Review of Chapters 12, 13 & 14

Transport Layer

Session Layer

Presentation Layer

Chapter 12

Transport Layer

Layers 1 - 3

Layer 1 allows bit streams to be created and to travel.

Layer 2 packages those data packets into frames to be converted to bit streams and makes LAN delivery possible.

Layer three packages data from upper layers in packets and makes routing and WAN delivery possible.

Layer 4 - Transport Layer

But we have made no provision for assuring our data reliably travels end-to-end across the often vast

network path. Layer 4 performs multiple functions to provide this "quality of service.”

Quality of Service

The phrase "quality of service" is often used to describe the purpose of Layer 4 - the transport layer. Its primary duties are to transport and regulate the flow of information from source to destination, reliably and accurately.

TCP & UDP

The emphasis of this curriculum is on TCP/IP Ethernet networks. The TCP/IP protocol of the OSI model Layer 4 (transport layer) has two protocols - TCP and UDP.

TCP

TCP supplies a virtual circuit between end-user applications. These are its characteristics:

connection-oriented

reliable

divides outgoing messages into segments

reassembles messages at the destination station

re-sends anything not received

reassembles messages from incoming segments.

UDP

UDP transports data unreliably between hosts. Following are the characteristics of UDP:

connectionless

unreliable

transmit messages (called user datagrams)

provides no software checking for message delivery (unreliable)

does not reassemble incoming messages

uses no acknowledgements

provides no flow control

TCP Segment Fields

Following are the definitions of some of the fields in the TCP segment:

source port -- number of the calling port

destination port -- number of the called port

sequence number -- number used to ensure correct sequencing of the arriving data

acknowledgment number - next expected TCP octet

window -- number of octets that the sender is willing to accept

TCP & UDP Port Numbers

Both TCP and UDP use port (or socket) numbers to pass information to the upper layers.

Numbers below 255 - are for public applications

Numbers from 255-1023 - are assigned to companies for marketable applications

Numbers above 1023 - are unregulated

3-Way Handshake

TCP hosts establish a connection-oriented session with one

another using a three-way handshake.

A three-way handshake synchronizes a connection at

both ends before user data is exchanged.

Window Size

Window size determines the amount of data that you can transmit at one time before receiving an acknowledgment from the destination.

Sequencing

TCP provides sequencing of segments with a forward reference acknowledgment.

If a sequence number is missing in the series, that segment is re-transmitted.

Chapter 13

The Session Layer

The Session Layer

The session layer establishes, manages, and terminates sessions between applications.

Dialog Control

When the session layer decides whether to use 2-way simultaneous or 2-way alternate communication it is called dialogue control.

The session layer uses two major forms of dialog control:

2-way alternate communication2-way simultaneous communication

2-Way Alternate and 2-Way Simultaneous Communication.

2-way alternate communication avoids interrupts by taking turns.

2-way simultaneous communication allows 2 messages to cross during a conversation.

Dialogue Separation

Dialogue separation is the orderly initiation, termination, and managing of communication.

Layer 5 Protocols

Layer 5 has a number of important protocols: Network File System (NFS) Structured Query Language (SQL) Remote

Procedure Call (RPC) X-Window System AppleTalk Session Protocol

(ASP) Digital Network Architecture Session Control

Protocol (DNA SCP)

Chapter 14

The Presentation Layer

Layer 6 - The Presentation Layer

The presentation layer is responsible for presenting data in a form that the receiving device can understand.

The presentation layer has 3 main functions:data formatting

data compression

data encryption

Data Formatting

To understand how data formatting works, imagine two dissimilar systems. The first system uses Extended Binary Coded Decimal Interchange Code (EBCDIC) to format text. The second system uses American Standard Code for Information Interchange (ASCII) to format text. Layer 6 provides the translation between these two different types of codes.

Graphic File Formats

The Internet uses two binary file formats to display images:

Graphic Interchange Format (GIF) Joint Photographic Experts Group (JPEG).

Any computer with a reader for the GIF and JPEG file formats can read these file types, regardless of the type of computer.

Multimedia File Format

The multimedia file format is another type of binary file, which stores sounds, music, and video.

Sound files may be completely downloaded, first, and then played, or they may download while they are playing. The latter method is referred to as streaming audio.

Markup Language

Another type of file format is markup language. This format acts as a set of directions that tell a Web browser how to display and manage documents. Hypertext Markup Language (HTML) is the language of the Internet.

Encryption & Compression

Layer 6 is responsible for data encryption. Data encryption protects information during its transmission.

The presentation layer is also responsible for the compression of files.

Conclusion

This concludes our review of chapters 12, 13 and 14. You are now ready to take the online exam!