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Lecture 5
Information Systems
Spring 2011
Convener:
Houman Younessi
1-860-548-7880
[email protected]
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Islands of
Information/Automation
Need for inter-communication
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Network:
A collection of links arranged so that messages may be passed from one part
to another usually over multiple links.
Data Communication Networks
Tele-Communication Networks
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Network Protocols
The Open Systems Interconnection Reference Model (OSI Model or OSI
Reference Model for short) is a layered, abstract description for communications
and computer network protocol design.
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Layer 1: Physical Layer
Defines all the electrical and physical specifications for devices including layout of
pins, voltages, and cable specifications. Hubs, repeaters, network adapters and
Host Bus Adapters (HBAs used in Storage Area Networks) are physical-layer
devices. Ethernet incorporates both this layer and the data-link layer (Layer 2). The
same applies to other local-area networks, such as Token ring, FDDI, and IEEE
802.11.
The major functions and services performed by the physical layer are:
• Establish/terminate a connection
• Contention resolution and flow control
• Modulation, or conversion between the representation of digital data
in user equipment and the corresponding signals transmitted over a
communications channel. These are signals operating over the
physical cabling—copper and fiber optic, for example—or over a radio link.
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Layer 2: Data Link Layer
Transfers data between network entities and to detect and possibly correct errors
that may occur in the Physical layer. The addressing scheme is physical which
means that the addresses are hard-coded into the network cards at the time of
manufacture. The addressing scheme is flat.
Examples are Ethernet, HDLC and ADCCP for point-to-point or packet-switched
networks and Aloha for local area networks. On IEEE 802 local area networks,
and some non-IEEE 802 networks such as FDDI, this layer may be split into a
Media Access Control (MAC) layer and the IEEE 802.2 Logical Link Control (LLC)
layer.
This is the layer at which the bridges and switches operate. Connectivity is
provided only among locally attached network nodes.
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Layer 3: Network Layer
Provides the functional and procedural means of transferring variable length data
sequences from a source to a destination via one or more networks while
maintaining the quality of service requested by the Transport layer. The Network
layer performs network routing, flow control, segmentation/de-segmentation, and
error control functions. Routers operate at this layer—sending data throughout
the extended network and making the Internet possible (there also exist layer 3
(or IP) switches). This is a logical addressing scheme – values are chosen by the
network engineer. The addressing scheme is hierarchical. The best known
example of a layer 3 protocol is the Internet Protocol (IP).
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Layer 4: Transport Layer
Provides transparent transfer of data between end users, thus relieving the upper
layers from any concern while providing reliable and cost-effective data transfer.
The transport layer controls the reliability of a given link. Some protocols are state
and connection oriented. This means that the transport layer can keep track of the
packets and retransmit those that fail. The best known example of a layer 4
protocol is the Transmission Control Protocol (TCP). It is the layer that converts
messages into TCP, User Datagram Protocol (UDP), Stream Control Transmission
Protocol (SCTP), etc. packets.
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Layer 5: Session Layer
Controls the dialogues (sessions) between computers. It establishes, manages
and terminates the connections between the local and remote application. It
provides for either duplex or half-duplex operation and establishes checkpointing, adjournment, termination, and restart procedures. The OSI model made
this layer responsible for "graceful close" of sessions, which is a property of TCP,
and also for session check-pointing and recovery, which is not usually used in
the Internet protocol suite.
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Layer 6: Presentation Layer
Transforms data to provide a standard interface for the Application layer. MIME
encoding, data compression, data encryption and similar manipulation of the
presentation is done at this layer to present the data as a service or protocol
developer sees fit. Examples: converting an EBCDIC-coded text file to an
ASCII-coded file, or serializing objects and other data structures into and out of
XML.
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Layer 7: Application Layer
Provides a means for the user to access information on the network through an
application. This layer is the main interface for the user(s) to interact with the
application and therefore the network. Some examples of application layer
protocols include Telnet, File Transfer Protocol (FTP), Simple Mail Transfer
Protocol (SMTP) and Hypertext Transfer Protocol (HTTP).
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Towards a Real-time Infra-structure:
Better Data – Better Decisions
Improved Process Visibility
Improved Process Efficiency
Sense-and-respond (respond to actual demand rather than
forecasted demand)
But for this to take place, we need to integrate more than just the production.
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