normally of
alternating, positive and negative polarity but
equal in amplitude, and in which binary zeros
are represented by signal elements that have
zero amplitude.
B8ZS
Bipolar 8 Zero Substitution.
Telco dictionary: Rather than inserting a one
for every seven consecutive zeros, B8ZS
inserts two violations of the bipolar line
encoding technique used for digital
transmission links.
Bipolar Violation: Two consecutive marks of
the same polarity on the T line.
ESF
Extended (Extended) Super Frame
T1 frames occur 8000 times a second,
each frame preceded by a framing bit.
ESF requires 2000 framing bits (which
are used for synchronization purposes).
The remaining 6000 bits are used for
error detection, data link monitoring,
cyclic redundancy checks, etc.
SONET
Synchronous Optical Network
Specifies the following:



Framing on optical circuits
Multiplexing smaller circuits on to larger
ones
How clocking information is sent alongside
data
SONET Framing
Why the Weird Size?
Take STS-1 for example:
51.840Mbps/125s = 6480 bits = 810
octets.
STS-3 is exactly 3 times that size, such
that 3 STS-1 frames can fit in a STS-3
frame!
To talk about in the future: SONET Ring