Video - The Upper Layers 5 Through 7 of the OSI Networking Model
This video by Kevin Wallace you learn about the Upper Layers 5 Through 7 of
the OSI networking model. Keven's videos teach networking in a light conversational
way, with emphasis on retaining the information for the purpose of passing the CompTIA
Network+ certification exam.
Transcript and my comments:
At the lower levels of the OSI model there are clearly defined data units. At the
upper levels it's more difficult to point to a specific layer protocol. When we point
up from the Session layer we're not quite sure where we're going next, we might not
even go to the session layer. That's because the OSI model protocol stack was not
intended just for TCP/IP. It was intended to be a generic model that could be applied
to a variety of protocols.
Actually, Kevin points out later in the video that the OSI model doesn't really
match up with TCP/IP stack either.
The session Layer
The session layer is responsible for setting up sessions, maintaining sessions,
and tearing down sessions. In setting up a session we might negotiate parameters
for the session. We might assign identifying numbers to a session.
For example in voice over IP you negotiate parameters like what port numbers are
going to be used for the session? What codec (software for encoding and decoding a
data stream. The word codec stands for "coder-decoder") are we going to use? What
rate are we going to use for fax transmission?
Maintaining a session involves the actual work of transferring data back
and forth. And if a session gets torn down, it involves re-establishing that session,
and maybe even acknowledging receipt of data. In addition to the acknowledgment
we get at layer 4, the transport layer.
Tearing down a session is notifying all parties involved that this session
is over. A session can be disconnected by both parties in a session, mutually agreeing
that the session is going to be torn down.
NetBIOS is a session layer protocol. It stands for Network Basic Input Output
System. NetBIOS is an API Application Programming Interface. Back in the early 1980s
it was used by an early IBM LAN technology called PC Network. NetBIOS could support
only about 80 computers on a LAN. IBM needed its networks to span more than 80 clients
There was an enhancement to NetBIOS called NetBIOS Extended User Interface which
we now call NetBEUI.
There's another stack besides the OSI stack called the TCP/IP stack. With the
TCP/IP stack there's less ambiguity as to which upper-layer protocol we're
going to be pointing to. With the TCP/IP stack the three upper layers of the
OSI model are consolidated into one generic Application layer.
Almost any network we deal with today is based on TCP/IP's suite of protocols.
So why not just consider the TCP/IP stack rather than the OSI model because
it has much lass ambiguity as to which protocol were pointing to after we leave
the Transport layer? Because it's important for certification and to understand
lots of the literature out there to understand the three upper layers of the OSI model.