The Discordant Opposition Journal Issue 6 - File 4
:Networks and Network Protocols:
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A local area network (LAN) is a data communications network spanning a limited geographical area a few miles at most. A LAN allows users to share information and computer resources. A LAN is made up of the following components:
- Network interface cards that fit inside the connected computers
- Cable to connect these computers together
- Protocol software to move data from computer to computer
- User interface software to connect the user and the network
- Operating system software to provide access to resources like files and printers
- End-user applications
Once a network spans more than a few miles, such as a campus environment, it can be referred to as a metropolitan area network (MAN). A MAN spans a region such as a city. A wide area network (WAN) brings companies into the sphere of networking by connecting computers in the entire enterprise, which may span over several cities, states, and countries. Although LANs were originally designed to share expensive printers and mass storage devices, networks have evolved into essential communications media.
A LAN protocol is a set of rules for communicating between computers. Protocols govern format, timing, sequencing, and error control. Without these rules, the computer cannot make sense of the stream of incoming bits. But there is more than just basic communication. Spose you plan to send a file from one computer to another. You could simply send it all in one single string of data. Unfortunately, that can cause problems. One potential problem with sending a file as a single string of data is that it would stop others from using the LAN for the entire time it takes to send the message. This would not be appreciated by the other users. Then, the blokes come and kick your ass. This is not a good thing.
Another potential problem with sending a file as a single string of data is that, if an error occurred during the transmission, the entire file would have to be sent again. To resolve both of these problems, the file is broken into small pieces called packets and the packets are grouped in a certain fashion. This means that information must be added to tell the receiver where each group belongs in relation to others, but this is a minor issue. To further improve transmission reliability, timing information and error correcting information are added.
Because of this complexity, computer communication is broken down into steps. Each step has its own rules of operation and its own protocol. These steps must be executed in a certain order, from the top down on transmission and from the bottom up on reception. Because of this hierarchical arrangement, the term protocol stack is often used to describe these steps. A protocol stack, therefore, is a set of rules for communication, and each step in the sequence has its own subset of rules.
A protocol is basically a set of rules followed by software that resides either in a computer's memory or in the memory of a transmission device, like a network interface card. When data is ready for transmission, this software is executed. The software performs different operations to follow the rules of a protocol. At the sending computer, the software prepares data for transmission and sets the transmission in motion. On the receiving end, software takes the data off the wire and prepares it for the computer by taking off all the information added by the transmitting end.
There are a lot of protocols, and this often leads to confusion. For example, there are four major protocols from four different vendors: Internetwork Packet Exchange/Sequenced Packet Exchange (IPX/SPX) [Novell], Network Basic Input/Output System (NetBIOS) [IBM], NetBIOS Extended User Interface (NetBEUI) [Microsoft], and DECnet [DEC].
Since the transmitter and the receiver have to use the same protocol, these four systems cannot talk directly to each other. And even if they could directly communicate, there is no guarantee the data would be usable once it was communicated. Anyone who's ever wanted to transfer data from an IBM-compatible personal computer to an Apple Macintosh computer realizes that what should be a simple procedure is definately not.
These two popular computers use very different file systems. That makes exchanging information between them impossible, unless you have translation software or a LAN. Even with a network, file transfer between these two types of computers isn't always easy. If two types of personal computers cant communicate easily, imagine the problems occurring between PCs and mainframe computers, which operate in vastly different environments and usually under their own operating software and protocols.
The original IBM PC's bus transmits data eight bits at a time. The 386 and 486 PCs have 32-bit buses, and mainframes have even wider buses. This means that peripherals designed to operate with one bus are incompatible with another bus, and this includes network interface cards (NICs). Similar incompatibilities also exist with software. Unix-based applications cannot be used on PCs operating under MS-DOS. Resolving some of these incompatibilities is where protocol standards fit in.
A protocol standard is a set of rules for computer communication that has been widely agreed upon and implemented by many vendors, users, and standards bodies. A protocol standard should allow computers to talk to each other, even if they are from different vendors. Computers don't have to use an industry-standard protocol to communicate, but if they use a proprietary protocol then they can only communicate with equipment of their own kind. Access method is the term given to the set of rules by which networks arbitrate their use. It is the way the LAN keeps data packets from crashing into each other on the network.
On a network, if two or more people try to send data at exactly the same time, their signals will interfere with each other, ruining the data being transmitted. The access method prevents this. Although many LAN standards exist, three major ones are most often used today. They are Ethernet, Token Ring, and Arcnet.
These standards are best known and best distinguished for the access methods they employ. Actually, these are wider-ranging standards that use particular access methods. They also define other features of network transmission besides the access method, such as the electrical characteristics of signals, and the size of data packets sent.
Ethernet
Ethernet is the most common network access method. It was developed by Xerox in the mid-1970s. It describes data transmission at 10Mbps using the CSMA/CD protocol. Ethernet gained its popularity in engineering, scientific, and university environments. The Ethernet access method is Carrier Sense Multiple Access with Collision Detection (CSMA/CD). This is a broadcast access method, which means every computer "hears'' every transmission, but not every computer "listens'' to every transmission.
Here's how CSMA/CD works. When a computer wants to send a message it does, no questions asked. The signal it sends moves up and down the cable in every direction, passing every computer on the network. Every computer hears the message, but unless the message is addressed to it, the computer ignores it.
Only the computer to which the message is addressed recognizes the message and sends an acknowledgment. The message is recognized because it contains the address of the destination computer. The acknowledgment can be correctly addressed because the original message also contained the address of the sending computer.
If two computers send at the same time, a "collision'' occurs. The signals become garbled and the messages can't be understood. When this happens, each of the colliding computers "backs off'' or waits for a random amount of time, then tries to retransmit. This wait/retransmission sequence repeats until the receiving computer acknowledges it got the message. The whole process takes a small fraction of a second.
Computers can tell if a collision has occurred because they don't hear their own message in a given amount of time, which is determined by the propagation delay of the network. [The propagation delay is the time it takes for a signal to travel to the end of the network and back.] Remember, messages move up and down the network in all directions. Every computer hears every message, even its own messages.
Ethernet's detractors characterize it as an inefficient access method because packets are prone to collisions. But while collisions occur, they don't mean very much in most cases. Since the whole process of transmitting, colliding, and retransmitting takes place so quickly, the delay a collision causes is minuscule. Of course, if the traffic approaches 35 percent of the total bandwidth, the number of collisions will mount and the network will slow considerably. This happens with some large-scale imaging or engineering applications, or network segments with too many nodes. Few Ethernet networks, however, have a traffic load of more than 10 to 20 percent, which means delay caused by collisions is seldom noticeable.
Token Ring
When Token Ring was introduced in 1984, it was not the first token-passing, ring network, but because it was endorsed by IBM, it has had a tremendous impact on the network industry. Token Ring is part of IBM's ultimate connectivity solution for all its computersópersonal, midrange, and mainframe. IBM's specifications follow those of the IEEE 802.5 standard.
Unlike the Ethernet market, IBM has a stranglehold on the Token Ring market, and supplies most of the network interface cards. The original Token Ring transmits data at 4Mbps; the newer specification calls for 16Mbps transmission. In Token Ring, the computers are arranged in a logical ring, but all data passing between work stations is routed through a hub. A multistation access unit (MAU) acts as the hub, and each work station is connected to it.
Token Ring uses a token-passing access method to prevent data collisionsóa token being a series of data bits created by one of the computers. The token moves around the ring, giving successive computers the right to transmit. If a computer receives an empty token, it may fill it with a message of any length as long as the time to send does not exceed the token-holding timer [this combination of token and data is called a frame].
As this message [frame] moves around the network, each computer regenerates the signal strength. Only the receiving computer copies the message into its memory, then marks the message as received. When the frame circulates back around, the sending computer removes the message from the frame and passes the empty token to the next computer on the ring.
Because each computer looks at the message and may act on it, a computer can perform certain tests to make sure the message is getting through correctly. Also, since the frame is copied and marked, rather than deleted, the sending computer can determine if the destination computer exists and if the message was received, by looking at the message when it comes back around.
Token Ring networks have a priority mechanism whereby certain computers can get the token faster than others. This important feature enables the network to give file servers priority over workstations. Token Ring's advantages include reliability and ease of maintenance. It uses a star-wired ring topology in which all computers are directly wired to a MAU. The MAU allows malfunctioning computers to be disconnected from the network. This overcomes one disadvantage of token-passing, which is that one malfunctioning computer can bring down the network since all computers are actively passing signals around the ring. In Token Ring, malfunctioning computers are simply disconnected by unplugging them from the MAU.
Arcnet
Arcnet was developed by Datapoint in the early 1970s. It is especially popular in very small networks, since it is inexpensive and easy to maintain. Arcnet uses a token-passing access method that works on a star-bus topology. Data is transmitted at 2.5Mbps. The network cable is laid out as a series of stars, each computer is attached to a hub which is the center of the star and the hubs are connected in a bus or line. When a computer wants to send data on an Arcnet network, it must have the token. The token moves around the network in a given pattern, which in Arcnet's case is a logical ring.
All computers on the network are numbered with an address from 1 to 254. The token moves from computer to computer in numerical order, even if adjacent numbers are at physically opposite ends of the network. When the token reaches the highest number on the network it moves to the lowest, thus creating a logical ring. Once a computer has the token it can send one 512-byte packet. A packet is composed of the destination address, the sending computer's own address, up to 508 bytes of data, and other information. The packet moves from node to node in sequential order until it reaches the destination node.
At the destination, the data is removed and the token released to the next node. Since one packet is often too small for an entire message, the token may need to make several rounds to complete a message or data transfer. The advantage of token passing is predictability. Because the token moves through the network in a determined path, it is possible to calculate the best and worst cases for data transmission. This makes network performance predictable.
It also means introduction of new network nodes will have a predictable effect. This differs from Ethernet, where the addition of new nodes may or may not seriously effect performance. However, a predictable network can be misleading ó for example, lost tokens will affect worst-case delivery times. A disadvantage of the token-passing access method is the fact that each node acts as a repeater, accepting and regenerating the token as it passes around the network in a specific pattern. If there is a malfunctioning node, the token may be destroyed or simply lost, bringing down the whole network. The token must then be regenerated.
Appletalk
The Apple Macintosh operating system, as its many supporters will tell you, is extremely easy to learn and use. Apple's AppleTalk network system brings the same kind of simplicity of use to Macintosh connectivity. Although not an official LAN standard, AppleTalk can be considered a de facto standard: With AppleTalk connectivity options built into each and every Macintosh, millions of Macs possess ready-made networking capabilities. This has not been lost on Mac aficionados, who have used AppleTalk to link thousands of Macs into efficient, cost-effective LANs. When you consider the kind of work performed by the typical Macintosh user, it's not surprising that Mac users have readily accepted networking.
Apple calls AppleTalk "a comprehensive network system" made up of hardware and software components. An AppleTalk network can consist of many different kinds of computer systems and servers and a variety of cabling and connectivity products. To support a variety of machines, Apple developed a suite of proprietary protocols that permits communication between the varying devices that users might need to attach to an AppleTalk network. However, AppleTalk is not a network operating system, a medium access control (MAC) method such as Ethernet, or a cabling system (LocalTalk is a trade name of Apple's cabling system). Rather, AppleTalk is a nonstandard suite of protocols that, while not fully compliant, still provides most of the functions spelled out by the International Standards Organization's Open Systems Interconnection [OSI] reference model.
The six-layer suite of AppleTalk protocols supports numerous connectivity options, including LocalTalk, Ethernet, and Token Ring. This set of protocols allows connections of virtually any computing device to an AppleTalk network. Here's how it works. Since AppleTalk's 1983 release, Apple and other third-party vendors have developed data link protocols to support Ethernet, Token Ring, and Arcnet networks, which exchange data at 10Mbps, 4Mbps, and 2.5Mbps respectively, all faster than LocalTalk's 230.4Kbps rate.
Despite its relative lack of performance, LocalTalk offers one major benefit these faster technologies lack: Every Macintosh computer that Apple has manufactured contains the built-in LocalTalk connection. LocalTalk, like Ethernet, uses a Carrier Sense Multiple Access (CSMA), medium access scheme to place data packets on the network wire. It does not rely on collision detection (CSMA/CD), as does Ethernet. It uses CSMA/CA, for Carrier Sense Multiple Access with Collision Avoidance. Stations on a CSMA/CA network, rather than sensing collisions between data packets sent by multiple stations, send out a small (three-byte) packet that signals their intent to place data on the wire. This packet tells all other stations on the wire to wait until the signalling node's data has been sent before they attempt to send data. If collisions between packets are going to occur, they will occur between the preliminary packets, not the actual data packets. This best effort packet-delivery system, managed by LLAP, does not guarantee that the packet reaches its destination, but it does ensure that all packets delivered are free of errors. The LLAP provides the data link access specifications and uses a dynamic address-acquisition method that enables AppleTalk's plug-and-play capabilities over twisted-pair wiring.
LocalTalk, though convenient, suffers from other limitations besides its slow data-transfer rate. For example, LocalTalk workgroups are limited to 32 nodes over a 1,000-foot cable run. Ethernet and Token Ring both support substantially greater numbers of nodes. The EtherTalk, TokenTalk, and Arcnet Link Access Protocols (ELAP, TLAP, and ALAP, respectively) manage AppleTalk network access to Ethernet, Token Ring and Arcnet networks. Apple developed EtherTalk and TokenTalk as extensions of the two protocols' industry-standard data link processes. Standard Microsystems developed ALAP.
One of the key responsibilities of ELAP, TLAP, and ALAP is mapping AppleTalk addresses into the standard data link Ethernet, Token Ring, or Arcnet address required for proper routing of data. Because the Ethernet, Token Ring, and Arcnet addressing schemes are incompatible with LLAP, AppleTalk node addresses must be translated into the appropriate format; the AppleTalk Address Resolution Protocol (AARP) handles this translation.
One layer up in the AppleTalk stack is the Datagram Delivery Protocol (DDP). The DDP works with the Routing Table Maintenance Protocol (RTMP) and AppleTalk Echo Protocol [AEP] to ensure data transmission across an internetwork. The DDP exchanges data packets called datagrams. Datagram delivery is the basis for building other value-added AppleTalk services, such as electronic mail. The DDP permits running AppleTalk as a process-to-process, best-effort delivery system, in which the processes running in the nodes of an interconnected network can exchange packets with each other.
The DDP provides these processes with addressable entities called sockets, and processes can attach themselves to one or more sockets in their nodes. The RTMP provides the logic that routes datagrams through router ports to other networks; it permits routers to dynamically learn routes to other AppleTalk networks in an internet. The AEP lets nodes send datagrams to any other nodes and to receive a copy, or "echo," of the datagram sent. This confirms the existence of a node and helps measure round-trip delays.
The ATP directs the AppleTalk transaction processes, in which sockets issue requests that require response (typically, status reports). ATP binds the request and response to guarantee a reliable exchange. The PAP sets up a connection-oriented service that sends print requests to AppleTalk-compatible printers. ASP opens, maintains, and closes transactions during a session, while ADSP provides a full-duplex, byte-stream service between any two sockets on an AppleTalk Internet.
At the highest layer of AppleTalk are the AppleTalk Filing Protocol [AFP] and the PostScript protocol. The AFP, built on top of ASP, permits users to share data files and applications on a server, while PostScript, a programming language understood by Apple's LaserWriter and numerous other output devices, provides a standard way of describing graphics and text data.
The AFP, which conducts the dialog between a user's computer and an AppleShare server, is one of the key AppleTalk protocols. AFP was designed to provide the tools that allow supporting different types of computersóthat is, Macs and IBM PCsóover an AppleTalk network. The AFP is also important because any network operating system that is fully compatible with it can operate transparently on any AppleTalk network. In turn, this means that such a NOS can support all AppleTalk-compatible applications.
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