It defines mechanical, electrical & timing interfaces to the network. Switching is another important task of physical layer. Two switching techniques are used. They are circuit switching & packet switching. Multiplexing is the process in which two or more signals are combined for transmission over a single communications path.
Multiplexing schemes, like TOM, FDM are developed by telephone companies to reduce the cost.
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Network Topologies
Topology is a term used to describe the way in which computers are connected in network.
The physical topology of a network refers to the configuration of cables, computers, and other peripherals.
Network Topologies are logical layouts of the network. The term "logical" used here marks a great significant.
That means network topologies depends not on the "physical" layout of the network. No matter that computer on a network are placed in a linear format, but if they connected via a hub they are forming a Star topology, not the Bus Topology.
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Network Topologies
Linear Bus Topology :
The type of network topology in which all of the nodes of the network are connected to a common transmission medium which has exactly two endpoints is called the 'bus'.
It consists a single main cable connects each node.
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Network Topologies
Ring Topology : The type of network topology in which each of the nodes of the network is connected to two other nodes in the network and with the first and last nodes being connected to each other, forming a ring.
That is the nodes are connected in a circle using cable segments. Each node is physically connected only to two others.
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Network Topologies
Star Topology : A star topology is designed with each node connected directly to a central network hub or concentrator. Data on a star network passes through the hub or concentrator before continuing to its destination.
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Network Topologies
Tree Topology : A tree topology combines characteristics of linear bus and star
topologies. Tree topologies allow for the expansion of an existing network, and enable schools to configure a network to meet their needs.
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Network Topologies
Topology Comparisons :
There are a number of factors to consider in making a choice of a topology. Feature Reliability Complexity Flexibility Expandability Cost Bus High Moderate High High Low Ring Low Low Moderate Moderate Moderate Star Low Low Low Low Moderate
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Switching
What is the purpose of switching ?
Communication is typically achieved by transmitting data from source to destination through a network of intermediate switching nodes. The switching nodes are not concerned with the content of data. Rather their purpose is to provide a switching facility that will move the data from node to node until it reaches the destination.
There are two types of switching 1. 2. 3. Circuit switching Message switching Packet switching
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Switching
1. Circuit switching :
A circuit switching network is one that establishes a dedicated circuit (or channel) between nodes and terminals before the users may
communicate.
Circuit switching is used for ordinary telephone calls. Communication using circuit switching involves three phases 1. Connection establishment: Before any signal can be transmitted, an end to end circuit must be established. 2. Data transfer: Information can now be transmitted from source through the network to the destination using the dedicated path established.
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Switching
Circuit switching : ( continued«)
2. Data transfer: Information can now be transmitted from source through the network to the destination using the dedicated path established.
3. Termination: After some period of data transfer, the connection is terminated
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Switching
2. Message switching :
Message switching was the precursor of packet switching, where messages were routed in their entirety and one hop at a time. Hop-by-hop Telex forwarding are examples of message switching systems. E-mail is another example of a message switching system.
When this form of switching is used, no physical path is established in advance in between sender and receiver. Instead, when the sender has a block of data to be sent, it is stored in the first switching office (i.e. router) then forwarded later at one hop at a time.
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Switching
Message switching :
As the figure indicates, a complete message is sent from node A to node B when the link interconnecting them becomes available. The message is stored at B until the next link becomes available, with another queuing delay before it can be forwarded. It repeats this process until it reaches its destination.
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Switching
3. Packet switching :
Packet switching splits traffic data (for instance, digital representation of sound, or computer data) into chunks, called packets.
Packet switching is similar to message switching. Any message exceeding a network-defined maximum length is broken up into shorter units, known as packets, for transmission.
Packet switching is used to optimize the use of the channel capacity available in a network, to minimize the transmission latency and to increase robustness of communication.
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Switching
Packet switching : ( continued«)
The most well-known use of packet switching is the Internet.
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Switching
Comparison of Communication Switching Techniques :
Sl No
Circuit Switching
Datagram Packet Switching
Virtual Circuit Packet Switching
1. Dedicated transmission path 2. Continuous transmission of data 3. Message are not stored 4. Fixed bandwidth 5. No overhead bits after call setup.
No dedicated path Transmission of packets
No dedicated path Transmission of packets
Packets may be stored until Delivered. Packets stored until delivered Dynamic use of bandwidth Overhead bits in each packet Dynamic use of bandwidth Overload bits in each packet
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Multiplexing
What is Multiplexing ?
Multiplexing is the process in which two or more signals are combined for transmission over a single communications path.
Multiplexing has made communications very economical by transmitting thousands of independent sig-nals over a single transmission line.
There are three predominant ways to multiplex: 1. 2. 3. Fre-quency Division Multiplexing (FDM) Wavelength Division Multiplexing (WDM). Time Division Multiplexing (TDM)
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Multiplexing
1.
Fre-quency Division Multiplexing (FDM):
Frequency Division Multiplexing. (FDM) is predominantly used in analog communica-tions.
In the FDM, modulated carrier frequencies are combined for transmission over a single line by a multiplexer (MUX). There is always some unused frequency range between channels, known as guard band.
At the receiving end of the communications link, a demultiplexer (DEMUX) separates the channels by their fre-quency and routes them to the proper end users.
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Multiplexing
Fig : Transmitting end of an FDM system
FDM was the first multiplexing scheme to enjoy wide-scale network deployment. FDM is widely used in FM stereo broadcast.
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Multiplexing
2. Wavelength Division Multiplexing (WDM) :
This is a form of frequency-division multiplexing (FDM) but it is commonly called wavelength-division multiplexing (WDM). With WDM,
the light streaming through the fiber consists of many colors, or wavelengths, each carrying a separate channel of data.
Wavelength Division Multiplexing (WDM) is a cost-effective way to increase the capacity of fiber optic communications.
The key elements of a WDM optical system are tunable semiconductor lasers, electro-optical modulators, multiplexing components, single-mode optical Figures and optical amplifiers.
While FDM has been used to great advantage in increasing system capacity, the use of TDM offers even greater system improvements.
TDM is protocol insensitive and is capable of combining various protocols and different types of signals, such as voice and data, onto a single highspeed transmission link.
It is more efficient than FDM, as there is no need for guard bands.
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Multiplexing
Time division multiplexing (TDM) : (continued..)
The main disadvantages of TDM are the greater complexity of digital systems and the greater transmission bandwidth required.
In order to use TDM, the transmission must be digital in nature so an essential component of TDM is the process of sampling the analog signal in time.
In order to transmit telephone conversations, speech, which is an analog signal, is con-verted to a digital signal, transmitted, and then reconverted into analog at the receiving telephone.
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Summary
Topology is a term used to describe the way in which computers are connected in network. The physical topology of a network refers to the configuration of cables, computers, and other peripherals. Different network topologies are Bus, Ring, Star and Tree The purpose of switch is provide a switching facility that will move the data from node to node until it reaches the destination. Different switching. Multiplexing is the process in which two or more signals are combined for transmission over a single communications path. switching technologies are circuit, message and packet