Introduction to Computer Networking

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Network Infrastructure
A computer network is made of

distributed applications
l provide services to users on other machines, or to other machines
l execute on computers

network infrastructure
l supports transport of data between computers where distributed applications reside
l in computers (Ethernet card, modem + software)
+ in special network devices (bridges, routers, concentrators, s witches)


A computer network is made of two distinct subsets of components
- distributed applications are programs running on interconnected computers
web server, a remote login server, an e-mail exchanger are examples. This is
the visible part of what people call “the Internet”. In this lecture we will
study the simplest aspects of distributed applications. More sophisticated
aspects are the object of lectures called “Distributed Systems” and
“Information Systems”.
- the network infrastructure is the collection of systems which are required
for the interconnection of computers running the distributed applications. It
is the main focus of this lecture.
The network infrastructure problem has itself two aspects:
- distance: interconnect remote systems that are too far apart for a direct
cable connection
- meshing: interconnect systems together; even in the case of systems close to
each other, it is not possible in non-trivial cases to put cables from all
systems to all systems (combinatorial explosion, cable salad management
problems etc.).


Protocol, service and other fancy definitions
Peer entities
l two (or more) instances of the same layer
Protocol and a PDU:
l the rules of the operation followed by peer entities
l the data exchanged is called PDU (Protocol Data Unit)
l there is one protocol (or more) at every layer
Service and a SDU
l the interface between a layer and the layer above - SAP (Service Access Point)
l the interface data is called a SDU (Service Data Unit)
Connection
l a protocol is connection oriented if the peer entity must be synchronized before
exchanging useful data (connection set up); otherwise it is connectionless.
A