Networks on the Net
Books used: Information Technology by David Cyganski, John Orr
Most of the existing telephone network was
designed to provide a communications
connection which is:
Way:Each party can speak to the other;
that is, this is a system meant for conversation
and not presentation or publication.
Time: A delay in excess of about 1/3 of a
second is disrupting to conversations, as it is a
large enough a time period to be confused
with vocal gestures (pause, doubt, surprise).
Pair wise:The conversation is an exchange
between two people and not an open forum
for anyone to join.
Circuit Switched Connection: Voice
connections are set up and taken down
frequently under control of the user (by
dialing a number) as opposed to being
permanent (in which case we would need a
telephone in our house for each party to
whom we ever intended to speak).
The TV System
Way: The television programming provider
sends its video product to your home and not vice
Time: As a viewer you don't really care or
notice if the program you are watching is 10
seconds late or more. Because the communication
is not two
way, there is no sense of delay, and
hence the provider can use slow channels
(satellites) or even manually mounted tape
recordings to provide the video feed.
Many: The provider does not individually
package and transmit TV shows to each user. A
single launch of information is made of all the TV
shows to all users.
Users, of course, can independently select
which information in the information stream
to show on their screens (and with some
modern TV sets, several of these can be
watched at the same time).
Permanent Connection: The connections
between the provider and the users are not
switched or directed. As noted above, the
same information is transmitted on one
network to all viewers. When the viewers
switch channels, they are only selecting
what part of the information to view and are
not altering what is being sent.
Circuit vs. Packet Switch Networks
based networks are those in which a
path is maintained between the users for the
duration of the call; and
based networks are those in which
individually addressed packets of information
are sent into a communications system, and
are individually forwarded until they reach the
based network with which the reader
might be best acquainted is the telephone system.
After a number has been dialed, a circuit is
established. That is, a path has been created
between your telephone and that of the person you
You can think of the telephone circuit and any other
based network connection as a hollow tube
running between two end points. Anything you drop
in one end of the tube will come out the other end
(for example, your voice in the case of the
If you want to communicate with a second person,
you need to establish a second circuit.
The destination of your message is
determined by the tube into which you
drop the message. In the case of cable
television networks (which are circuit
based) the provider drops the same
messages into all the tubes.
Packet Switch Networks
based network with which the reader is
probably most acquainted is the postal system. To
send a packet of information (a letter) to someone,
you must put an address on an envelope. Then,
you drop the letter into a mailbox.
This mailbox no longer acts like a tube following a
single path to the destination. In fact, you can drop
many messages with different addresses into the
same mailbox and they will each be directed to the
correct recipient, in stark contrast to the telephone
example above. In fact, not only do you not need a
separate box for each possible recipient, you can
actually use any mailbox you find on any street to
send the same piece of mail.
Packet and Circuit Switch
The telephone companies comprise
what has commonly been called the
Public Switched Telephone Network, or
The PSTN has traditionally been in the
business of providing temporary circuit
based connections on demand for voice
One of the first and most popular digital
permanent circuit services sold by the PSTN in
North America was the so
called T1 carrier.
This transmission system was devised originally
sets of 24 simultaneous
digitized phone conversations
pairs of copper wire over distances of up to
approximately 50 miles.
Repeaters were placed 6,000 feet apart along
the path of the cable to regenerate the signal,
which otherwise would suffer severe
degradation due to cable losses.The overall bit
rate of this digital data transmission system is
1.544 Mbps per channel.
It is typical telephone parlance to refer to
the format of the digital signal involved in
T1 transmission not as a T1 signal, but
rather as a
DS1 (Digital Services 1)
, because this same format can be
used on essentially any kind of cable.
Thus, T1 refers to both the physical
medium (the twisted pair cable) and the
DS1 format of the digital data being used
to move the data on that cable.
The digitized voice telephone signals transmitted
by a T1 carrier are represented by 8,000 samples
per second quantized to 8 bits per sample.
Actually, some of these data bits are ``stolen'' to
provide synchronization information and
``signaling'' information, which is for routing the
data at the various switch points.
Also, one additional bit is inserted into the stream
for every 192 bits (24 voice channels bits), which is
used to establish overall synchronization of the
channels within the stream. Thus, 193 bits
samples/sec= 1.544 Mbps are transmitted.
T1 in Europe
The transmission hierarchy is different
in Europe, where groups of 30 voice
channels rather than 24 are the basis
for the hierarchy.
In the late 1980s, the PSTN gave users direct
access to a switched digital service. This
service, which parallels the regular voice
telephone network, became widely available in
a form that has been named
Services Digital Network,or ISDN
However, for reasons of cost (high), data rate
(not very high), and installation difficulties,
ISDN has not been widely accepted, and will
likely be eclipsed by cheaper and/or faster data
ISDN services are available in a number of versions
with various cable connection types and bit rates. We
will briefly discuss the two forms are now readily
available in most larger cities throughout the world. In
particular, we will discuss the formats used in North
ISDN BRI (basic rate interface) is available worldwide
and offers a 192 kbps digital service divided in a
fashion that separates it into one 16 kbps signaling
data streams and two 64 kbps data streams that can
be used as two computer data streams, two digital
phone streams, or one of each. The BRI service is
often called the 2B+D service,indicating that two ``B''
channels and one ``D'' channel are available on a
ISDN BRI services are delivered via the local
wiring that had been used before to simply provide
the analog telephone technology connection for your
home and office phone.(Actually, not all existing
telephone wiring is ``good enough'' to support these
data rates, and the PSTN company with whom one
contracts for ISDN service will test the lines prior to
selling the service.)
Once ISDN services have been purchased, users can
connect a special interface box to their telephone wall
plate for connection to computers, printers,
telephones, and so forth.
This device will support the call set
and allow users to enter (either on the front
panel of the device or through a computer
communications link and user interface
software) the telephone number of another
ISDN device anywhere in the PSTN.
Thus, just as with analog telephone service,
many end points can be serviced from any one
line. In fact, the two B channels can be dialed
into two different end points simultaneously.
Asynchronous Transfer Mode
ATM “squeezes” overhead out of the process of
handling data streams by making the following
concession: every packet (now called a cell) is
of fixed size
with 48 bytes devoted
to data and the remainder to addressing and
controlling the virtual circuit.
By breaking the data into small cells it becomes
possible to pack those cells into standardized
divided data streams, ``on
Furthermore, the small cell size leads to a
lessening of the storage needed in the store
forward architecture of the network.
we also abandon any guarantees of packet
delivery and let the end
users apply their own
techniques to recover from missing cells.
based virtual circuit packet transmission
services for speeds up to 155 Mbps, are readily
available from PSTN companies.
ATM equipment is now commercially available
that handles data at 622 Mbps. And,new
integrated circuit chips have also been
previewed that handle ATM data rates up to
switching technology was developed that uses
a special packet addressing scheme named the
The term datagram is derived from the word
telegram. A telegram is an addressed letter that is
transmitted electronically over several long distance
hops into the local area of the destination. At that
point a printed copy is rapidly and personally delivered
by a special messenger service to the doorstep of the
That is, the idea behind the telegram was fast service,
multiple retransmissions, and rapid direct delivery to a
Along the way, the movement and handling of themes
sage was determined (much like mail) by the address
on the telegram. At no time was an arrangement
made for a future delivery (a connection). In fact,
telegrams were used in the past not only when speed
was needed,but also before the time of universal
telephone and mail service.
Because no connections were made, it was also
possible for the telegram service to discover that the
! In that case, a
message was rapidly returned to the originator.
While a seeming disadvantage in these cases,
the benefit is that most messages arrive more
quickly because of the removal of interactions
that would otherwise be needed.
Furthermore, the end station in the telegram
often try several times to
locate the recipient before giving up
the sender could be worrying about other
communications rather than waiting for the
destination availability to be verified.
A datagram is a packet of information that is self
That is, in addition to the message content itself, it
contains the entire universal address of the sender
These datagrams are then delivered on a``best
effort'' basis. In other words, the delivery service
takes responsibility forgetting it there, but will
abandon the attempt after a certain amount of time
has passed or a number of attempts at delivery have
In the usual datagram implementation there is not
even feedback in case of failed delivery.
The Institute of Electrical and Electronic Engineers
(IEEE) modified the specifications for Ethernet
protocol and published it as IEEE Standard 802.3 in
The Ethernet was originally intended as a backbone
for the operation of the office of the future. In such an
office, no unnecessary paper changes hands, and
paper is not used as the universal medium of
exchange between office equipment. Instead,after a
letter is typed and stored as data, that data could be
transferred to a printer, a copier, an overhead
projector, or to the archive file system.
To make such an office possible, it would be
necessary for all office equipment
manufacturers to absolutely agree upon data
formats and to make equipment that could be
pulled out of a box and instantly plugged into
the network anywhere and without difficult
up by specially trained
Compare to these ideals, Ethernet has not been
Ethernet II provides means of transmitting 10
Mbps data streams among large numbers of
computers distributed over a 2.5 km (1.5 mile)
Later revisions of the Ethernet scheme have
realized data speeds as high as 1 Gbps.
Next figure shows an Ethernet card and cabling
in the original ``daisy chain'‘ configuration.
Preamble: An 8
byte sequence that is always
the same. This acts as a flag that a receiving
system can watch for and synchronize itself
with, so that the body of the message can be
The preamble provides an important protocol
service: it stands out in a way that cannot be
mistaken for any other part of a message.
Every recipient knows what information to
expect and in what order immediately after
seeing the preamble.
Destination Address: A 6
byte address that uniquely
identifies the recipient of the datagram from all
recipients throughout the world. That's right: A 48
number is big enough that every computer can have
its own address without overlap.
Source Address: Another 6
byte address that identifies
the sender of the datagram.
Type Field: A 2
byte identifier of the type of handling
that this datagram should receive by the recipient.
There are groups that issue standards regarding how
this number is used. For example, a type value of
0800 Hex identifies an Internet Protocol packet as
would be generated by Internet
Data: From 46 to 1500 bytes of data intended for use
by the destination system. This would contain perhaps
a fragment of a picture being moved through the Web
or a piece of text from an e
Frame Check Sequence: A 4
byte number constructed
from processing of all the data that follows the
preamble in a certain way. The chances that the same
number would result from similarly processing an
altered version of this data are very, very, small. Thus,
the recipient can process the received data in the
same way and check its FCS calculation with the one
that was received. If they differ, the message can be
discarded as being in error.
The Ethernet addresses, which are the key to
datagram operation, are
uniquely issued and
permanently attached to Ethernet hardware
(the piece of electronics in your computer that
attaches to the network wire).
The IEEE was given the duty in January of 1986 to
issue these addresses, which they renamed
Organizationally Unique Identifiers (OUIs). Every
manufacturer of Ethernet equipment submits a
request to the IEEE, which grants the use of blocks of
these addresses. The manufacturer then installs each
address exactly once into each device manufactured.
Thus, no two Ethernet devices ever built, share the
same OUI. Hence, a datagram launched from one
machine to another cannot be misdelivered.
With a 48 bit address size, there are approximately
that is, about 20,000,000
addresses per person on the face of the Earth.
Ethernet Local Area Network technology is based
upon an efficient message delivery scheme (for small
collections of computers) in which all messages are
simply transmitted on a single shared cable and the
unique addresses (names) on the datagram alert the
How to send
Imagine 30 people names who may each occasionally
need to send information to another person from the
Suppose each person has a desk placed somewhere in
a large room, but we don't tell anyone where they
must sit and they occasionally change positions to get
better window views or to sit closer to the coffee
For this example, suppose the names of the people
are Alice, Bob,Carol, and so on. Now, suppose Dave
has a message for George. Dave could write a
telegram, write George on the outside of the
envelope, and hand it off to a delivery person who
circulates the room.
The delivery person has no idea where anyone will be
seated in general, so at each point in a delivery round,
he has to check the nameplate on the desk and look
through all the telegrams. Eventually every telegram
gets delivered, but the system is not exactly fast.
to our delivery problem:
Dave stands up and shouts ``George!'' and then
reads the message (Figure 19.5). George receives the
message instantly. Everyone else hears but ignores
the message. The seating arrangement had no impact
and all that mattered was that the names of the
people were unique.
Of course, this delivery approach would have a
problem if there were many more people in the room.
With enough people trying to send telegrams in this
way, at some point there will not be any empty time
between shouts to add any more telegrams.
The Ethernet is based on exactly the scheme just
described. All the computers in an Ethernet LAN are
connected to a single piece of cable (it may be broken
into smaller pieces for convenience, but these are
hooked up to act like a single piece of wire).
When one computer wishes to send a message
to another, it simply transmits the datagram by
causing a variation of the voltage on the
wire.This variation can be seen by all the
computers attached to that wire. It is as if the
computer shouted the telegram in a room (the
cable) occupied by all the computers.
Thus we see that the uniqueness of the OUI
addresses assigned to Ethernet hardware is
essential to the proper operation of this
We also see that the single cable that
connects all the computers is actually
being shared by those computers. Thus,
when a sufficient number of computers
are connected to a single LAN, the
available bandwidth begins to drop.
We also see here one reason that LANs
need to be kept relatively small in size.