II. Physical Layer
A.Transmission Media
1.Any communication signal
must occupy a finite bandwidth.
a.Nyquist showed (in 1924) that if signal has V discrete levels, then
V must increase exponentially for a linear increase in the data rate.
b.The rate at which signal levels change is the baud rate.
c.One cannot distinguish arbitrarily close voltages because any real communication
channels has noise.
Shannon showed (in 1948) that
2.The bandwidth of a medium
is limited ot its carrier frequency; so, it is advantageous to use higher
carrier
frequencies
to transmit higher data rate signals.
The radiation
that carries signals can be guided or un-guided.
Fig. 2-1. The
electromagnetic spectrum and its uses for communication.
a.Guided Waves
twisted pair (used in telephones and have a very low data rate)
coaxial cable(used for computer connections; cable tv)
fiber optics (used for high data-rate trunks in telecommunications systems)
[Note the big gap
between the central frequency of optical fibers and everything else.]
Videophones and other high-speed communications over phones are not possible
at high quality because
the bandwidth of twisted pair.
Where do we go?
coaxial cable [owned by cable tv companies-- and they can sell phone service
too, maybe]
fiber to the home [expensive but would deliver almost in finite bandwidth]
fiber to the curb [with twisted wire or coax to the home, the bandwidth
would be significantly
increased at a reasonable cost.]
b.Unguided Waves
radio, tv, sattelite, microwave
As the frequency increases, the waves become less capable of going around
obstacles and through
buildings. [They are less wave-like and more particle-like.]
The switch-over point is at about 100 Mhz--1Ghz.
B.The Telephone System
1.Why study the telephone
network?
a.It is the network "par excellence" connecting people in places all over
the world.
b.A large amount of computer communication is done over the phone system
via modems. A dialup
connection has one error in every 10^5 bits (with a maximum data rate of
10^4 bits/sec), so dealing with
the phone system's limitations is important as well.
2.Connecting everyone directly
is impossible because too much wiring is required. (With n users, there
would be n!
connections!).
The system developed
from completely connected -> connected through swithcing offices -> 5-level
hierarchy (at
AT&T before
breakup).
Fig. 2-2. The AT&T
telephone hierarchy. The dashed lines are direct trunks.
a.Each telephone is connected by twisted wire through the local loop to
the end office.
End offices are connected to switching centers called toll offices or tandem
offices (if they are in the
same local area) by toll connecting trunks.
Toll offices are connected to higher levels via intertoll trunks.
b.Connections are made at the lowest possible level of the tree. Also,
there are some special, direct links for
very busy paths (New York to Los Angeles).
Fig. 2-3. The relationship of LATAs, LECs, and IXCs. All the circles are
LEC switching offices. Each
hexagon belongs to the IXC whose number is in it.
c.In 1984, AT&T was broken up into one long-distance company and 7
RBOC's (Regional Bell Operating
Companies).
The United States was divided into 160 LATA's (Local Access and Transport
Areas). In most LATA's
there is one LEC (Local Exchange Carrier), but there can be several (there
are more than 1500
independent carriers in the US).
Long-distance companies (called Inter-eXchange Carrier = IXC) carry traffic
between LATA's. Each can
build a switch in any LATA where it wants to carry traffic called a point-of-presence
(POP) which is
connected to the tandem office or the end offices.
d.In 1995, things were organized again to allow even more competition.
3.Analog-to-Digital Conversion
a.All local loops are analog (twisted pair).
b.Almost all trunks are digital (microwave or optical fiber)-- but some
are still analog.
c.Digital links cost less and have higher fidelity. (The issue is replacing
current analog links).
d.Since local loops are analog, one must take a digital computer signal,
turn it into analog at the loop (via a
modem), then turn it digital again (via a codec) at the end office.
Fig. 2-4. The use
of both analog and digital transmission for a computer to computer call.
Conversion is done by
the modems and
codecs.
4.Modems
a.Information is transmitted by modulating a sine wave carrier between
1,000-2,000 Hz.
Fig. 2-5.
One can transmit information by altering:
amplitude (AM=amplitude modulation)
frequency (FM=frequency modulation)
phase (PM=phase modulation)
b.The basic line rate (baud rate) is 2400. To get higher data rates, one
can use a combination of phase and
amplitude modulation. Using an appropriate combination of phases and amplitudes,
it is possible to raise
the bit rate. The pattern on the left (ITV V.32) corresponds to 9600 bps.
Fig. 2-6. (a) 3 bits/baud modulation. (b) 4 bits/baud modulation.
c.To get higher data rates, it is possible to use a variety of sophisticated
error correction and data
compression schemes.
5.Multiplexing
Fig. 2-7. Frequency
division multiplexing. (a) The original bandwidths. (b) The bandwidths
raised in frequency.
(c) The multiplexed
channel.
a.Old analog trunks multiplex different conversations (channels) together
using FDM (Frequency Division
Multiplexing). Voice channels each occupy 4Khz (3Khz for data and 500Hz
on each side as a guard
band). 12 channels = group; 5 groups = supergroup; 5 or 10 supergroups
= mastergroup
b.Digital trunks use TDM (Time Division Multiplexing).
1.The incoming signal is sampled in a codec (coder-decoder) at 8000 samples
per second
(125 µ sec/sample) which is the Nyquist limit.
2.In the United States: Each sample produces 7 bits of data (corresponding
to 2^7 = 128 amplitude
levels) and one additional bit that is used for control for a total of
8 bits. Twenty-four channels are
combined with eight bits per channel to make a T1 or DS1 frame. One additional
bit is used at the
beginning of the frame for synchronization in the pattern 101010...
Fig. 2-8. The T1 carrier (1.544 Mbps).
3.The T1 streams are time-multiplexed together, along with extra framing
and recovery bits to form
successively higher and higher data rate channels.
Fig. 2-9. Multiplexing T1 streams onto higher carriers.
4.This is the North American Digital Hierarchy. Other schemes are used
elsewhere (see
Tanenbaum).
c.SONET/SDH (Synchronous Optical NETwork/Synchronous Digital Hierarchy)
1.Different US IXC's had different, proprietary optical TDM systems to
connect to fibers--not to
mention the European and Asian systems. To make things uniform, Bellcore
(the RBOC's research
arm--just sold to SAIC) developed SONET. CCITT came up with the SDH standard
which is
compatible.
2.Design goals:
a.Standardize all carriers world-wide.
b.Access individual channels without completely demultiplexing the data
stream.
c.A straightforward path to Gbit/sec systems.
To do that, the system is synchronous--controlled by a master clock to
one part in 10^9. This level
of synchronism is only feasible in optical fibers!
3.A SONET system has repeaters and multiplexers between the source and
destination.
Fig. 2-10. A SONET path.
Path = source-to-destination link; Line = multiplexer link; Section = repeater
link; A repeater is
where information is electronically regenerated.
4.SONET frames are transmittted at 8000 frames/sec, corresponding to the
rate at which voice
channels are sampled. Each frame has 810 bytes (6480 bits) organized conceptually
in 9 rows of 90
bytes each. The first three columns are for section overhead (the first
three rows) and line overhead
(the next six rows). The remaining 87 columns are for data. The gross data
rate is 51.84 Mbps and
the user data rate is 50.112 Mbps.
Note that the data can begin anywhere and can extend between frames. The
data is referred to as
the SPE (Synchronous Payload Envelope). Its first column is the path overhead.
The flexibility is
very important in dealing with ATM.
Fig. 2-11. Two back-to-back SONET frames.
5.The multiplexing of multiple data streams, called tributaries, are important
in SONET.
The base rate is STS-1 (electrical) or OC-1 (optical). The tributaries
are byte-interleaved, not
bit-interleaved, so the channels are separable. THERE IS NO ADDITIONAL
OVERHEAD. So,
the rate of STS-3 = 3*STS-1.
6.The base level of SDH (referred to as STM-1) corresponds to STS-3 or
OC-3 and runs at 155.52
Mbps. The levels of SONET are multiplexed in multiples of 3 for compatibility
with SDH.
OC-3c (where "c" is for concatenated) means that there is a single data
stream, rather than 3
multiplexed together. The OC-3c channel is the basic transport vehicle
for ATM.
d.Optical WDM
At rates higher than OC-48 (2.56 bits/sec) physical effects in the fiber
make it hard to transmit TDM
signals over long distances. To reach higher data rates and to use optics
in networks, OC-48 channels at
different wavelengths are multiplexed. Systems using this approach are
just now evolving.
Fig. 2-12.
6.Switching
a.Circuit-switching vs. packet-switching
Fig. 2-13. (a) Circuit switching. (b) Packet switching.
1.In a circuit-switched network, a physical layer, hard-wired connection
is set up before the
communication can begin. (It may be copper, fiber, sattelite, or a combination.)
Thus, there is a
delay before data transmission, but afterward, transmission is continuous.
This is good for voice but
bad for data.
Note: A circuit-switched network is necessarily connection-oriented at
all layers (to the extent that it
makes sense to speak of layers in this case--another issue).
2.In a packet-switched network, the packets contain much if not all of
the routing information.
Transmission can begin immediately and be connectionless as in IP
or
Transmission can be connection-oriented and sequential, requiring setup,
as in ATM.
Note: It is the flexibility of this approach that makes data and voice
in one network possible.
b.Switching Architectures
Fig. 2-14. (a) A crossbar switch with no connections. (b) A crossbar switch
with three connections set up:
0 with 4, 1 with 7, and 2 with 6.
1.The simplest switch conceptually is a crossbar switch. A switch with
n inputs and n outputs has n^2
possible connections, referred to as crosspoints. This switch is non-blocking
since every possible
connection can be made simultaneous but requires many possible connections.
2.By splitting the switch into stages, it is possible to build space-division
switches with far fewer
crosspoints. With N inputs and crossbars with n inputs, we need N/n input
crossbars and N/n
outputs. The second stage has k N/n x N/n crossbars.
What is the number of crosspoints? 2(N/n)nk + k(N/n)^2 = 2kN + k(N/n)^2.
With N=1000, n=50,
k=10, Number of crosspoints = 24,000. But: the switch can block! As usual,
it is an optimization
problem.
Fig. 2-15. Two space division switches with different parameters.
3.Time-division switches use time slot interchangers to carry out a similar
operation in time.
Fig. 2-16. A time division switch.
c.ISDN, B-ISDN, and ATM
1.ISDN (Integrated Services Data Network) began in 1984 when the world's
phone companies met
under the auspices of the CCITT to plan end-to-end digital services.
a.Enhanced voice services with direct links to computers are possible (when
a customer calls a
stockbroker, his portfolio could become available on the computer at the
same time).
b.A basic rate ISDN channel (also referred to as N-ISDN service) contains
two 64-Kbps
PCM channels for voice or data and a 16-Kbps channel for out-of-band signalling.
c.This service is growing slowly. There isn't enough bandwidth to allow
for video or a large
enhancement of services, but it is MUCH better at 144-Kbps in connecting
to the Intenet
from home, and it is affordable.
2.B-ISDN (Broadband ISDN)
a.This technology is based on ATM which is packet-switched and transmitted
at 155-Mbps
(STS-3 = STS-1).
b.Installing B-ISDN is a long-term, costly prospect because it requires
replacing the local loops
which cannot handle 155 Mbps. It also means replacing the current telco
space and time
division switches which cannot handle packet switching.
c.Virtual Circuits are used to route packets (ATM cells). When a route
is chosen through a
network, entries are made in router tables so that when a cell with an
appropriate header
comes along it is sent along the correct path. Resources can also be allocated.
The arrival rate
depends on the application; data can be bursty while voice is smooth (allowing
both), but
sequence must be preserved.
Fig. 2-17. The dotted line shows a virtual circuit. It is simply defined
by table entries inside the
switches.
d.Building switches that:
rarely drop cells
never reorder cells
at the high data rates required (typically 16 to 1024 cells every 2.7 µsec)
is a real challenge.
e.How do we deal with contention when several inputs want the same output?
General principle: Output queuing is more efficient than input queuing.
Tanenbaum discusses
several switch designs that use output queuing based on this principle.
Fig. 2-18. Input queueing at an ATM switch.
Fig. 2-19. Output queueing at an ATM switch.
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