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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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