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)
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