CS3502,
Data and Computer Networks:
the physical layer-4
Synchronization
 to
transport bits from X to R, R must know when
X is transmitting, in order to correctly interpret the
signals; 2 standard ways are synchronous and
asynchronous.
 asynchronous
transmission
 small
groups of bits (5-10 bits)
 each small group synchronized separately
 simple signaling (NRZ)
 short distances only; eg, PC to printer
 start and stop bits mark the bit group
 how
much overhead? how efficient is this?
synchronization
 synchronous
transmission
 start,
end of data marked by flag byte (01111110)
 flag pattern must not appear inside frame; bit-stuffing
takes care of this
 encoding
-> need self clocking codes
 exercise:
give a FSM for bit stuffing for the flag
01110, and to unstuff bits at the receiver
 what
is the overhead (efficiency)?
interfacing
 this
means translating from 1 physical protocol to
another
 digital devices usually have a very limited data
transmission/reception capability - not able to
transmit onto a network directly
 examples:
 digital
to analog (modems)
 digital to digital (PC to LAN)

4 parts of standard interface:
 mechanical
 electrical
 functional
 procedural
interfacing : EIA -232 standard
 terminology
 DTE
- data terminal equipment -the device which we
wish to connect to the network
 generic term for data source, data terminal (sink), or
both
 examples: PC, computer terminal, workstation
 DCE
- data circuit terminating equipment - the device
which interfaces with the network
 creates,
maintains and terminates connection with
network
 signal
conversion and coding
 example: modem
interfacing : EIA -232 standard
 25
pin connector; most apps. don’t use all
 signal/line
types: data, control, timing, ground.
(note Table 5.1 list)
 15
meters max distance
 +3
to +25 volts for 0; -3 to -25 V for 1.
 unbalanced/asymmetric
completed by ground).
1
connection (circuit
data line each way, so full duplex possible
 more
details in text;Tanenbaum p114.
interfacing : ISDN physical connector
 standard
for ISDN connections (Integrated
Services Digital Network)
 ISDN
basic data rate: 144 Kbps
 symmetric
 more
2
- this gives better electrical properties
logic, less circuits: 8 pins
data pins each way = 4 data pins
 date
circuits carry both data and control information
 other
pins for power sources
multiplexing
problem: a transmission line operates at 1.544 Mbps,
but 1 connection needs only 64 Kbps; so rest is
wasted.... since 1.544 Mbps costs about $2K/
month.
solution: share the link among many users, each
paying only their part.
purpose: to utilize as much of the line as possible
3 techniques: FDM, synch TDM, statistical TDM
multiplexing : FDM
 analog
signals with high bandwidth
 TV Cable channels; broadcast radio; voice trunks *
have Wlink > wi
than sum of channels.
 must
 main
i.e.,link capacity greater
carrier is a composite of many subcarriers.
each subcarrier may be modulated with 1 channel
 example: a carrier has a total bandwidth of 240
MHz, from 54 to 294 MHz. Subcarriers are
centered every 6 MHz; each forming 1 channel.
 guard band necessary to avoid interference
multiplexing : FDM
 FDM
problems
 crosstalk
- can occur between neighboring channels, if
overlap too close
 intermodulation
over distance
noise - possible on high capacity links
 noise,
clarity - over distance, analog signals more
vulnerable than digital; gradually being replaced in
most areas.
 switching
- not as efficient with analog signals
multiplexing : TDM
 two
types: synchronous and statistical
 synchronous
TDM
 digital
data
 signal - usually digital; can be analog signal coded
digitally
 data rate of link must be greater than sum of inputs
 similar to timesharing computers
 example:
T1 multiplexer
 standards: DS0, DS1 (T1), DS3 (T3); OCn; EC1
multiplexing : TDM
 synchronous
TDM
 time
slot to each input line
 1 slot for synchronization
 unused time slots lost
 slot size 1 bit or 1 byte, in general
 physical layer; no error or flow control
 Q:
 Q:
how much buffer space needed?
what capacity needed for 24 voice channels? how
many voice channels possible on a T3 line? OC3? OC12?
how many T1 lines on an OC12? OC48?
statistical TDM
 another
way of assigning time slots
 if
input rates irregular, varied, synch TDM could
be wasteful; stat. TDM could be more efficient
 slots
are assigned dynamically, as needed;
 requires
more intelligence; more of a data link
layer function
 frames
 show
 more
must have more control information;
fields of a possible frame
overhead than synch. TDM; closer to a MAC
type protocol
comparison: stat and synch TDM
 synch
TDM
 fixed
number slots per round
 can waste slots
 timing simpler, fixed
 format simpler
 stat
TDM
 variable
number slots per frame
 doesn’t waste slots
 more overhead, complexity; similar to data link function
 Q:
how much buffer space needed for stat TDM?
stat TDM - buffer space summary

average input rate  must be less than link capacity ; but 
may exceed temporarily.

buffer space stores temporary overflows

buffer size must be estimated based on expected input
rates and their arrival distribution. Given these we can
calculate buffer size (queue length); but in reality never can
be completely sure.

link utilization is given by  a standard queueing
formula

as approaches 1, queue (buffer) size becomes very large,
quickly; approaching infinity as  reaches or exceeds 1

utilization  of no more that 0.8 is good rule of thumb
the voice channel and telephone system
 basic
telephone network designed to deliver
quality voice service;
 voice emits analog signal - sound waves - from 30
to 10,000 Hz. Human ears detect up to 20K Hz.
 most energy in 200-3500 Hz range; Standard
analog voice channel is 4000 Hz. This key number
selected many years ago by phone company.
 standard PCM digital voice channel is 64 Kbps.
 most local telephone loops still analog
 all long distance in US is digital; majority is fiber.
the voice channel and telephone system
 voice
not very sensitive to most noise and
distortion; for this and other reasons, local telco
loops not well suited to modern data networks
 However,
the local telco networks are one of few
comm. links between homes, businesses and rest
of the world

Structure of U S Telephone networks /companies
 local
loops “last mile” and telcos
 long distance networks and companies
 network equipment
video channels and the cable TV system
 TV
cable system established only recent decades
 switching
 standard
equipment designed for broadcast TV
TV - needs 6 MHz per channel
 copper
coaxial cables capable of ~500 MHz; carry
many TV channels.
 these
cables have capacity to carry thousands of
voice channels and/or high speed data -- but need
appropriate switching equipment at home office,
and in homes
 already
becoming a reality . Will threaten existence
of old telcos. (note pending merger of ATT, TCI)