TweetFollow Us on Twitter

Oct 01 Network Management

Volume Number: 17 (2001)
Issue Number: 10
Column Tag: Network Management

Networks 201 pt. 7

by John C. Welch

Layer 5: The Session Layer, and Layer 6: The Presentation Layer

Refresh

Well, we've made it into the last three layers, and things should go a bit faster now. We are also going to group Layers 5 and 6 in this one article, as they are quite thin, and often rolled into other layers. The first OSI layer we will talk about is Layer 5, the Session Layer. This Layer is quite similar in function to Layer 4, and in many situations, it is rolled into the Transport Layer, and is not even a separate part of the networking model. In fact, there is only one protocol that really makes specific use of the Session Layer, and that is AppleTalk. (Other protocols, notably IPX/SPX have aspects that use the Session Layer, but AppleTalk makes more specific use of it than most any other protocol.). In fact, TCP/IP almost completely ignores the Session Layer. So if I seem to talk about AppleTalk more than other protocols here, there's a reason for it.

The Session Layer

The basic function of the Session Layer is to manage communications flow during a session. Now, this is much of what Layer 4, the Transport Layer does, so why have a separate layer? Scope is the reason. The Session Layer is not concerned with things like error checking, reliability, etc. It is assuming that those things are being taken care of by the Transport Layer.

Instead, the Session Layer is concerned with managing the connections or sessions from a higher level point of view. It is concerned with the way traffic is flowing in the session, and making the session run smoother. It is dealing more with the application using the session, then the low - level mechanics of the session. So the Session Layer is said to control the dialog between two applications on a connection.

One of the aspects of this dialog control is the duplex level of the connection's data flow. That is, can the applications at both ends send and receive data simultaneously, (full duplex), or do they have to send and receive in separate actions, (half duplex). Deciding which to use, and handling it is a Session Layer function. An example of a protocol that deals with this type of functionality is the Apple Data Stream Protocol, or ADSP. ADSP's job is to handle connections between devices, and manage the bi-directional flow of data between them. It also allows the data to flow as a continuous stream from sender to receiver, and can handle things like out of order data problems. In that sense, ADSP is also performing some Transport Layer functions too.

Another function of the Session Layer is grouping. An example of this could be the AppleTalk Zone Information Protocol, or ZIP. This is the protocol that takes the information from the Name Binding Protocol, (NBP) and uses it to present the names of devices on an AppleTalk network in the correct logical groups, or zones. The Chooser is a classic use of ZIP and NBP.

Finally, the Session Layer deals with recovery, or what to do when the session fails for some reason. Now, there are two ways to handle this. The first is to just discard all the data on the receiving end, and retransmit all of it. For small pieces of data, this is not a big deal, but for say a five hundred page print job, that gets to be intolerable. So what the Session Layer does is allow for the insertion of checkpoints into the data. So now, if the connection dies for some reason, all that has to happen is for both ends to synchronize which checkpoint was last successfully transmitted, and proceed from there. An example of this functionality, although it occurs at a different layer is the ability to resume an interrupted FTP transfer. Another example of a protocol using this is the Printer Access Protocol, or PAP. When you have to stop and restart an AppleTalk print job, and it asks you "resume printing on page X", that's what we are talking about.

As you can see, there is not a lot to the Session Layer. The protocols that use the layer can be quite complex, but Layer 5 is, in and of itself, rather thin.

Layer 6: The Presentation Layer

This layer is different from all the previous layers, as it is the first layer that is not at all concerned with moving data from point A to point B. Instead, the Presentation Layer cares about how the data is sent up to the Application Layer, and how it is sent down to the Session Layer. Remember, so far, regardless of how we have moved the data, it has all been nothing but a series of bits. However, applications, and the users of those applications don't want ones and zeros. We want letters, numbers, colors, etc.

That's what the Presentation Layer is for. It handles things like ASCII encoding, EBCDIC encoding, converting between the two formats, etc. It also handles other things like dealing with different floating point formats and the representation of those formats. If you are doing things like data compression, or encryption via a protocol, then this is where you would handle that sort of thing.

In other words, the way data is handed off, and how it is encoded, or how it looks to the other layers is a Presentation Layer function. One example of a use for the Presentation Layer is Postscript printing. Although the actual data transfer would be handled by the Session Layer and below, when you send Postscript data to a printer, it has to be encoded correctly, it has to ‘look' right. Otherwise, the print job could fail.

Conclusion

Well, this was certainly a short article. Again, these two Layers really are light weight. They don't' really interact with the user or the user applications, as does the Application Layer. They also don't directly deal with the mechanics of data transfer. As we pointed out, the Presentation Layer doesn't deal with this aspect at all. Still, since AppleTalk makes use of the Session Layer, and almost all protocols deal with the Presentation Layer in one fashion or another, it's good to know why they are even there. Next time, it's the series conclusion, the end of the journey...Layer 7, the Application layer.

Bibliography and References

As already mentioned, you may wish to include a bibliography at the end of your article, especially if you wish to cite work in the body of the article. Here is some typical bibliography format, to serve as a model:

  • Tannenbaum, Andrew S. Computer Networks. Third Edition Prentice Hall, 1996.
  • Sportack, Mark. Networking Essentials Unleashed. SAMS Publishing, 1998.
  • Anders, James K. Live Wired A Guide to Networking Macs. Hayden Books, 1993.
  • Apple Computer, Inside Macintosh: Networking With Open Transport. Apple Computer, Inc. 1997.
  • Stallings, William, Local and Metropolitan Area Networks. Fifth Edition Prentice Hall, 1997.

John Welch jwelch@aer.com is a Mac/PC/Unix administrator and Mac writer living in the wilds of that part of Massachusetts that is not Boston. He has over fifteen years of experience at making computers work. His specialties are figuring out ways to make the Mac do what nobody thinks it can, and showing that the Mac is the superior administrative platform.

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

BBEdit 11.1.1 - Powerful text and HTML e...
BBEdit is the leading professional HTML and text editor for the Mac. Specifically crafted in response to the needs of Web authors and software developers, this award-winning product provides a... Read more
CrossOver 14.1.3 - Run Windows apps on y...
CrossOver can get your Windows productivity applications and PC games up and running on your Mac quickly and easily. CrossOver runs the Windows software that you need on Mac at home, in the office,... Read more
Little Snitch 3.5.3 - Alerts you about o...
Little Snitch gives you control over your private outgoing data. Track background activity As soon as your computer connects to the Internet, applications often have permission to send any... Read more
OmniGraffle Pro 6.2.3 - Create diagrams,...
OmniGraffle Pro helps you draw beautiful diagrams, family trees, flow charts, org charts, layouts, and (mathematically speaking) any other directed or non-directed graphs. We've had people use... Read more
OmniFocus 2.2 - GTD task manager with iO...
OmniFocus helps you manage your tasks the way that you want, freeing you to focus your attention on the things that matter to you most. Capturing tasks and ideas is always a keyboard shortcut away in... Read more
Cocktail 8.4 - General maintenance and o...
Cocktail is a general purpose utility for OS X that lets you clean, repair and optimize your Mac. It is a powerful digital toolset that helps hundreds of thousands of Mac users around the world get... Read more
PDFKey Pro 4.3 - Edit and print password...
PDFKey Pro can unlock PDF documents protected for printing and copying when you've forgotten your password. It can now also protect your PDF files with a password to prevent unauthorized access and/... Read more
Kodi 15.0.beta1 - Powerful media center...
Kodi (was XBMC) is an award-winning free and open-source (GPL) software media player and entertainment hub that can be installed on Linux, OS X, Windows, iOS, and Android, featuring a 10-foot user... Read more
DiskCatalogMaker 6.4.12 - Catalog your d...
DiskCatalogMaker is a simple disk management tool which catalogs disks. Simple, light-weight, and fast. Finder-like intuitive look and feel. Super-fast search algorithm. Can compress catalog data... Read more
Macs Fan Control 1.3.0.0 - Monitor and c...
Macs Fan Control allows you to monitor and control almost any aspect of your computer's fans, with support for controlling fan speed, temperature sensors pane, menu-bar icon, and autostart with... Read more

Moleskine Timepage – Calendar for iCloud...
Moleskine Timepage – Calendar for iCloud, Google & Exchange 1.0 Device: iOS iPhone Category: Productivity Price: $4.99, Version: 1.0 (iTunes) Description: The most elegant calendar for your pocket and wrist, Timepage is a... | Read more »
QuizUp Gets Social in its New Update
Plain Vanilla Corp has released a new and improved version of their popular trivia game, QuizUp. The app now emphasizes social play so you can challenge friends from all over the world. [Read more] | Read more »
The Deep (Games)
The Deep 1.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0 (iTunes) Description: Swipe Controls Delve into the deep in this retro rogue-like! Swipe to move your diver around and keep away from the enemies as you... | Read more »
Battle of Gods: Ascension (Games)
Battle of Gods: Ascension 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: TURN-BASED TACTICAL COMBATFight tactical battles against the forces of Hades! In Battle of Gods: Ascension you play... | Read more »
Shadowmatic's Latest Update Adds a...
Shadowmatic's shadowy shadow-ness is getting a little shadowy-er thanks to a recent update that adds an Arcade Mode. [Read more] | Read more »
Sunrise Calendar and Slack Have Assimila...
Wunderlist is perhaps one of the most populat and beloved productivity apps on the App Store - and now it's gone and incorporated itself into other useful services like Sunrise Calendar and Slack. [Read more] | Read more »
Crossy Road Devs Hipster Whale are Bring...
Hipster Whale, the minds behind the rather popular (and rather great) Crossy Road, have teamed-up with Bandai Namco to create PAC-MAN 256: an absolutely bonkers looking maze runner chaser thing. | Read more »
Meet the New Spotify Music
Spotify Music  has a lot going on. They're introducing 3 new modes to serve all your musical needs, with the "Now" start page  gives you curated playlists based on your particular tastes. As you listen the app will learn more about your tastes and... | Read more »
What the Apple Watch Gets Right, and Wha...
| Read more »
Celebrate PAC-MAN's 35th Birthday W...
BANDAI NAMCO Entertainment America is celebrating PAC-MAN's 35th anniversary by releasing updates for PAC-MAN and PAC-MAN Lite for iOS. [Read more] | Read more »

Price Scanner via MacPrices.net

Apple refurbished 2014 13-inch Retina MacBook...
The Apple Store has Apple Certified Refurbished 2014 13″ Retina MacBook Pros available for up to $400 off original MSRP, starting at $979. An Apple one-year warranty is included with each model, and... Read more
What Would the ideal Apple Productivity Platf...
For the past four years I’ve kept a foot in both the Mac and iPad camps respectively. my daily computing hours divided about 50/50 between the two devices with remarkable consistency. However, there’... Read more
PageMeUp 1.2.1 Ten Dollar Page Layout Applica...
Paris, France-based Softobe, an OS X software development company, has announced that their PageMeUp v. 1.2.1, is available on the Mac App Store for $9.99. The license can be installed on up to 5... Read more
Eight New Products For USB Type-C Application...
Fresco Logic, specialists in advanced connectivity technologies and ICs, has introduced two new product families targeting the Type-C connector recently introduced across a number of consumer... Read more
Scripps National Spelling Bee Launches Buzzwo...
Scripps National Spelling Bee fans can monitor the action at the 2015 Spelling Bee with the new Buzzworthy app for iOS, Android and Windows mobile devices. The free Buzzworthy app provides friendly... Read more
13-inch 2.5GHz MacBook Pro on sale for $120 o...
B&H Photo has the 13″ 2.5GHz MacBook Pro on sale for $979 including free shipping plus NY sales tax only. Their price is $120 off MSRP, and it’s the lowest price for this model (except for Apple’... Read more
27-inch 3.3GHz 5K iMac on sale for $1899, $10...
B&H Photo has the new 27″ 3.3GHz 5K iMac on sale for $1899.99 including free shipping plus NY tax only. Their price is $100 off MSRP. Read more
Save up to $50 on iPad Air 2, NY tax only, fr...
B&H Photo has iPad Air 2s on sale for up to $50 off MSRP including free shipping plus NY sales tax only: - 16GB iPad Air 2 WiFi: $469 $30 off - 64GB iPad Air 2 WiFi: $549.99 $50 off - 128GB iPad... Read more
Updated Mac Price Trackers
We’ve updated our Mac Price Trackers with the latest information on prices, bundles, and availability on systems from Apple’s authorized internet/catalog resellers: - 15″ MacBook Pros - 13″ MacBook... Read more
New 13-inch 2.9GHz Retina MacBook Pro on sale...
B&H Photo has the 13″ 2.9GHz/512GB Retina MacBook Pro on sale for $1699.99 including free shipping plus NY tax only. Their price is $100 off MSRP, and it’s the lowest price for this model from... Read more

Jobs Board

*Apple* Solutions Consultant - Retail Sales...
**Job Summary** As an Apple Solutions Consultant (ASC) you are the link between our customers and our products. Your role is to drive the Apple business in a retail Read more
*Apple* Watch SW Application Project Manager...
**Job Summary** The Apple Watch software team is looking for an Application Engineering Project Manager to work on new projects for Apple . The successful candidate Read more
Engineering Manager for *Apple* Maps on the...
…the Maps App Team get to take part in just about any new feature in Apple Maps, often contributing a majority of the feature work. In our day-to-day engineering work, we Read more
Senior Software Engineer - *Apple* SIM - Ap...
Changing the world is all in a day039s work at Apple . If you love innovation, here039s your chance to make a career of it. You039ll work hard. But the job comes with Read more
Lead *Apple* Solutions Consultant - Retail...
**Job Summary** Job Summary The Lead ASC is an Apple employee who serves as the Apple business manager and influencer in a hyper-business critical Reseller's store Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.