Please enable JavaScript.
Coggle requires JavaScript to display documents.
Networks & Data Communications_1 (Channel Characteristics (Data…
Networks & Data Communications_1
Data Communications
Terminology
Protocols
establish accurate & appropriate meaning to the messages that are understood by both senders & receivers
Physical connection
that is
independent of the messaging
Message sharing "Connection"
btwn applications at the
senders & the receiver ends
Physical connection with signalling
that represents the messages being
transported btwn the sender & the receiver
Communications channel
that can capably
& reliably transport messages
Examples
POTS - Plain Old Telephone Service;
communication btwn two end users
Web servers & web browsers
-HTTP format
Data
- messages to be shared btwn sender & receiver
Model of a communication channel
Messages
We assume that the message
is represented digitally
Data is described as a
byte stream
because communications
are predominately serial
They can take many forms such as
data, a program, a file, or multimedia
Limitation of using message as a
communication tool is the varying
message length
-Long messages could tie up a
communication channel indefinitely,
creating problems for other messages
that share that channel
Communication btwn cooperating
applications at each end node
Packet Header
Provides a description of the packet;
also known as the preamble
Contains
Destination address of the receiver
Source address of the sender
Description of the packet
Info about the data being sent
Packets
Consist of data encapsulated by
the packet header, which contains
information
about the packet
Used to solve problems of channel
availability & maximum unitilisation
Can take turns using the channel,
allowing
sharing
of the channel
for different messages
Equivalent to an envelope
that contains pages of data
A group of related units that
make up a single message;
these units are called
packets
Sometimes referred to as a
frame
or
datagram
, which
identifies their purpose
Advantages
of Packets
Packets from several sources
can share a single channel
Each sender/receiver pair
appears
to have a channel to itself
Offer an alternative to dedicating
a channel for the entire
length of the message
Receiving computer can process
an entire block of data instead
of a character or byte at a time
Represent a reasonable unit
for
routing
of data
Simplifies synchronisation of the
sending & receiving systems
by providing clear start & stop points
Simplifies operations &
incr communications efficiency
Channel Characteristics
A multi-Link Channel
Data transmitted
directionally
Half-duplex
- messages are
carried in both directions
but only one direction at a time
Full duplex
- messages are
simultaneously carried in both directions
Simplex
- messages are
carried only in one direction
Medium
Guided - communications limited
to a specific path constrained to
a
cable
of some sort
Can be either electrical or optical;
includes various forms of
wire & fibre-optic cables
Unguided - communications not
limited to a specific path
Radio waves transmitted from
an antenna are unguided
Also includes cellular phone,
broadcast radio, wireless networking
Number of
connections
A communication channel can
be
point-to-point
or
multipoint
Choice is decided by
the nature of medium
Bandwidth
It is the bit rate of overall channel
Rate at which the data
can be moved successfully
through the channel
Point-to-point
-Fibre optics are usually point-to-point;
difficulty of tapping into a fibre-optic cable
Links
Intermediate nodes btwn
the links forward packets
from one link to the next
Interfaces at each end
of the connection may be different
The communication channel
is divided into segments called links
Multipoint
-Wireless networking is multipoint by necessity
The
communication channel
May include intermediate nodes
that forward packets to the next node
Interfaces at each end of
the connection may be different
Provides the path for
the message btwn two
communicating end nodes
End node interfaces
Wired or wireless ethernet
Bluetooth, WiMax, DSL, cable,
satellite link, telephone modem
Protocols
allow globally interconnected
data communications capability
TCP/IP & Ethernet
Network Topology
Bus Topology
Similar to
multipoint buses
Each node is tapped into
the bus along the bus
To communicate, each node
"broadcasts" a message that
travels along the bus
Every node on the bus receives the
message but it is ignored by all nodes
except the one whose node matches the
delivery address in the message
Transmission from any station travels
entire medium (both directions)
Termination required at ends of bus
to prevent the signal from echoing
Branches can be added to a bus,
expanding it into a tree but messages
are still broadcast throughout the entire tree
Bus Network Implementation
Traffic congestion
is a major issue
Rarely used in designs of new
networks except for wireless
Only requires a single
pair of wires from one end
of the network space to the other
Easiest to wire of the network topologies
Low cost
Because of the unguided nature
of radio waves, wireless networks
require some form of bus topology
Mesh Topology
Failure of an indiv intermediate node
will slow but not stop the network as
long as an alternative path is available
Large networks that use
switches
&
routers
are typically partial
mesh networks
Multiple paths btwn end nodes
Full mesh network
Impractical
due to large number
of connections needed
Number of connections
= nodes x (nodes - 1) / 2
Direct point-to-point channel
connecting every pair of nodes
500 computer nodes would require
125,000 interconnecting cables!
Star topology
Switching in the central device connects pairs of nodes together to allow them to communicate directly
Central device can steer data from one node to another as required
Nodes communicate through the central device
Most modern switches allow multiple pairs to communicate simultaneously
All nodes are connected point-to-point to a central device
Failure of central device causes
entire network
to go down
Primarily used for
local area networks
& sometimes used to connect satellite offices to a central office
NT
Affects availability, speed & traffic
congestion of the network
Local topology - operational relationship
btwn the various network components
Describes the path or paths btwn
any two points in the network
Physical topology - actual layout of
the network wiring
Describes the fundamental layout
or configuration of a network
Ring Topology
Last node is connected back to the first to form a closed ring
Packets are placed on the loop at a node, & travel from node to node until the desired node is reached
Point-to-point connection from each node to the next
Although the ring is inherently unidirectional, it is possible to build a bidirectional ring network
Popular in the past because they provided a controlled way in which to guarantee network performance (legacy token-ring bidirectional networks)
Used in some FDDI fiber optic backbone & metropolitan area networks