Continuing on with the OSI model series I will discuss about Layer 5, the session layer. Layer 5 of the OSI model, as the name implies, handles sessions between two devices. The purpose of a session is to start communication between two devices and end the communication when all of the data is passed. Layer 5 also provides services between layer 4, the transport layer, and layer 6, the presentation layer.
Unlike the lower layers that I discussed about in the OSI model, information at layer 5 and above is just called ‘data’ (data is the name of the PDU for layers 5 and up.) Session communication can be done in three different ways: simplex mode, half-duplex or full-duplex. Simplex mode is used in one-way communication. The source device sends data to the receiving device, while the receiving device does not send anything back to the source. Video streaming is a service that can benefit greatly with simplex. Half-duplex is used when the source and destination takes turns in communicating with each other, while full-duplex allows the source and destination communicate with each other at the same time.
There are a lot of protocols on layer 5. Most are based on RPC (Remote Procedure Protocol). Most are also platform specific. Unix usually relies heavily on NFS (Network File System.) Windows uses SMB (Server Message Block.) Apple, by default, uses ASP (AppleTalk Session Protocol) or AFP (Apple Filing Protocol) as its choice.
Going over the specifics about each protocol could probably take many posts. Instead, I will just briefly go over SMB, since that is the most popular. It is the most popular because in Windows, it is really your only choice, while other platforms usually support SMB. So, in mixed environments with Windows, SMB is usually the logical choice.
If you have Windows and ‘file and print sharing’ is enabled, you are using SMB. If you have a non-Windows platform but want to share files with a Windows computer, you can use Samba. Samba is an open source standard used by Unix and Linux in order to communicate with Windows machines.
According to Microsoft, SMB (Server Message Block) offers the following services over a network:
- Dialect negotiation
- Determining other Microsoft SMB Protocol servers on the network, or network browsing
- Printing over a network
- File, directory, and share access authentication
- File and record locking
- File and directory change notification
- Extended file attribute handling
- Unicode support
- Opportunistic locks
The great thing about SMB (or most of the other layer 5 protocols) is that it is pretty much automatic. However, it is also not very configurable.
In order to share files, the source computer needs to know the name of the destination. For older Windows platforms, this name was called a NetBIOS name (Network Basic Input Output System.) Each computer on a LAN needed to have a unique NetBIOS name. Newer Windows platforms use DNS names (Domain Name System,) but are also usually assign a NetBIOS name for backwards compatibility.
We are almost done with the layers of the OSI model! Please stay tuned to the next post, when I discuss about Layer 6.
Resources
http://support.apple.com/kb/ht1549 http://www.axis.com/products/video/about_networkvideo/audio.htm http://www.samba.org/ http://msdn.microsoft.com/en-us/library/aa365233%28v=vs.85%29.aspx http://technet.microsoft.com/en-us/library/bb727013.aspx
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