Introduction
• Computer networks:
– definition
– computer networks from the perspectives of users
and designers
– Evaluation criteria
– Some concepts:
– packet switching & circuit switching
– point-to-point networks & broadcast networks
– multiplexing
– performance metrics: delay & throughput
– LAN & WAN
• Definition :
– Something that interconnects computers,
allowing computers to exchange digital
information (bits).
– Hardware or software?
• What do computer networks offer?
–
–
–
–
Information collection / distribution
access to shared computing resources
provide high reliability
…...
• Computer Networks from the perspectives
of network users/programmers and network
designers:
• Perspective from a user:
– What do you care?
• What services? Email, FTP, reliable communication,
unreliable.
• Service quality: How fast?
• Service cost
• Service interface: send_packet, receive_packet
• Perspective from a designer:
– What do you care?
• How to make users happy?
• How to achieve that at the lowest price? (how to
provide the services effectively.)
– Design issues
– Design alternatives
– Considerations in deciding which alternative is better?
– Example: How to interconnect the machines in
a network?
• Pairwise interconnection (complete graph)
• point-to-point networks.
– No direct connections between all pairs of nodes.
Messages must pass a number of intermediate nodes to
reach the destination. E.g. Tree, Mesh.
– point-to-point networks.
• No direct connections between all pairs of nodes.
Messages must pass a number of intermediate nodes
to reach their destinations. E.g. Tree, Mesh.
• How to do this?
– Packet switching and circuit switching
– Packet switching:
» Messages are cut into packets
» When an intermediate node receives a packet, it
determines the next node and forward the packet
(store-and-forward networks).
– Circuit switching networks:
» set up a circuit before transmission
» all data from one session (connection) follow the
same circuit.
– Broadcast networks.
• Each node can send messages to all nodes in the
system. E.g. bus connection.
• Contention problem: two nodes may want to send
messages simultaneously.
• Resolve the contention: multiplexing.
– Time division multiplexing: divide time domain into time
slots, different nodes use different time slots to transfer.
– Design choice # 1: static assignment
» A uses slot 1, B uses slot 2, …
– Design choice # 2: dynamic assignment
» dynamically allocate slots to communication
channels.
» Need a mechanism to allocate channels, both ends
must agree a slot for communication
– How to evaluate these design choices?
– Evaluation criteria
• Application requirements:
– functionality: Can I do it?
– performance: How fast I can send data?
• Cost constraints: the cost of the network has to be
within certain budget.
– Application performance requirements:
• delay: time from the data are transmitted to the time
the data are received.
– Propagation delay + queueing delay + transmission delay
– propagation delay = distance / signal propagation speed
speed of light (in vacuum): 3.0*10^8 m/s = 300km/ms
propagation delay across continental USA (NY to LA):
3000 miles = 4800km, delay = ???
Geosynchronous Satellite: 36000km, delay = ?
– Application performance requirements:
• delay: time from the data are transmitted to the time
the data are received.
–
–
–
–
Propagation delay + queueing delay + transmission delay
propagation delay = distance / signal propagation speed
queueing delay: depend on the network load
transmission delay: how fast can you put bits on the wire?
» E.g. Ethernet 10Mb/s (bandwidth)
» 500KB message, transmission delay = ?
• Throughput: amount of data transmitted / time
• High throughput = low delay?
– E.g: transfer 10Kb data using 10Mb/s link, delay = ?
– Using 1Gb/s link, delay = ?
– Conclusion?
– Applying the criteria, some case studies:
– Case 1: topology choices for local area
network(LAN) and wide area network (WAN).
• LAN: computer networks that cover a small area (a
single building up to a few kilometers) with a small
number of computers.
– Topology choices: all directly connected, point to point
and broadcast?
• WAN: computer networks that cover a large area
(continent) with a lot of computers.
– Topology choices: all directly connected, point to point
and broadcast?
– Case 2: the switching technique for telephone
networks and computer networks
application
tele.
Interactive
voice
computer rlogin
traffic
throughput
continuous
smooth
64kbps
bursty
varied
delay
<200ms
web
ftp
telephone networks: circuit switching or packet switching?
Circuit switching + TDM
Computer networks: circuit switching + TDM?
Integrated Service Digital Network (ISDN): supporting both
telephone traffic and computer traffic. ATM.
varied
– Case 3: Network for Military use
• military concerns: survivability
– I still need to hear your voice even when most of the
network components were gone.
– Idea: each packet can find its way through the network,
end system recover from the lost packets (due to the
damaged links or congestion).
• How ARPANET works:
– End system chops messages into packets, each packet has
a header with destination address.
– Each packet finds its way to the destination independently.
– End system put together the message.
• Why not circuit switching + TDM?