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Network Architecture COSC 5110 Mark Zieg
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Network Architecture
COSC 5110
Mark Zieg
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BRIDGE
Prof. David Metcalf
October 13, 1997 Page 2
Chapter 2: The Electrical Interface
2.1
Give a brief description of the application and limitations of the following types of transmission
media:
a.
Two-wire open lines
b.
Twisted-pair lines
c.
Coaxial cable
d.
Optical fiber
e.
Microwaves
a. Open lines
These lines are cheap and easy to build and represent the wide variety of straight-through cables used in
connecting peripherals to computing equipment. Such cables include classic serial (8, 9, 15, or 25-pin DB or DIN
connectors), parallel (50-pin Centronics, ribbon cords), and silver satin phone cable. All these cables are
customarily very short due to the ease with which they can pick up interference either from outside sources or from
neighboring wires (crosstalk).
b. Twisted-pair
It turns out that by twisting two wires together you can greatly cut down on the amount of interference the wires
pick up. This is because any interference inherited by one wire is almost guaranteed to affect the other wire when
they are so closely intertwined. Since the main problem of signal interference is how it changes the voltage
differential between the signals carried by two wires, it can be easier to try to equitably distribute interference
universally than to try to block it altogether (via shielded cables). Standard unshielded twisted-pair lines are
identified by category (Cat-3, Cat-5, etc), where the category refers to how many times each wire pair is twisted
every inch: if you carefully stripped back the insulation on a 2-foot length of Cat-5 cable, you should expect to find
at least 120 twists in each pair.
The main problem with this kind of cable is the
skin effect. This refers to a physical phenomenom in which
attenuation and media resistance increase at high signal frequencies, limiting the useful bandwidth of UTP over
long distances. However, UTP is becoming extremely popular in LAN connections due to its low cost and the ease
with which it may be wrapped, stripped, and crimped.
c. Co-axial
Co-axial was very popular for many years, and remains entrenched in the walls of many networked facilities.
Unlike open cable, co-ax has natural shielding which minimizes interference, and unlike UTP co-ax can be used for
extremely long runs at high bandwidth. For this reason, many early networks adopted co-axeither thicknet or
thinnetfor their backbone or ring network topologies. Co-ax is also the
de facto standard for cable TV companies,
who need its high bandwidth and reliable signal strength.
The main problem with co-ax is that its a little hard to work with maintenance-wise. Its stiff, heavy, expensive,
and awkward to crimp. Therefore many networks are gravitating to UTP for workstation connections and leaving
co-ax (or more often fiber) for their building-to-building links and backbones.
d. Fiber
Fiber optical cable is a dream come true for bandwidth-hungry technology visionaries. It carries scads of data,
quickly, reliably, and securely, with none of the interference or power problems evinced by electrical conductors.
For instance, a single pair of fiber has been used to transmit data at over 3Gbps--try that with copper! Fiber is also
much harder to tap than copper, especially in systems employing a carrier signal. Nor can electrical discharges,
such as lightning bolts, propogate along fiber strands, since glass makes a fairly poor electrical conduit. For the
same reason, you can run a fiber pair right past massive electrical generators, turbines, nuclear power plants, you
name it, with only a thin layer of cladding to protect the integrity of the signal. Neat stuff.
The only real problems with fiber are cost. Termination gear is very expensive compared to copper variants,
and F-type network cards still cost at least five times their copper equivalents. Finally, the lines themselves will
remain expensive until volume picks up. Page 3
e. Microwave
Microwaves represent the high-bandwidth end of the wireless spectrum. As such, their priniple advantage is
that they can be used to form data links between locations where physical cable connections would be prohibitively
expensive, inefficient, or unsightly. There are two main kinds of microwave communication: satellite relay, in which
ground signals are sent skyward to geosynchronous staellites for retransmission to one or more earth-based
receiving stations, and terrestrial, in which microwave towers communicate directly over line-of-sight.
Besides freedom from cable connections, microwave communications are popular due to their relatively high
bandwidth, option of coarse or narrow transmission, and broadcast capability. On the other hand, they are not
suitable for every application. Transmitters/receivers can be both expensive and bulky, and of course satellites
dont grow on trees. Signals can be subject to interference from weather, and microwave towers are subject to
control by local zoning authorities.
2.2
With the aid of sketches, explain the differences between the following transmission modes used
with optical fiber:
a.
Multimode stepped index
b.
Multimode graded index
c.
Monomode
a. Multimode stepped index
Multimode stepped index suffers from a problem involving internal reflection. As light enters an optical core, it
disperses in several directions. Any light not traveling straight down the fiber core strikes and is reflected by the
inner surface of the cladding. This light keeps bouncing around, from wall to wall, until it reaches the end of the
core. However, light bouncing at high angles of incidence will take somewhat longer to reach the cable end than
light bouncing at low angles of incidence, due to the additional trips it must take across the core diameter. This
causes some light pulses to arrive at their destination faster than others, with the result that data bits can overlap or
arrive out of sequence at high pulse rates.
This is diagrammed in figure 2.2.1, which shows a stream of bits being transmitted through a section of fiber
core, each color-block representing a frame of 1000 bits. The original light pulse is dispersed at three different
angles, ranging from about 30° (triangles) to 60° (diamonds) from true. As you can see, although the frames
remain mostly aligned at the beginning of the cable, by the end they are out of phase by as much as three full
frames, creating a significant data jumble at the receiving end.
Figure 2.2.1
The simplest solution to this problem is to slow down the signal transmission rate so that all of the optical
receiver can be certain that it has received all of the reflected impulses of one bit before the next is scheduled to
arrive, preventing overlap. However, this decreases available bandwidth.
b. Multimode graded index
Multimode graded index cable improves over stepped index by using a graduated core material with a variable
refractive index. Light traveling near the axis of the core moves more slowly than light traveling toward the rim.
Therefore, although light may be dispersed toward the cladding and thus have a longer distance to travel than light
sent directly down the center of the core, the refracted light is allowed to move more quickly near the surface and
thus arrives at the destination at almost the same time as the inner signal. The refractive traits of the core may be
visualized as shown in Figures 2.2.2 and 2.2.3. Page 4
Figure 2.2.2
Figure 2.2.3
c. Monomode
Monomode cable is a second solution to the dispersion/refraction problem. By narrowing the core down to a
diameter equal to a single wavelength of light, there is literally nowhere for pulses to go except for straight forward.
By solving the bouncing problem without slowing either the pulse rate or the transmission medium, very high bit-
rates can be attained. I assume, however, that monomode cable is significantly more expensive than other kinds,
and possibly more fragile and difficult to work with (although cladding could probably solve that).
2.3
The maximum distance between two terrestrial microwave dishes, d, is given by the expression:
d
Kh
=
7 14
.
where h is the height of the dishes above ground and K is a factor that allows for the curvature fo
the earth. Assuming K = 4/3, determine d for selected values of h.
As you can see in Figure 2.3.1, the advantage of adding height to a microwave relay tower tapers off
rapidly, with a 600 tower p