Simplifying Network Messages Or Who said that? And what does it mean?
When all goes well, APL timesharing is just the dialogue between you at your terminal and the APL system to which you're connected. But, when problems arise, you get messages. The messages originate at the various waysta-tions between you and APL, or from APL itself. Once you understand where they originate, it's much easier to understand what they mean.
This winter (for our northern hemisphere clients; summer down under) we are introducing some changes in the messages that originate in the SHARP APL Network. This article first explains where messages come from, and then lists and explains the messages.
What is the network?
With the SHARP APL Network, users in different parts in the world can reach APL by making a local phone call. The network consists of a set of minicomputers with at least one minicomputer located in each city where I.P. Sharp has local telephone access. The minicomputers are linked together by cables. Usually the cables are private lines leased from the various telephone authorities.
Each minicomputer is a node of the network. A node is connected to at least one other node, and sometimes to several. Each node is able both to communicate with its neighbouring nodes, and also to accept calls from individual terminals.
To link your terminal to a network node, you may dial the number of a phone directly connected to the node. Alternately, your terminal may have a cable going directly to the node (thus being a "hard-wired" terminal). The device at the node which accepts a call is called a port. The minicomputer must have one port for each terminal session signed on through it.
Packet switching
One job of the node is to break the message from your terminal into segments. Each segment is called a packet. A packet is labelled to show from whom it comes and where it is destined. It also has a sequence number to help reassemble packets in the right order. At the receiving end, the node reconstructs the message from the packets it receives.
Breaking the transmission into packets makes it much easier to verify (and, if necessary, correct or repeat) individual packets. It also makes it much easier for a few longdistance links among the nodes to process the work of many different users, and hence to cut the costs of long-distance communication.
Multiple destinations
The SHARP APL Network serves only users of SHARP APL. But SHARP APL is offered at a number of different computer centres. So when you dial a node of the network, the first thing you must tell the node is which SHARP APL system you want to talk to.
Your first transmission includes the character ). Following the parenthesis, you may type letters to indicate which SHARP APL system you want. Each node has its own default rules about what to assume if you just type ) but don't specify an APL system. (That default rule permits the majority of users of a particular node to reach the system they want without having to name it explicitly.)
Alphas and 3705s
The minicomputer at each node may be either of two types: a Computer Automation model Alpha LSI-2, or an IBM 3705. These names are universally abbreviated by the people who work with them to "Alpha" and "3705", and this article will do likewise.
Any node, be it an Alpha or a 3705, can both transmit data to the nodes neighbouring it, and handle calls originating from individual users. However, only a 3705 has the additional ability of being able to communicate with the computer (CPU) that runs an APL system.
The network concept diagram doesn't show a particular network, but illustrates the sort of connections that are possible. You can see that the Alphas (a) and 3705s are connected to each other in an arbitrary pattern, except that only a 3705 is connected to an APL system. Some large APL systems may be served by several 3705s.
Asynchronous ports are for connection to normal interactive terminals. Bisync (binary synchronous) ports are specialized ports for batch mode file transfers.
The chain from you to APL (and back)
The second diagram illustrates the various links that usually exist between you and the APL system you're using. When you first try to connect to APL, you begin by reaching a node on the network. (You may dial the node directly, or, if you link via a public network such as Tymnet, Telenet, or Datapac, you use that network to reach a node of the I.P. Sharp Network.)
The first node you reach must decide what type of terminal you're using, so that it can understand your transmissions and make intelligible replies. It has to identify both the speed of transmission (baud rate) and the data encoding. You must send it the APL character ) because that character is sent differently in the various encodings, and thus serves to reveal which one your terminal provides.
Next, the node needs to know which APL system you're trying to reach. If the APL system you ask for cannot be reached, the node sends you the message NO PATH. That's a new message; it partially replaces the message LINES DOWN.
No path may exist for either of two reasons: (1) You've asked for a real APL system, but for the moment there is no way to reach it, perhaps because of an interruption in the link from one node to another. (2) The APL system you've asked for doesn't exist—perhaps because you've mistyped the name. The node you reach doesn't have a way of distinguishing these two situations; it knows only that it can't place a call to the system you've asked for, and so it says NO PATH.
A node may also send out the message LINES DOWN. With the change, it will do that only if it has successfully connected you to an APL system, but then finds that it has been cut off by a communications failure somewhere between it and APL.
When your local node is able to address a call to the APL system you want, your message is sent to a 3705 serving that CPU. The 3705 passes on to the APL supervisor your request to sign on.
The 3705 may not be able to reach the APL supervisor. In some cases this is because the 3705 itself is swamped and can't accept more calls. You then get the message APL FULL. Or it may be that APL isn't running, in which case you get APL DOWN. If you have successfully signed on and subsequently the 3705 notices that APL has been terminated (and has reset its links to the 3705) the 3705 sends you the message APL RESET. You are no longer connected to the system.
Messages that originate in the 3705 may be followed by a slash and an abbreviation intended to help the staff diagnose the problem. These needn't concern you (save to note that the infamous AWAITING FUNCTION FOUR of former years now appears as APL DOWN /F4).
Messages from the APL system itself
You can divide APL into approximately two parts: the supervisor that manages the APL service, and the interpreter that actually does your work after you've signed on. The following messages are sent by the supervisor. When you see one of these, you know that your call has reached an APL system.
INCORRECT SIGN-ON: means that the first transmission the supervisor receives doesn't match the required format. Your sign-on must be in the form:
)1234567 :PASSWORD
where 1234567 and PASSWORD represent your user number and password respectively. The parenthesis and colon are obligatory; embedded blanks are forbidden. You usually get INCORRECT SIGN-ON: when the sign-on includes blanks, the user number includes non-numeric characters, or you've omitted the colon.
APL NOT AVAILABLE is a message that the supervisor sends during part of its start-up procedure. It means that APL is indeed running, but isn't yet ready to accept sign-ons. APL should be ready soon.
PATIENCE PLEASE is sent when you've been using the system and have been disconnected. Possibly your terminal has been disconnected or network problems have prevented your signing off. When you try to sign on again, on the same account number, you receive this message. You will be able to sign on shortly.
NUMBER NOT IN xxx SYSTEM arises either because you aren't enrolled at all, or because the password you've supplied is incorrect. (For xxx, substitute the name of the particular system you've reached.)
NUMBER LOCKED OUT OF xxx SYSTEM means that the operators have locked your account to prevent anyone's signing on.
There are two messages that the APL supervisor can send both before and after you've signed on. They are ENTRY ERROR and RESEND. ENTRY ERROR arises when the supervisor is unable to decide what APL characters you intended, either because you have sent tab characters without defining tab positions, or because you have constructed overstrikes that don't form valid characters. RESEND indicates a transmission that the supervisor is unable to handle, usually because of its extreme length, or, rarely, because of a problem in transmitting it.
Once your call has passed through a node, a 3705, and the APL supervisor, and you've signed on to an APL system, at last you're free to make APL mistakes, and receive messages such as VALUE ERROR, SYNTAX ERROR, and so on •
Paul Berry, Palo Alto
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Summary of New Message
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New Error Message |
Origin
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Meaning |
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NO PATH |
Alpha |
SHARP APL cannot be reached. |
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LINES DOWN |
Alpha |
After you are successfully connected to an APL system, there is a communications failure between the node and APL. |
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APL DOWN |
3705 |
APL isn’t running on the host computer that the 3705 is connected to. |
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APL FULL |
3705 |
The 3705 cannot accept any more calls. |
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APL RESET |
3705 |
You are successfully connected to an APL system, but APL has been terminated. |
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JANUARY/FEBRUARY 1983—VOL.11 N0.1