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Computer Communications & Networking
Week 2ISO/OSI Model
5th September 2012
OSI/ISO Seven Layers Model To address the problem of networks being
incompatible and unable to communicate with each other, the International Organization for Standardization (ISO) researched network schemes like TCP/IP in order to find a set of rules.
As a result of this research, the ISO created a network model that would help vendors create networks that would be compatible with, and operate with, other networks.
Rather than developing protocols, they identified which networking functions had related uses and collected those functions into discrete groups that became the layer
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The OSI reference model
The OSI reference model, released in 1984, was the descriptive scheme they created. It provided vendors with a set of standards that ensured greater compatibility and interoperability between the various types of network technologies that were produced by the many companies around the world.
In the OSI reference model, there are seven numbered layers, each of which illustrates a particular network function. This separation of networking functions is called layering.
Each layer defines a family of functions distinct from those of the other layers
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Advantage of seven layers modelDividing the network into these seven layers
provides the following advantages:
It standardizes network components to allow multiple-vendor development and support.
It allows different types of network hardware and software to communicate with each other.
It prevents changes in one layer from affecting the other layers, so that they can develop more quickly.
It breaks network communication into smaller simpler parts to make learning it easier to understand.
Encapsulation The data portion of a packet at layer (n-1) carries
whole packet (including data and headers) from layer n
A packet at layer 7 is encapsulated in a packet at layer 6, the whole packet of layer 6 is encapsulated into a packet at layer 5 and so on
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Application layer
The application layer is responsible for providing services to the user.
Layer 7: The Application Layer
It provides network services to the user’s applications.
It differs from the other layers in that it does not provide services to any other OSI layer, but rather, only to applications outside the OSI model.
Example browsers (http protocol), e-mail client like outlook express (pop3 protocol, smtp protocol), file transfer (ftp protocol), remote access (telnet) and NFS etc.
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Layer 7: The Application Layer
Remote login
ftp
Mail services
Presentation layer
The presentation layer is responsible for translation, compression, and encryption.
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Layer 6: The Presentation Layer
The presentation layer ensures that the information that the application layer of one system sends out is readable by the application layer of another system.
If necessary, the presentation layer translates between multiple data formats by using a common format.
If you want to think of Layer 6 in as few words as possible, think of a common data format.
Provides platform independence
Layer 6: The Presentation Layer
Code ConversionASCII to EBCDIC
Encryption For secure messages encryption at sender
end and decryption at recipient end
Compression To improve performance by reducing size
of the message.
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Session layer
The session layer is responsible for dialog control and synchronization
Layer 5: The Session Layer
As its name implies, the session layer establishes, manages, and terminates sessions between two communicating hosts.
The session layer provides its services to the presentation layer.
It also synchronizes dialogue between the two hosts' presentation layers and manages their data exchange.
In addition to session regulation, the session layer offers provisions for efficient data transfer, class of service, and exception reporting of session layer, presentation layer, and application layer problems.
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Layer 5: The Session Layer
Dialog control It allows communication between two processes
to take place
Synchronization Allows a process to add check points or
synchronization points e.g. if transferring a file of 1000 pages, a checpoint may be inserted after every 10 page unit to ensure received acknowledgement
In case of disconnection, the transferred pages should not be re-transmitted.
Layer 4: The Transport Layer The transport layer segments data from the sending
host's system and reassembles the data into a data stream on the receiving host's system. The boundary between the session layer and the transport layer
Whereas the application, presentation, and session layers are concerned with application issues, the lower three layers are concerned with data transport issues
The transport layer attempts to provide a data transport service that shields the upper layers from transport implementation details.
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Source-to-destination delivery
Transport layer
The transport layer is responsible for the delivery of a message from one process to another
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Reliable process-to-process delivery of a message
Layer 4: The Transport Layer
Specifically, issues such as how reliable transport between two hosts is accomplished is the concern of the transport layer.
In providing communication service, the transport layer establishes, maintains, and properly terminates virtual circuits.
In providing reliable service, transport error detection-and-recovery and information flow control are used.
If you want to remember Layer 4 in as few words as possible, think of quality of service, and reliability.
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Layer 4: The Transport Layer
Program Addressing Segmentation and Reassembly Connection control
Connection oriented transport protocol establishes connection with the recipient machine’s transport layer before exchange of message
Flow control Flow control is performed end-to-end rather than
across a single link
Error control Error control from process to process rather than
across a single link
Hop-to-hop deliveryThe network layer is responsible for the delivery of individual packets from the source host to the destination host.
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Network layer
Layer 3: The Network Layer The network layer is a complex layer that provides
connectivity and path selection between two host systems that may be located on geographically separated networks.
If you want to remember Layer 3 in as few words as possible, think of path selection, routing, and addressing.
Logical Addressing Physical addresses can be used for data
transmission within the same network and if a packet passes boundaries of the network, then we need another addressing scheme
Routing In internetwork routers are used to route packets
from source to destination
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Data link layer
The data link layer is responsible for moving frames from one hop (node) to the next.
Layer 2: The Data Link Layer
The data link layer provides reliable transit of data across a physical link.
In so doing, the data link layer is concerned with physical (as opposed to logical) addressing, network access, error notification, ordered delivery of frames, and flow control.
If you want to remember Layer 2 in as few words as possible, think of frames and media access control.
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Layer 2: The Data Link Layer Framing
Converts network layer packets into frame
Physical Addressing
Flow Control Sender and receiver should communicate at mutually
accepted data rate
Error Control Error detection mechanism
Access Control When two or more devices are connected to the same
link, then which device should have control over shared medium
Layer 1: The Physical Layer
The physical layer defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between end systems.
Such characteristics as voltage levels, timing of voltage changes, physical data rates, maximum transmission distances, physical connectors, and other, similar, attributes are defined by physical layer specifications.
If you want to remember Layer 1 in as few words as possible, think of signals and media.
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Physical Layer Physical characteristics of medium
Representation of bits
Data rate
Synchronization of bits
Line configuration Connection of devices to the media – point to point
(dedicated) or multipoint – shared
Physical topology
Transmission mode Simplex
Half duplex
Full duplex
Physical layer
The physical layer is responsible for Movements of individual bits from one hop (node) to the next.
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Summary of layers
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2-4 TCP/IP PROTOCOL SUITE
The layers in the TCP/IP protocol suite do notexactly match those in the OSI model. The originalTCP/IP protocol suite was defined as having fourlayers: host-to-network, internet, transport, andapplication. However, when TCP/IP is compared toOSI, we can say that the TCP/IP protocol suite ismade of five layers: physical, data link, network,transport, and application.
Physical and Data Link LayersNetwork LayerTransport LayerApplication Layer
Topics discussed in this section:
TCP/IP and OSI model
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2-5 ADDRESSING
Four levels of addresses are used in an internet employingthe TCP/IP protocols: physical, logical, port, and specific.
Physical AddressesLogical AddressesPort AddressesSpecific Addresses
Topics discussed in this section:
Addresses in TCP/IP
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Relationship of layers and addresses in TCP/IP
In Figure 2.19 a node with physical address 10 sendsa frame to a node with physical address 87. The twonodes are connected by a link (bus topology LAN). Asthe figure shows, the computer with physical address10 is the sender, and the computer with physicaladdress 87 is the receiver.
Example 2.1
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Physical addresses
As we will see in Chapter 13, most local-area networksuse a 48-bit (6-byte) physical address written as 12hexadecimal digits; every byte (2 hexadecimal digits) isseparated by a colon, as shown below:
Example 2.2
07:01:02:01:2C:4B
A 6-byte (12 hexadecimal digits) physical address.
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Figure 2.20 shows a part of an internet with two routersconnecting three LANs. Each device (computer orrouter) has a pair of addresses (logical and physical) foreach connection. In this case, each computer isconnected to only one link and therefore has only onepair of addresses. Each router, however, is connected tothree networks (only two are shown in the figure). Soeach router has three pairs of addresses, one for eachconnection.
Example 2.3
IP addresses
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Figure 2.21 shows two computers communicating via theInternet. The sending computer is running threeprocesses at this time with port addresses a, b, and c. Thereceiving computer is running two processes at this timewith port addresses j and k. Process a in the sendingcomputer needs to communicate with process j in thereceiving computer. Note that although physicaladdresses change from hop to hop, logical and portaddresses remain the same from the source todestination.
Example 2.4
Port addresses
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The physical addresses will change from hop to hop,but the logical addresses usually remain the same.
Note
Example 2.5
As we will see in Chapter 23, a port address is a 16-bitaddress represented by one decimal number as shown.
753
A 16-bit port address represented as one single number.
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The physical addresses change from hop to hop,but the logical and port addresses usually remain the same.
Note
