Download Data Communication Network

333: DISCUSS THE FUNDAMENTALS OF
NETWORKING
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1. Discuss networking concepts (20 hrs)
2. Discuss hardware & software requirement
to setup a Local Area Network (20 hrs)
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PERFORMANCE STANDARD
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Objectives:
◦ Given a scenario, identify correctly the hardware and
services required for a Network
◦ Define a Node/Host
◦ Discuss functional & geographical relationship among
nodes
◦ Discuss services provided by a common carrier such as
 Leased line
 Switched line
◦ Describe switching techniques
 Circuit
 Message
 Packet
◦ Discuss ISO/OSl reference model
◦ Explain network communication protocols
Describe switching
techniques
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Networks are used to interconnect many devices.
We have checked with Local Area Networks.
Now, wide area networks
◦ Since the invention of the telephone, circuit switching has
been the dominant technology for voice communications.
◦ Since 1970, packet switching has evolved substantially for
digital data communications. It was designed to provide a
more efficient facility than circuit switching for bursty data
traffic.
 Two types of packet switching:
 Datagram (such as today’s Internet)
 Virtual circuit (such as Frame Relay, ATM)
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The figure shows that if the data from station
A intended for station F are sent to node 4.
They may then be routed via nodes 5 and 6
or nodes 7 and 6 to the destination.
Some nodes connect only to other nodes
(e.g., 5 and 7).
Their sole task is the internal (to the network)
switching of data. Other nodes have one or
more stations attached as well.
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Node-station links are generally dedicated pointto-point links.
Node-node links are usually multiplexed, using
either frequency division multiplexing (FDM) or
time division multiplexing (TDM).
Usually, the network is not fully connected; that
is, there is not a direct link between every
possible pair of nodes.
However, it is always desirable to have more than
one possible path through the network for each
pair of stations.
This enhances the reliability of the network.
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In large networks there might be multiple
paths
linking
sender
and
receiver.
Information may be switched as it travels
through various communication channels.
There are three typical switching techniques
available for digital traffic.
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Circuit Switching
Message Switching
Packet Switching
Switching Techniques
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Circuit switching is a technique that directly connects
the sender and the receiver in an unbroken path.
Telephone switching equipment, for example,
establishes a path that connects the caller's telephone
to the receiver's telephone by making a physical
connection.
With this type of switching technique, once a
connection is established, a dedicated path exists
between both ends until the connection is terminated.
Routing decisions must be made when the circuit is
first established, but there are no decisions made
after that time
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Circuit switching in a network operates almost
the same way as the telephone system works.
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A complete end-to-end path must exist before
communication can take place.
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The computer initiating the data transfer must
ask for a connection to the destination.
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Once the connection has been initiated and
completed to the destination device, the
destination device must acknowledge that it is
ready and willing to carry on a transfer.
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Circuit switching:
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Communication via circuit switching has three phases:
◦ There is a dedicated communication path between two stations
(end-to-end)
◦ The path is a connected sequence of links between network
nodes. On each physical link, a logical channel is dedicated to
the connection.
◦ Circuit establishment (link by link)
 Routing & resource allocation (FDM or TDM)
◦ Data transfer
◦ Circuit disconnect
 Deallocate the dedicated resources
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The switches must know how to find the route to the
destination and how to allocate bandwidth (channel) to
establish a connection.
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Inefficiency
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Delay
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Developed for voice traffic (public telephone
network) but can also applied to data traffic.
◦ Channel capacity is dedicated for the whole duration of a
connection
◦ If no data, capacity is wasted
◦ Long initial delay: circuit establishment takes time
◦ Low data delay: after the circuit establishment, information
is transmitted at a fixed data rate with no delay other than
the propagation delay. The delay at each node is negligible.
◦ For voice connections, the resulting circuit will enjoy a high
percentage of utilization because most of the time one
party or the other is talking.
◦ But how about data connections?
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Advantages:
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Disadvantages:
◦ The communication channel (once established) is
dedicated.
◦ Possible long wait to establish a connection, (10
seconds,
◦ more on long- distance or international calls.) during
which no data can be transmitted.
◦ More expensive than any other switching techniques,
because a dedicated path is required for each
connection.
◦ Inefficient use of the communication channel, because
the channel is not used when the connected systems are
not using it.
Subscribers: the devices that attach to the network.
Subscriber loop: the link between the subscriber and the network.
Exchanges: the switching centers in the network.
End office: the switching center that directly supports subscribers.
Trunks: the branches between exchanges. They carry multiple voicefrequency circuits using either FDM or synchronous TDM.
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A circuit-switched network consists of a set of
switches connected by physical links. A connection
between two stations is a dedicated path made of one
or more links. However, each connection uses only
one dedicated channel on each link. Each link is
normally divided into n channels by using FDM or
TDM.
Note
A circuit-switched network is made of a
set of switches connected by physical
links, in which each link is divided
into n channels
A trivial circuit-switched network
Note
In circuit switching, the resources need
to be reserved during the setup phase;
the resources remain dedicated for the
entire duration of data transfer until the
teardown phase.
Example 8.1
As a trivial example, let us use a circuit-switched
network to connect eight telephones in a small area.
Communication is through 4-kHz voice channels. We
assume that each link uses FDM to connect a maximum
of two voice channels. The bandwidth of each link is
then 8 kHz. Figure 8.4 shows the situation. Telephone 1
is connected to telephone 7; 2 to 5; 3 to 8; and 4 to 6. Of
course the situation may change when new connections
are made. The switch controls the connections.
Figure 8.4 Circuit-switched network used in Example 8.1
8.22
Example 8.2
As another example, consider a circuit-switched
network that connects computers in two remote offices
of a private company. The offices are connected using a
T-1 line leased from a communication service provider.
There are two 4 × 8 (4 inputs and 8 outputs) switches in
this network. For each switch, four output ports are
folded into the input ports to allow communication
between computers in the same office. Four other output
ports allow communication between the two offices.
Figure 8.5 shows the situation.
Figure 8.5 Circuit-switched network used in Example 8.2
Figure 8.6 Delay in a circuit-switched network
8.25
Note
Switching at the physical layer in the
traditional telephone network uses
the circuit-switching approach.
8.26
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Problem of circuit switching
◦ designed for voice service
◦ Resources dedicated to a particular call
◦ For data transmission, much of the time
the connection is idle (say, web browsing)
◦ Data rate is fixed
◦ Both ends must operate at the same rate during
the entire period of connection
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Packet switching is designed to address
these problems.
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With message switching there is no need to
establish a dedicated path between two
stations.
When a station sends a message, the
destination address is appended to the
message.
The message is then transmitted through the
network, in its entirety, from node to node.
Each node receives the entire message, stores
it in its entirety on disk, and then transmits the
message to the next node.
This type of network is called a store-andforward network.
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A message-switching node is typically a
general-purpose computer. The device needs
sufficient secondary-storage capacity to store
the incoming messages, which could be long.
A time delay is introduced using this type of
scheme due to store- and-forward time, plus
the time required to find the next node in the
transmission path.
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Advantages:
◦ Channel efficiency can be greater compared to
circuit-switched systems, because more devices are
sharing the channel.
◦ Traffic congestion can be reduced, because
messages may be temporarily stored in route.
◦ Message priorities can be established due to storeand-forward technique.
◦ Message broadcasting can be achieved with the use
of broadcast address appended in the message.
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Disadvantages
◦ Message switching is not compatible with
interactive applications.
◦ Store-and-forward devices are expensive, because
they must have large disks to hold potentially long
messages.
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Packet switching can be seen as a solution
that tries to combine the advantages of
message and circuit switching and to
minimize the disadvantages of both.
There are two methods of packet switching:
Datagram and virtual circuit.
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Data are transmitted in short packets
◦ Typically at the order of 1000 bytes
◦ Longer messages are split into series of packets
◦ Each packet contains a portion of user data plus some
control info
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Control info contains at least
◦ Routing (addressing) info, so as to be routed to the
intended destination
◦ Recall the content of an IP header!
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store and forward
◦ On each switching node, packets are received, stored
briefly (buffered) and passed on to the next node.
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Line efficiency
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Data rate conversion
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◦ Single node-to-node link can be dynamically shared by
many packets over time
◦ Packets are queued up and transmitted as fast as possible
◦ Each station connects to the local node at its own speed
In circuit-switching, a connection could be blocked
if there lacks free resources. On a packet-switching
network, even with heavy traffic, packets are still
accepted, by delivery delay increases.
Priorities can be used
◦ On each node, packets with higher priority can be
forwarded first. They will experience less delay than lowerpriority packets.
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A station breaks long message into packets
Packets are sent out to the network
sequentially, one at a time
How will the network handle this stream of
packets as it attempts to route them through
the network and deliver them to the intended
destination?
◦ Two approaches
 Datagram approach
 Virtual circuit approach
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Each packet is treated independently, with no
reference to packets that have gone before.
◦ Each node chooses the next node on a packet’s
path.
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Packets can take any possible route.
Packets may arrive at the receiver out of
order.
Packets may go missing.
It is up to the receiver to re-order packets
and recover from missing packets.
Example: Internet
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In virtual circuit, a preplanned route is
established before any packets are sent, then
all packets follow the same route.
Each packet contains a virtual circuit
identifier instead of destination address, and
each node on the preestablished route knows
where to forward such packets.
◦ The node need not make a routing decision for
each packet.
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Example: X.25, Frame Relay, ATM
A route between stations is set
up prior to data transfer.
All the data packets then
follow the same route.
But there is no dedicated
resources reserved for the
virtual circuit! Packets need to
be stored-and-forwarded.
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Virtual circuits
◦ Network can provide sequencing (packets arrive at the
same order) and error control (retransmission between two
nodes).
◦ Packets are forwarded more quickly
 Based on the virtual circuit identifier
 No routing decisions to make
◦ Less reliable
 If a node fails, all virtual circuits that pass through that node fail.
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Datagram
◦ No call setup phase
 Good for bursty data, such as Web applications
◦ More flexible
 If a node fails, packets may find an alternate route
 Routing can be used to avoid congested parts of the network
Comparison of
communicatio
n switching
techniques
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The size of the packet can vary from 180 bits,
the size for the Datakit® virtual circuit switch
designed by Bell Labs for communications
and business applications; to 1,024 or 2,048
bits for the 1PSS® switch, also designed by
Bell Labs for public data networking; to 53
bytes for ATM switching, such as Lucent
Technologies' packet switches.
Question?