topic 6 data network (part 1)
DESCRIPTION
Data Network (Part 1)TRANSCRIPT
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DEPARTMENT OF
ELECTRICAL
ENGINEERING
EP601 DATA
COMMUNICATION
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At the end of this learning session, student must be able to;
Define :
a. Data Network
b. Value Added Network (VAN)
c. Packet Switching Network
Identify types of common switching network:
a. Circuit switching
b. Message switching
c. Packet switching
Compare among circuit switching, message switching, packet switching network
Categorize packet switching methods:
a. Datagram
b. Virtual circuit
Explain protocol used in packet switched services:
a. X.25
b. Frame relay
C. Asynchronous transfer mode (ATM)
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Definition:
A data network is an electronic communications process
that allows for the orderly transmission and receptive of
data, such as letters, spreadsheets, and other types of
documents.
What sets the data network apart from other forms of
communication, such as an audio network, is that the data
network is configured to transmit data only.
This is in contrast to the audio or voice network, which is
often employed for both voice communications and the
transmission of data such as a facsimile transmission.
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Definition:
A value-added network adds value to the services or facilities provided by a common carrier to provide new types of
communication services.
Examples of added values are error control, enhanced connection
reliability, dynamic routing, failure protection, logical multiplexing
and data format conversion.
Examples of value-added networks are GTE Telnet, DATAPAC,
TRANSPAC and Tymnet Inc.
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Definition :
A digital communication network which operates by dividing
each piece of information to be sent into discrete packets.
These packets are then sent individually across the network
and reassembled, in order, at the information's destination.
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.
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Data is transmitted as stream of bits, no packetizing is needed.
Circuit switching:
o There is a dedicated communication path between two stations (end-to-end)
o The path is a connected sequence of links between network nodes. On each physical link, a logical channel is dedicated to the connection.
Communication via circuit switching has three phases:
Circuit establishment (link by link)
Routing & resource allocation (FDM or TDM) Data transfer
Circuit disconnect
Deallocate the dedicated resources
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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Example of circuit switching network to connect
eight telephones in a area.
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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 voice-
frequency circuits using either FDM or synchronous TDM.
Normal telephone service is based on a circuit-switching technology, in which a dedicated line is allocated for transmission between two parties.
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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-and-forward network.
Application : Mail Delivery
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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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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.
A station breaks long message into packets. Packets are sent out to the network sequentially, one at a time (individually) and can even follow different routes to its destination. Once all the packets forming a message arrive at the destination, they are recompiled into the original message.
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
store and forward On each switching node, packets are received, stored briefly (buffered) and
passed on to the next node.
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Most modern Wide Area Network (WAN) protocols, including TCP/IP, X.25, and Frame Relay, are based on packet-switching technologies.
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No Circuit switching Message switching Packet switching
1 dedicated transmission
path
no dedicated transmission
path
no dedicated transmission
path
2 continuous transmission
of data
transmission of messages transmission of packets
3 operates in real time not real time near real time
4 messages not stored messages stored messages held for short
time
5 path established for entire
message
route established for each
message
route established for each
packet
6 call setup delay message transmission
delay
packet transmission delay
7 blocking may occur blocking cannot occur blocking cannot occur
8 no speed or code
conversion
speed or code conversion speed or code conversion
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There are two approaches:
Datagram Virtual circuit
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Datagram - Connectionless service :No handshaking, each packet is sent and routed independently and can follow different paths to reach to the destination. The full address of the source and destination must be attached to each packet.
o No setup delay
o Packets are not guaranteed to arrive in the order they were sent
o Robust: If a router crashes only packets inside the router will be lost, other packets can follow other path
It is up to the receiver to re-order packets and recover from missing packets.
Example: Internet
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Virtual circuit - Connection-oriented : A connection
(handshaking) between the sender and the receiver is
established and the complete path for the packets to reach
to the destination is determined before transmission of
any packets. This path is called virtual circuit or a
connection and the address given for each packet is the
sequence number of the virtual circuit called Virtual
Circuit Identifier (VCI)
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o No dedicated path: the path can be used by other virtual
circuits
o Data is packetizied before transmission
o Packets are guaranteed to arrive in the order they were
sent
o Packets are logically connected to each other, packets travel
one after the other
o The virtual circuit has to be terminated after all packets of
a message have been arrived
o If the virtual circuit router crashes all virtual circuits that
go through the router are terminated and paths are lost
o Used in WAN (Frame relay, ATM)
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Virtual circuits
o Network can provide sequencing (packets arrive at the same order) and error control (retransmission between two nodes).
o Packets are forwarded more quickly
Based on the virtual circuit identifier
No routing decisions to make
o Less reliable
If a node fails, all virtual circuits that pass through that node fail.
Datagram
o No call setup phase
Good for bursty data, such as Web applications
o More flexible
If a node fails, packets may find an alternate route
Routing can be used to avoid congested parts of the network
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There are divided in three services:
X.25 Frame Relay Asynchronous Transfer Mode (ATM)
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In case of packet switching networks, the attached stations must organize their data into packets for transmission.
This requires a certain level of cooperation between the network and the attached stations.
X.25 is an ITU-T standard that specifies on interface between a host system and packet switching network.
Implemented at the network layer.
Implements extensive error correction and flow control due to early unreliable links.
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X.25 Layers in Relation to the OSI Layers
Note:
Link Level (LAPB- Link Access Protocol Balanced) Packet level (PLP-Packet Layer Protocol)
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Layer 1 - Physical level:
Physical level deals with the physical interface between an
attached station and the link that attaches that station to the
packet switching node.
Layer 2 - Link level:
The link level provides for the reliable transfer of data across
the physical link, by transmitting the data as a sequence of frames.
The link level standard is referred to as LAPB (link access protocol
balanced), LAPB is subset of HDLC (High-level Data Link Control).
Layer 3 - Packet level:
The packet level provides a virtual circuit service.
This service enables any subscriber to the network to setup
logical connections called virtual circuits, to other subscribers.
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X.25 is a standard for interface between the host system
with the packet switching network in which it defines
how DTE is connected and communicates with packet
switching network.
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Note:
S-Frames flow and error control in the frame layer
U-Frames- used to set up and disconnect the links between a DTE
and a DCE. In the frame layer, communication between a DTE -
DCE involves three phases:
1: Link Setup ; 2: Packet Transfer ; 3: Link Disconnect
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Internationally, Frame Relay (FR) was standardized by the
International Telecommunication UnionTelecommunications Standards Section (ITU-T).
FR originally was designed for use across Integrated Service Digital Network (ISDN) interfaces, allows for digital voice communication (VOFR).Today, it is used over a variety of other network interfaces as well.
Improvement of previous technology X.25
Operate only at the Physica and Data link layer. Error
detection at the data link layer. No flow control or error correction
control (less overhead).
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FR Layers in Relation to the OSI Layers
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Layer 1 : Physical layer
o No specific protocol, it is left to the implementer to use
whatever is available
o Supports any of the protocols recognized by ANSI
Layer 2 : Data link layer
o Employs a simplified version of HDLC called core LAPF
(Link Access Procedure for Frame Mode Bearer Services) with
no extensive error and flow control fields.
LAPF core: minimal data link control
Preservation of order for frames
Small probability of frame loss
LAPF control: additional data link or network layer end-to-end
functions
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Uses asynchronous time division multiplexing.
Designed to take advantage of the bandwidth of
optical fiber transmission media.
Many of the protocol functions are implemented in
hardware (not software) to insure the best possible
performance.
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ATM uses fixed packet lengths of 53 bytes (5 bytes of overhead and 48 bytes of user data), which is more suitable
for voice transmissions.
ATM provides extensive quality of service information that enables the setting of very precise priorities among
different types of transmissions (i.e. voice & video, internet,
etc).
ATM provides connection-oriented services only.
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Note :
All cells are 53 bytes
5 byte header
48 byte data payload
An ATM Cell
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ATM is implemented in the physical and data link
layers.
Any physical layer carrier can carry ATM cells (wired, optical,
wireless).
The Data Link layer for ATM provides :
Routing/Switching
Multiplexing
Flow control (quality of service)
Error detection and correction
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ATM Layers
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ATM layers in endpoint devices and switches
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Architecture of an ATM network
Note :
UNI user to network interfaces NNI network to network interfaces
Cell switched architecture