On Monday we began talking about Direct Link Networks.
We looked at the hardware involved in network nodes.
We discussed how the CPU communicates with the network
adapter using the CSR and can send and receive using either DMA or PIO.
We made the point that CPU speeds are increasing faster than memory
access speeds, as well as bus speeds, and this can limit our ability to send data as fast as we might want.
Also, it shows we don't want to perform too many reads or write to memory in transferring data.
Another issue we discussed is that our memory is finite and so this limits our ability to buffer on a given machine.
On Monday we also start talking about links, frequency, wavelength, and the EM spectrum.
We now look more into the details of implementing links...
From Waves to Bits and Channels
So far we have described how a physical medium carries signals in the form of waves...
We often want to transmit digital information like 0's and 1's. How we piggy-back binary data
on waves is called an encoding.
It has two layers:
A lower layer which is concerned about modulation -- varying the frequency, amplitude, or
phase of the signal to effect transmission of information. For example, one might vary the power (amplitude) of a single wavelength between two values -- the "high" signal and the "low" signal.
An upper layer which is concerned with encoding bits using these two different signal values. (What we'll be interested in for this class.)
Another issue about links is how many bit-streams can be encoded on a link at a given time. If only one, we have to have a
mechanism for sharing access such as what we will describe for multiple-access links. For point-to-point links, if the answer is two, then one signal can be used to communicate from Host A to Host B and the other for the way back. This is called a
full-duplex link. If the answer is one then Host A and B must take turns and the channel is called
half-duplex.
Cables
One way to connect two machines is to physically string a cable between the two.
We next consider a couple of different cable types...
Twisted Pair
Twisted pair wire consists of two insulated copper wires
typically about 1mm thick which are twisted together.
Other a single cable will consist of a group of four such pairs each twisted at a
different twist rate.
As a charge travels down a wire it gives off EM radiation,
which is power lost.
Twisting causes the waves that are radiated to cancel
preventing the wires from radiating as effectively; the different twist rates of the four
pairs minimize cross-talk between pairs.
This basic set-up is called unshielded twisted pair (UTP), you can also have different
kinds of shielded twisted pair wires (screen or foil).
Old-school telephones were typically connected to the phone company by
twisted pair wires.
A signal can be sent down such a wire without
amplification for several kilometers. For longer distances a
repeater is necessary.
The bandwidth of these wires is on the order of
megabits/sec, and they are very cheap.
The most common kind of twisted pair wires used in networks
today is some flavor of Category 5 (Cat-5). More recently, Cat-5e might be used for
gigabit networks and Cat-6 for gigabit and deca-gigabit networks.
Coaxial Cable
This kind of wire has better shielding then twisted pair
wires, so it can span wider distances at higher speeds.
It is often called coax.
There are two types of coax cables 50 and 75 ohm.
50 ohm was intended for digital transmission; whereas, 75
ohm was intended for both digital as well as analog
tramsmission such as TV.
Modern cables are capable of bandwidth around 1GHz.
In addition to cable TV use, these cables used to be used
by the phone company over long distances before fiber
optics became more popular.
Fiber Optics
This is the largest bandwidth transmission technology.
Currently systems can operate at 10Gbps, with lab
examples running at 100Gbps over a single fibre.
Optical transmission has three components: the light
source, the medium, and the detector.
A pulse of light is used to indicate a 1, the absence of a
pulse a 0.
The transmission medium is an ultra-thin fibre of glass.
A detector generates electrical pulses when light fall on it.
Light does not leak out of the fiber because it get reflect
off the sides of the fibre back into the fibre
Angles of incident light at which this ricochet effect will
work are called modes of the fiber.
You can have single as well as multi-mode fiber.
A Table Comparing Common Cables and Fibers
Cable
Typical Bandwidths
Distance
Cat-5 Twisted Pair
10-100Mbps
100m
Thin-net Coax
10-100Mbps
200m
Thick-net Coax
10-100Mbps
500m
Multimode Fiber
100Mbps
2km
Single Mode Fiber
.1-10 Gbps
40km
Leased-Lines
If the two nodes you want to connect are on the opposite sides of the country, or even across town, it is not
practical to install the link yourself.
You could lease such a dedicated link from the phone company called a "leased line".
Below is a table of the various "leased line" services the phone company might provide:
Service
Bandwidth
DS-1
1.544 Mbps
DS-3
44.736 Mbps
STS-1
51.840 Mbps
STS-3
155.250 Mbps
STS-12
622.080 Mbps
STS-24
1.244160 Gbps
STS-48
2.488320 Gbps
More on Leased-Lines
DS1 and DS3 (sometimes called T1 and T3, DS := digital signal) are older technologies originally defined for copper-based
transmission media.
DS1 is equal to the aggregation of 24 digital voice circuits of 64kbps each and DS3 is equal to an aggregration
of 28 DS1 links.
All the STS-N links are optical fiber (STS stands for Synchronous Transport Signal).
STS-1 is the base link speed and STS-N has N times the bandwidth of STS-1.
STS-N is sometimes called OC-N (optical carrier).
Typically, the phone company does not implement a leased link using a single cable, but instead uses its own internal network.
Last-mile Links
Instead of a dedicated leased line, it is often cheaper to connect two hosts by connecting each of them to a network using
a last-mile link.
Common last-mile services are: modem (POTS - plain old telephone service), Integrated Services Digital Network (ISDN) - two 64Kbps channels stuck together one for voice the other for data (can ditch voice if want), xDSL (some flavor of digital subscriber line ADSL or VDSL) goes over POTS but where the voice filter on local loop has been removed at end office, and cable modem.
Below is a table of common last-mile links and their bandwidths:
Service
Bandwidths
POTS
28.8-56Kbps
ISDN
64-128Kbps
xDSL
128Kbps-100Mbps
CATV
1-40Mbps
Wireless Links
Wireless links transmit electromechanical signals - radio, microwave, infrared, or even optical light (optical telegraphy was used by Napoleon).
The space through which these signals are transmitted is shared.
Since signal strength falls off with the square of the distance, a given source attenuates with distance and this can be used to limit who hears who in space.
The other way we can control who uses this common resource is by coming up with protocols about which frequency to send data on.
In the U.S., the FCC (Federal Communications Commission) is responsible for reserving certain frequency bands for certain uses: AM radio, FM radio, TV, etc.
It also licenses specific frequencies within these bands to organizations for a given area. For example, KQED.
There are license-exempt frequencies that have different restrictions on their use: low power, etc.
There are several techniques for sharing a limited spectrum of frequencies among many users so as to reduce interference (spread spectrum techniques):
frequency hopping -- sender and receiver share a pseudo-random number generator (prng) seed, and use this seed to compute
a sequence of frequencies to briefly communicate on.
direct-sequence -- each bit of data is represented by multiple bits in the transmitted signal, so if some bits are damaged by interference
it is often still possible to figure out if the code was for a 1 or a 0. For each bit the sender wants to transmit, the sender actually
XORs that bit with n random bits generated by a (prng) known to the sender and receiver. The transmitted value is known as an n-bit chipping code.
Spread spectrum techniques are used with 802.11 (Wi-fi), 802.15.1 (Bluetooth), 802.16(Wi-Max).
Encoding
We want to be able to write 0 and 1's using our ability to have send the signals low and high between machines.
NRZ Encoding
In the nonreturn to zero (NRZ) encoding we use high for a certain amount of time to represent a 1 and low for a certain amount of time to represent 0.
The receiver usually keeps an average of the signal seen so far, so if several consecutive 0's or 1's are sent it skews the average, making it hard to distinguish between 0's and 1's. (baseline wander)
The sender and receiver need to synchronize their clocks in order to know how long a single 0 or 1 is. To do this the receiver typically relies on seeing 0 to 1 or 1 to 0 transitions from the sender so it can align its clock. However, if too many 0's or 1's in a row are sent this is not possible.
NRZI Encoding
In the nonreturn to zero inverted (NRZI) encoding, to send a 1 the sender transitions from the current signal value to the opposite; to send a 0 the sender stays on the current signal.
This solves the problem of consecutive 1's, but does not help with consecutive 0's.
Manchester Encoding
In the Manchester encoding, one sends the XOR of the clock and the NRZ-encode data.
This results in a 0 being encoded as a low to high transition and a 1 being encoded as a high to low transition.
The drawback is the rate at which the signal changes (the baud rate) to encode a single bit has doubled.
The bit rate/baud rate is only 50% which is somewhat inefficient.
