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This article contains a series of questions based on material in "Inside AppleTalk" with answers. References to "Inside AppleTalk" are from the March 1989 edition (ISBN 0-201-19257-8), published by Addison-Wesley.
DISCUSSION What's a Protocol Stack Question: "Inside AppleTalk" suggests that a protocol stack is an implementation of a protocol suite, like AppleTalk for VMS or AppleShare PC. Most other literature refers to a protocol stack as a subset of the OSI model. An example is TCP, which does not include layers 6 and 7 or selected protocols with AppleTalk. Which is more accurate? The implementation idea or the subset idea? Answer: Protocol stacks often implement only a portion of the OSI 7-layer model. A protocol stack is a suite of protocols that implements the feature set for a particular networking scheme. The term "protocol stack" typically refers to the actual implementation of a networking scheme. TCP/IP's protocol stack only implements the layers that it needs, whereas AppleTalk implements all of the layers. The OSI Model and Protocols Question: Often, I have heard (and saw documented in "Inside AppleTalk") that the OSI model is a general description of protocol usage and not the protocols themselves. Yet page I-22 of "Inside AppleTalk" cautions that AppleTalk is not necessarily compatible with the forthcoming OSI protocols. What are these forthcoming protocols? What is OSI, if not a reference to follow for integration into heterogeneous networks? What protocols for OSI have already been established? Are these based on CCITT protocols like X.25 and X.400? (It seems to me that although we have protocols that match every layer of OSI, we are only loosely tied to those layers. Is this so?) Answer: The OSI 7-layer model has become a standard in the industry for describing network architectures. This model was originally designed as a blueprint for the OSI network protocols that were and still are in the process of being implemented. Many of the lower layers have already been implemented, and there are even products available for the Macintosh that provide connectivity solutions to these protocols. The press has often described the OSI architecture as the replacement for TCP/IP. Although we have a network architecture that fits nicely within the OSI model, AppleTalk cannot be considered an implementation of OSI any more than it can be considered an implementation of TCP/IP. We use the model, because it provides a concise and elegant way to describe our network. We do not use the OSI architecture, because many of its layers remain undefined. AFP an Application? Question: "Inside AppleTalk" shows AFP to be mostly Presentation and part Application. What aspects of AFP are Application? Wouldn't AppleShare File Server and AppleShare Workstation be applications? Answer: We are not sure why AFP is listed as falling partially into the application layer. We assume that it has to do with the fact that the description of AFP in later chapters includes actual network services. AFP does define many of the features that need to be implemented in AFP servers. If this is the reason AFP falls partially in this layer, then AppleShare implements the highest part of the application layer, putting the interface on the services that have, at least, been partially defined by AFP. CMSA/CA Question: I understand that CSMA has at least two flavors: 1-persistent and non-persistent. 1-persistent is when a deferring sender grabs the channel as soon as it's released, contending with another sender who grabs the channel at the same time. Non-persistent throws in a random period of time to back off before attempting to grab the channel. Is this what we call CSMA/CA? Or, is CA something different? Or, is it non-persistent including handshaking? (See page 1-3, "Inside AppleTalk.") Answer: The CA in CSMA/CA stands for "Collision Avoidance". It does implement the random period of time you mentioned as opposed to collision detection (CD). Ethernet throughoutput Question: They say that the 10MBps of Ethernet is only used under heavy loads and that its throughput does not come through at that rate. What does this mean? Are nodes communicating at 10MBps or not? Is Ethernet a broadband network? How are the channels within the broadband used? (You can refer me to some other documentation on that last one.) Answer: "Throughput" is a term used to describe the actual rate of transfer achievable over a network. This is a relative term, similar in reliability to "MIPS" or "megaflops." Network throughput is often measured by copying large files across a network and calculating the amount of data transferred in a specified period. As you can see, this throughput depends on the overhead of the protocols being used and the capability of the servers to transmit data to the client. The 10MBps specified for Ethernet refers to the ideal maximum possible over coaxial cable and is never achieved in real-world situations. Standard Ethernet coax is baseband cable, not broadband. However, these days, you can run Ethernet over many different media, including broadband cable, unshielded twisted-pair telephone wiring, T1 telephone circuits, satellite links, and leased lines. These have different speeds and characteristics. Bridge Question: A bridge is a datalink layer device only. It separates networks but does not use network numbers. Right? Answer: A bridge is used to connect two physical networks of the same type into one logical network. As far as any node on the network is concerned, this is a single network. Router Question: A router is a higher-level device (either transport or network layer) that routes like-protocols from one network to another. The networks on either side are identified by their network numbers. Like-protocol routing includes AppleTalk-to-AppleTalk, TCP/IP-to-TCP/IP, SNA-to-SNA, and so on. Right? Answer: A router is used to divide two physical networks into separate logical networks using the same protocols. Each logical network has its own unique network identifier(s) distinguishing that network from other networks on the same Internet. Networks and Bridges Question: AppleTalk is a network protocol suite. A network is a specified physical cable and datalink, like Ethernet and Token Ring. A physical network is a cable. Are these accurate? Does a bridge create an Internet? Are the segments on either side of the bridge networks or just physical networks? Are physical networks and segments synonymous? Answer: The physical network, or layer, typically refers to the physical medium in use. This can be cable, twisted-pair wiring, and so on. If you go with the definition above, a bridge does not create an Internet. A router provides the necessary functionality and acts at a high-enough level to create an Internet. A "segment," as you are using it, is a piece of cable or set of wiring that is part of a physical network. This usually means as the physical medium on one side of a bridge or router, but it could be a smaller part. |
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| Document Information | |
| Product Area: | Communications-Networking |
| Category: | MacX25 |
| Sub Category: | General Topics |
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