Lecturer Slides3

8/9/2019 Lecturer Slides3

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LECTURER SLIDES

NETWORK CORE

Department of Electrical Engineering

Faculty of Engineering

University of Hasanuddin

2009


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Network Core

The mesh of routers interconnect the Internets end systems

Circuit Switching Packet Switching

Message Switching


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Circuit Switching A circuit The network must establish a connection between

the sender and the receiver

Telephone mechanism

Sample: 3 circuit switches interconnected by two links

Each links has n circuits can support n simultaneous connections End systems, e.g. PCs, connected to one switch

Hosts may have analog access or digital access to the switches.(Analog modem)

To communicate end-to-end circuit between two hosts Each end-to-end circuit over a link = 1/n of the links bandwidth

for the duration of the circuit


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Circuit Switching A circuit is implemented with FDM or TDM

FDM, the frequency spectrum of a link is shared among theconnections established across the link Telephone, the bandwitdh is 4 kHz (traditional)

TDM, time frames of fixed duration and each frame a fixednumber of time slots.

A connection is established, the network allocated one slot in everyframe to the communication

Transmission rate of the frame = frame rate * n- number of bits in a slot

Dedicated circuits are idle during silent periods disadvantage

Establishing end-to-end circuits and reserving end-to-end bandwidth complex and complicated signaling software for coordinatingswitches operations


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Circuit Switching Sample:

Time required to send a file of 640 Kbits from host A tohost B over a circuit-switched network

e.g. TDM with 24 slots with a rate of 1536 Mbps

e.g. 500 msec to establish an end-to-end circuit


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Packet Switching The source breaks long message into small packets

Traverse along communication link and packet switch,known as what?

Transmitted over comm. link with full transmission rate

Applied store-and-forward transmission Switches receive the entire packet before start transmitting the

first bit of packet onto the outbound link

Applied store-and-forward delay at the input of each link

proportional to the packets length in bits, A packet consist of L bits

Forwarded onto the outbound link of R bps

S-a-f delay at the switch = L/R seconds


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Packet Switching Solving:

e.g. assuming there Q links between the two hosts, at rate ofR bps

Queuing delays and end-to-end propagation delays are

neg igi e an no time require or connection esta is ment First, transmit the packet from A to the first link, takes L/R

second

Second, Q-1 is the remaining links to be transmitted

Third, stored and forwarded Q-1 times

Thus, the total delay = QL/R


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Packet vs. Circuit Switching Packet disad:

Not suitable for real-time services

Packet advan:

Simpler, more efficient and less costly to implement


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Message Switching Packet switching:

The source host segments long messages into smallerpackets and send to the network

The receiving host reassembles the packets back to the

original message Resembles parallel transmission

Message switching messages stay intact as they

traverse the network Resembles sequential transmission


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Packet vs. Message Switching Sample:

Message A message with 7.5 Mbits long

Transmitted at a rate of 1.5 Mbps

Time taken to traverse to the receiver with two packetswitches and three links

Packet

A source breaks into 5000 packets with 1.5 Kbits in length Time required to move to destination host (no congestion)


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Packet vs. Message SwitchingBit errors as it transit the network

Packet: If one bit is corrupted in one of the packets one packet isdiscarded

Each packet carry important message, message header

t e amount o ea er is approximate y equa or amessage/ packet, the amount of header overhead per byteof data is high

Message:

If the entire message is in one packet, an error in a packet,will discard the entire packet message is discarded

Amount of header is small


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Routing in Data Networks Two broad classes of packet switched networks:

Virtual Circuit Network Datagram Network


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Virtual Circuit Network Routing the packets according to virtual circuit

numbers VC consists of:

source to destination hosts

VC numbers: one number per each link along the path

(interface numbers)

Entries in VC number translation tables in each packetswitch along the path


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Virtual Circuit Network Sample:

Host A requests the network establish a VC betweenitself and host B

e.g., the path A-PS1-PS2-B and assigns VC numbers 12,

22,32 to the three links in the path


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Virtual Circuit Network Every new VC, an entry is added to the VC-number table

Every a VC terminates, the entries in each table along itspath are removed

Reasons to maintain similar VC numbers: Replacing the number from link to link reducing the length of

ie

Permitting a different VC number per each link along the pathwill simplified the network management function

The network switches maintain the state information ofthe ongoing connections (updating VN number table)

Sample: the Internet (Connection-oriented andConnectionless)


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Datagram Networks Analogous to postal services

Each packet that traverse the network

its header addressof the destination

Hierarchical structure

address and forward the packet to adjacent switchpacket arrived

Each packet switch has a routing table maps destination

addresses to an outbound link Do not maintain connection-state information in their switches

Sample: X.25, Frame relay, ATM (connection-oriented)


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