Data Communication Networks

Packet switched nets are store and forward nets.

ckt switched nets have dedicated communication paths (i.e. telephone)

For example:

A Broadcast Comm. Net can be WAN, the media is unshared:

LAN Examples:

Ethernet with a bus topology:

Ring topology net:

Note: no switching nodes, media is shared in LAN

For packet radio, every station can hear every other, the media is shared so this is a LAN technology.

This course addresses the communication systems parts between the input and output devices.

Computer Communications Architecture

The Three Layer Model

Application run on computer which use networks

network layer - xchange of data between computer and network

transport layer - error detection and correction

application layer - file xfer etc.

File xfer modules

xmits passwords

file commands

file records

communications service module

error correction

encryption

keeps track of data to ensure delivery

Network Access Module

net specific stuff

(SAP) service access points 1 2 and 3 are used to uniquely identify the application communicating

Network access layer is told which computer but not which SAP

The transport layer will break data into chunks adds control information and this is called a PDU or protocol data unit

Protocol Data Units

TPDU1 = transport protocol data unit :=
transport hdr, chunk1

TPDU2 = transport hdr, chunk2

NPDU = network protocol data unit = network hdr, TPDU

Application data = chunk1 + chunk2 + ...

Destination SAP (service access points)

TPDU has destination SAP so that appropriate application obtains data

Sequence # TPDU's may be sent in any order, and by different routes and may arrive at different times.

TPDU may have error detection or correction codes built-in

NPDU

destination address (i.e. IP address)

facilities requires (i.e. priority)

Transmission Terminology

point-to-point :- guided transmission is unshared

I.e. O(N**2) connectivity (completely connected)

= number of links

Multipoint guided transmission allows shared medium

ANSI Defs:

simplex - one direction only

half-duplex - two-way but not at the same time.

full-duplex - simultaneous two way transmission

CCITT

CCITT vs ANSI

simplex = half-duplex

duplex = full-duplex

CCITT = international telegraph and telephone consultive committee

Analog and Digital Data Transmission

data - entities which convey meaning

signals - encoding of data

signaling - act of sending a signal

transmission - communication of data by propagation and processing of signals

digital data - takes on discrete values, i.e. text integers.

analog data - takes on continuously varying patterns of intensity, i.e.. sound

But what is digital? What is Analog? How do we defines these things?

s(t) is continuous (i.e. analog) iff

for all a

A signal, s(t) is discrete iff

for some a

For example, the function s(t) has multiple values or does not exist and there fore the limit does not exist.

Let the function f be defined on the set S. f is continuous at a point P0 of S iff

whenever .

If f is defined on an interval [a, b] and is continuous at then given an

Data and Signals

Modem (modulator/demodulator)

modulation involves two waveforms: The modulating signal (the message) and the carrier wave which is altered by the modulating signal

The complementary process is called demodulation

Bits <-> modem <-> analog signal <-> modem <-> bits

Codec := coder-decoder

analog <-> codec <-> bits

In General a digital waveform has infinite bandwidth.

Band limited channels low-pass filter square waves, this rounds their corners.

Rule of thumb: if a signal is W bps, you may need 2W Hz bandwidth.

Sigal Strength

attenuation := a loss of signal strength

signal strength falls off logarithmically so the decibel is used to measure the loss

Decibel gains and losses are computed via simple addition and subtraction.

Let number of decibels

Example:

loss=10log(5/10)=-3 dB

dBw=decibel watt

About Those Exponents...

The Laws of Exponents give us the laws of logarithms