Today, we will look at OSI view of protocols and services.
OSI reference model contributed to the development of
a unified view of layers, protocols, and services.
Similar requirements occur at the different layers in a network architecture.
The view and the terminologies are very helpful in our learning in this specialization.
A layer is a set of related communication functions
that can be managed and grouped together.
Layer n in one machine interacts with layer n in
another machine to provide a service to its upper layer n+1.
The entities comprising the corresponding layer
on different machines are called peer processes.
The processes at layer n are referred to a layer n entity.
The machines at the same layer use a set of precise rules called as a layer-n protocol.
Layer-n peer processes communicate by exchanging protocol data units called PDUs.
Communication between peer processes is virtual and actually indirect.
Layer n+1 transfer the information by invoking the services provided by layer n.
Services are available at service access points.
Each layer passes data and control information to the layer
below it until the physical layer is reached and transfer occurs.
The data passed to the layer below is called a Service Data Unit (SDU).
SDUs are encapsulated in PDUs.
The figure illustrates the layered services in which
SDUs are exchanged between layers while PDUs are exchanged within a layer.
For more illustrations, readers may refer to the example of HTTP
used in TCP service in the lesson of examples of protocol and services.
When a SDU from layer above is encapsulated in
a PDU as payload, a header and, sometimes, a trailer are appended.
This process is called encapsulation.
The header often carries information such as addresses.
Trailer may contain the check bits, say,
CRC for error control.
In the layering architecture,
each layer adds a header and
possibly a CRC checksum trailer to the block of the information.
It accepts from the layer above.
The figure shows the headers and the trailers that are added as
a block of information data works its way down the seven layers.
A header of protocol carries information such as addresses,
sequence number, flag bits,
lengths, and so on.
At the destination, each layer reach
its corresponding header to determine what actions to take and it
eventually pass the block of information to the layer
above after removing the header and the trailer.
So, to transfer a block of application data from the application layer, indeed,
the data link layer will transfer several headers and,
possibly, one CRC checksum trailer in addition to the application data.
Thus, one can easily tell the bandwidth utilization in this case.
As an example, we show the encapsulation process in TCP/IP.
A HTTP request would have been encapsulated at TCP layer,
further to IP layer, and finally,
at the physical network, say, ethernet.
Three headers and a one check bits trailer would be appended to the HTTP request itself.
A layer may impose a limit on the size
of data blocks that it can transfer for implementation or other reasons.
Thus, a layer-n SDU may be too long to be handled as a single unit by a slower layer.
At the sender side,
SDU is segmented into multiple PDUs.
At the receiver side,
SDU is reassembled for sequence of PDUs.
The communication service provided by
a layer can be connection-oriented or connectionless.
A connection-oriented service has three phase: connection set up,
SDU transfer, and a connection release.
A connectionless service doesn't require connection set up
and as each SDU is transmitted directly.
Internetworking is to build a network of networks or Internet.
It can operate over multiple,
coexisting, different network technologies,
provide ubiquitous connectivity through the IP packet transfer,
achieve huge economic of scale while provisioning universal communication service.
What glues Internet together is Internet Protocol (IP).
IP packets transfer information across the Internet.
IP layer in each router determines next router.
Network interface transfer IP packets across networks.
As a quick summary,
the unified view enables a common understanding of
the protocols and services that are found at the different layers.
Next lesson, we will study TCP/IP architecture in details.
Xiaobo ZhouProfessor, Interim Dean
