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Types of Switching
 Switching is a process of moving the data through a
series of intermediate steps rather than moving from
the start point directly to the end point.

Switching

Circuit Switching Message Switching Packet Switching

Circuit Switching Technique
• Direct physical connection established between sender and
receiver before data transmission
• Allows a fixed rate of transmission
• Major drawbacks are:
– Unused transmission is wasted
– Unused bandwidth cannot be used by any other
transmission

Message Switching
• No direct physical connection established between
sender and receiver
• Message routed through the intermediate nodes
• This technology is also known as store and forward
method

Packet Switching
• Message to be transmitted broken into units called
packets
• Packet contains addressing information
• Packets are carried on virtual circuits. Virtual circuits are
temporary connections over which the sending and
receiving stations communicate

Connectionless Services
• Before transmitting the packets, actual connection not
established between the sender and the receiver
• Each packet considered as an independent unit
• Each packet treated as a complete message
• Packets follow different routes to reach destination

Connection-oriented Services
• Before transmitting packets, communication link
established
• Packets follow same route to reach destination
• For transmission, uses either Switched Virtual Circuit
(SVC) or Permanent Virtual Circuit (PVC)

Public Switched Telephone Network
• Telephone system that uses copper wires to carry analog
voice data is called Public Switched Telephone Network
(PSTN)
• Telephone services carried by the PSTN are often called as
Plain Old Telephone Service (POTS).
• Services offered by POTS are follows:
– Bi-directional – full duplex
– Ringing signals and dial-tone
– Operator services
– Conference calling assistance
Uses electromechanical switches and now a days it has been
made digital

Structure of Telephone System – I
• Digital transmission is more reliable than analog and less
prone to noise and interference and also it is cheaper
and easier to maintain
Telephone system consists of following major components:
– Local loops – Analog twisted pairs going into houses and
businesses
– Switching offices – Place where the calls moved from one
trunk to another
– Trunks – Switching offices connected using digital fiber
optics called trunks

Structure of Telephone System – II

• Typical circuit route for a medium-distance call:

Structure of Telephone System – III and Local
Loops
• Switching center is known as a toll office
• Different tolls communicate with each other
using the high bandwidth intertoll trunks

Local loop is sometimes referred to as last
mile
Uses analog signaling

Multiplexing
• Link is the physical path between sender and
receiver whereas channel is the portion of a
link that carries a transmission
• Three types of multiplexing:
– Time Division Multiplexing (TDM) – used for digital
data
– Frequency Division Multiplexing (FDM) –
– Wavelength Division Multiplexing (WDM) – used
for optical carrier signals

Mobile Telephone System
• Passed through three stages:
First Generation (Analog voice) – known as cellular
mobile radio telephone
used in US
advanced mobile phone system (AMPS) was launched
commercially
Second Generation (Digital Voice) – Uses FDMA, TDMA and
CDMA technologies
Third Generation (Digital Voice and Data) – intended for
true multimedia cell phone called as smart phones
Data Transfer Rate (DTR) is 3 Mbps

Point to Point WANs
• Two remote devices connected using a line available
from a public network
• Public network can be a telephone network
• Services basically provided at the physical layer
• Users responsible for the data link layer protocols

Physical Layer
• To accomplish point-to-point connection between two
devices at the physical layer, use the following services:
– Modem technology (56K Modem)
– Digital Subscriber Line (DSL) – uses existing
telecommunication networks to achieve high speed
bandwidth
– Cable modem
– T-line (digital) – T1 – 1.544 Mbps T3 – 44.736 Mbps
– E1 Lines (digital) – 2.048 Mbps
– SONET – 51.84 Mbps (ANSI)

Data Link Layer
• Concerned with data transfer
• Protocol needed for reliable connection
• A protocol is needed at this layer to have a reliable point-
to-point connection. For reliability, uses Point-to-Point
Protocol (PPP)
• For establishing, maintaining and terminating the link,
uses Link Control Protocol (LCP)
• For providing flexibility to PPP, uses Network Control
Protocol (NCP)

X.25 Protocol
• Connection-oriented packet-switching protocol at the
network layer
• Defines the way in which the connections between the
user devices and the network devices are established
and maintained.
• Used in the packet-switched networks (PSNs) such as the
telephone companies.

X.25 Devices - I
• X.25 network devices are:
– Data terminal equipment (DTE)
– Data circuit-terminating equipment (DCE)
– Packet-switching exchange (PSE)

X.25 Devices - II
• X.25 Session establishment – a full duplex
communication is established between two
devices and can be terminated by either of the
two devices
• Packet Assembler/Disassembler (PAD) – main
function is buffering and adds an X.25 header
to the packet

X.25 Virtual Circuits - I
• X.25 virtual circuits are:
– Switched Virtual Circuits (SVCs)
– Permanent Virtual Circuits (PVCs)

X.25 Virtual Circuits - II
• Multiple virtual circuits also called as logical
connectors
• Source DTE devices specifies the virtual
circuits to be used in the headers of the
outgoing data packets

Show from CBT Frame Relay
• High performance WAN protocol, faster than X.25
• Operates at the physical and data link layer of the OSI
reference model
• Does not involve error correction and network flow
control operations
• Frame relay virtual circuits are:
– Switched Virtual Circuits (SVCs)
– Permanent Virtual Circuits (PVCs)

Frame Relay Devices
• The devices attached to the Frame Relay WAN for
transmission of data packets are, Data terminal
equipment (DTE) and Data circuit-terminating
equipment (DCE).

Asynchronous Transfer Mode (ATM)
• Cell-switching and multiplexing technology which
combines the benefits of both circuit switching and
packet switching
• Transmits data, voice and video signals simultaneously
over the same communication lines
• Before transmission, this information is converted into
fixed size cells
• Cell consists of 53 octets or bytes
• Header information is contained in the first 5 bytes and
48 bytes contain user information

ATM Devices
• ATM network consists of two devices:
– ATM Switch
– ATM endpoint

ATM Network Interface
• Uses two types of interfaces for
interconnection
• User Network Interface (UNI)
• Network to Network Interface (NNI)
• The UNI and NNI are classified on the basis of
whether the switch is owned and located at
the customer’s premises or are publicly owned
and operated by the telephone company

ATM Services

ATM Services

Permanent Switched
Virtual Virtual Connectionless
Circuits Circuits Service
(PVC) (SVC)

ATM Reference Model
• Reference model comprises of the following:
– ATM layers
– ATM planes

Integrated Services Digital Network (ISDN)

• Involves digital telephony and data-transport services
offered by the regional telephone carriers
• Permits transmission of voice, data, text, graphics, music
and video over existing telephone lines
• Consists of following devices:
– Terminals
– Terminal Adapters
– Network Termination Devices

ISDN Services
• The information transmitted over the ISDN network travels
through the three logical digital communication channels:
– B-Channel - Carries user service information that includes
digital data, video and voice. It is the basic user channel
and operates at 64 kbps
– D-Channel - Carries signals and data between the user and
the network.
– H-Channel - Performs the same functions as that of the B-
Channels and operates at data rate of 64 Kbps.
– ISDN BRI service – operates 192 kbps
– ISDN PRI service – operates at 1.544 mbps

Broadband ISDN (BISDN)
• Extension of ISDN
• Used for technologies such as video conferencing and file
transfer and operates at 600 Mbps
• Provides two types of services as follows:
– Interactive Services
– Distributive Services

Fiber Distributed Data Interface
• Based on ring topology and token passing
• Two optical fibers used as follows:
– Multimode optical fiber
– Single-mode optical fiber
• Two types of copper cables used as follows:
– Category 5 Unshielded Twisted Pair copper wiring
– IBM Type 1 Shielded Twisted Pair copper wiring

Satellite Communication
• Two parts of satellite communication are:
– Uplink – Transmitter consisting of a ground-based part
– Transponder – The satellite-based part reflecting signals
towards receivers
• Advantages:
– Satellites can cover large areas of
earth
–I t is commercially attractive
– It is preferred instead of cables as maintenance of
cables is expensive and difficult

Polling
• Communication technique which determines when a
terminal is ready to send data
• Round robin sequence is used by the computer to
continuously interrogate its connected terminals
• Communication system comprises of the following:
– A Master station
– Number of slave stations each communicating with the
master station
– A two-way transmission line connecting the master station
and the slave stations

ALOHA
• A simple communications scheme in which each
transmitter or source in a network sends data whenever
there is a frame to send is called as ALOHA
• Next frame sent only if the first frame reaches the
destination successfully
• If the frame fails to reach the receiver, it is sent again
• Types of ALOHA:
– Pure aloha
– Slotted aloha

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