Transcript ECE 478/578 Lecture 2

Classes of Networks
 LAN
(Local Area Network)
 Maximum
distance not more than a few kms
 Ownership by a single organization
 Transmission speed of at least several Mbps
(tens to hundreds are economical)
 Often broadcast, shared media based
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Classes of Networks - LANS
 Some
widely used standards include:
 IEEE
803.3 - Ethernet
 IEEE 803.5 - Token ring
 FDDI
 ATM
 An
important issue in broadcast LANs is the allocation
of the shared channel (media access control)
 Control may be static (time division multiplexing) or
dynamic (contention or arbitration)
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Classes of Networks - MANS
 MAN
(Metropolitan Area Network)
 Distances
between 5 and 50 kms
 Data rate above 1 Mbps
 Standards: IEEE 802.6 DQDB, FDDI, and ATM
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Classes of Networks - WANS
 WAN
(Wide Area Network)
 Spans
entire states or countries
 Data rate of 1.544 (T1), and 45 (T3) Mbps common
 Higher data rates are available with the wide
deployment of ATM backbone networks
 Often owned by multiple organizations
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Classes of Networks - WANS
 Usually
separate communications functions from
application functions
 Transmission lines: circuits, channels or trunks
 Switching elements:
 Specialized computers connecting two or more
circuits
 These elements receive data on an incoming
circuit and transmit it on an outgoing circuit
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Classes of Networks - WANS
 Intermediate
Systems, Packet Switching
Node, Data Switching Exchange, Router, etc.
 Intermediate systems store a complete packet
before forwarding it
• store-and-forward; packet switched; point-topoint network
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Classes of Networks - Internetworks
 Internetworks
 Unlike
WANs they often interconnect different,
incompatible networks
 Use special types of intermediate systems
called Gateways
 Gateways translate between different types of
physical media and network software
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Network Software
 Network
software is highly structured
 This technique has been immensely successful
 The key is Layered design
 Each
layer provides a service to the layer above
 Each layer hides details of how the service is
provided to the layer above
 The Nth layer on one machine “talks to” or
interacts with the Nth layer on another machine
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Network Software - Protocols
 Conventions
and rules governing this
interaction are specified by the Layer N
Protocol
 A protocol
is an agreement about how
communications are to proceed
 Without a protocol, communication can be difficult
or even impossible
 E.g. Telephone conversation, Postal addresses
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Network Software - Protocols
Peer “entities”
Layer 5
Layer 5 Protocol
Layer 5
Layer 4 Protocol
Layer 4
Layer 3 Protocol
Layer 3
Layer 2 Protocol
Layer 2
Layer 1 Protocol
Layer 1
Interface
Layer 4
Interface
Layer 3
Interface
Layer 2
Interface
Layer 1
Physical Medium
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Network Software - Protocols
 Information
is not actually transferred directly
between peer layer N entities
 Peer
layer N entities carry on a virtual communication
using the services of the layers below
 Layer N passes data and control information down to
(or receives data and control from) Layer N-1 until the
physical medium is reached
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Network Software - Interfaces & Services
 Interfaces exist between each layer
 Interface defines which primitive functions and
services layer N-1 provides to layer N
 Want layers to:
 Perform
a well defined, logically related set of
functions
 Minimize the amount of information needed to pass
between layers
 Keep interfaces “clean” to allow easy and transparent
replacement of layers
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Network Software - Protocols
 The
set of protocols and layers together make
up the Network Architecture
 A Network Architecture
Specification must
provide enough information to allow
implementation in hardware/software
 Implementation specific details are not part of the
architecture and should be irrelevant for interoperation
 With one protocol per layer we have a Protocol
Stack
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The benefits of Layered Protocols
 Network Architectures, Protocols and Protocol
stacks are the Fundamentals of Computer
Networks
 They
form the foundation for the very considerable
success of computer networks in the real world
 Multilayer communications protocols allow
 ready
adaptation of successful protocols to new
technology (prevent obsolescence)
 migration of protocols from software implementation
(slow) to hardware (fast) as they evolve
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More Benefits of Layered Protocols
 Separate
data and control information
 Support differing levels of abstraction (message,
packet, frame) with different sizes
 Allow segmentation of large messages
 Peer process abstraction facilitates reduction of
difficult design task (a network architecture) into
smaller manageable tasks (protocol layer
architecture)
 Typically lower layer protocols of “network
software” are implemented in silicon (hardware)
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Understanding Services and Protocols

Protocol is set of rules about the format and
meaning of data units exchanged by peers
 Protocol
is used by entities to implement
services
 Protocol and/or its implementation can change
and as long as the Service (interface) remains
unchanged, higher layers are happy and
continue to work
 Like in abstract data types or object orientation,
we decouple interface and implementation
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Layering concept elsewhere
 Layering
is used in other software (e.g. Operating
systems) - UNIX
 For Network Software the important difference is
that we are not allowed to violate layering (Layer
5 cannot directly access Layer 1)
 For Network Software, its important Layers don’t
“peek” into headers of other layers and rely on
protocol data of other layers
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