Re: The "Presentation" Layer

Well, first, i'm no expert on this.

But reading you're explanation, you could say that the workstation controller is simply another peer in the network and acts as a "display" server to the program (the client) AND acts as a client to the terminal (which takes on a server role).

But, logically, the client program requests the terminal to display a format, and therefor, by definition, the terminal acts as a server with regard to the program. The workstation controller is simply an implementation detail.

Date: Fri, 18 Jul 2008 07:25:16 -0700
From: nandelin@xxxxxxxxx
To: web400@xxxxxxxxxxxx
Subject: Re: [WEB400] The "Presentation" Layer

From: john e
A green-screen terminal is also a peer in the network,
but in practice it acts more as a server, and the program
running on the host is the client. E.g. the program requests
the terminal to display a format. So most of the time a
terminal takes on the server role. However, at first, when
logging into the system, a terminal initiates a session and
has the client role.

John,

It seems to me that the problem with your definition of 5250 protocol is that it fails to take into account the role of the Workstation Controller, which is server based, and acts as an intermediary between the terminal (or terminal emulator), and the application program.

My understanding is that the terminal never functions as a server; only a client. The Workstation Controller acts as an intermediary server to both the terminal and the application program. The terminal is never requested to display a format. That's the role of the Workstation Controller. The Workstation Controller generates the 5250 data stream for the terminal, and the terminal displays it.

The terminal is never in control of the conversation. The Workstation Controller is. It can say, "Your session is expired", for example.

So I view 5250 as a client-server protocol, comparable to HTTP. The main difference is that the "intelligence" of the browser is vastly superior.

Nathan.



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