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"L" Networking Definitions & Concepts...

LAN (Local Area Network) .. to .. LocalTalk

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LAN (Local Area Network):

A network in one location. The size of the location can vary; for example, a network on one floor of an office building is a LAN. So is a network connecting all the buildings of a college campus. Networks connected by modems and telephone lines, however, are not LANs. Thus, a network connecting buildings in different cities is not a LAN (although the networks in each of the buildings probably are). Contrast WAN (Wide Area Network).

A LAN is a collection of two or more computers that are located within a limited distance of each other and that are connected to each other, directly or indirectly. LANs differ in the way the computers are connected, in how information moves around the network, and in what machine (if any) is in charge of the network. The figure below "Context and properties of a LAN" summarizes some of the features of LANs.

The computers in a LAN may be PCs, Macintoshes, minicomputers, mainframes, or machines with other architectures. However, there are restrictions and the combinations that are feasible and sensible.

LAN Security:

Connectivity and flexibility of a local area network should not be accomplished at the expense of security. If data and user communication is allowed to be accidentally or intentionally disrupted, then the LAN loses its integrity.

This LAN should have provisions for ID and password security mechanisms. File security should be enforced with the use of read, read-write, execute, and delete attributes. Additionally, virus detection mechanisms should be always in place, at every workstation.

Security should also be extended to hardware devices attached to the network. The LAN should be able to restrict access to hardware devices to only those users that have proper authorization.

LAN (Typical hardware components):

There are two major items that must be considered when planning or installing a LAN:

Network hardware components and network software:

There are three major categories of devices that make up the hardware components of a local area network. These are:

  • The Server,
  • The LAN communication system, and
  • The Workstations.

  • Servers--

  • Servers are computers on the network that are accessible to network users. They contain resources that they "serve" to users that request the services. The most common type of server is a file server (see file server). Most LANs have at least one file server, and they often have multiple file servers.

    If a LAN relies on the server for all of its functions, then this type of technique is called a dedicated server approach. Other LANs do not require a distinction between a user workstation and the file server. This type of approach is called a peer -to-peer network. In a peer-to-peer network any microcomputer can function as the file server and user at the same time, sometimes called a nondedicated server. An example of the peer-to-peer network is AppleTalk via AppleShare.

  • Workstations--

  • The typical LAN workstation is a microcomputer. Terminals can also be used to communicate on a LAN, but the cost of a personal computer is usually low enough to be justifiable, since a complete computer with increased capabilities is obtained. Once the microcomputer is connected to a LAN, it is used in similar fashion to a microcomputer in stand-alone mode. The LAN usually replaces the locations from where files and applications are retrieved. Some LANs, such as those that use Novell NetWare, can have workstations from different vendors, such as IBM and Apple. Users of NetWare can attach an IBM PC or clone and a Macintosh and use their machines the same way they used them in their stand- alone configuration.

    The responsibility of the PC or MAC workstation is to execute the application served by the LAN file server. On most LANs the workstation typically does the processing. On distributed LAN networks, the file server and the workstation can share the processing duties. This scenario is typically found on LANs dedicated to database functions only.

    After an application is served to the MAC or PC workstation, the application begins execution. During the execution of the program, the user may want to store a file or print a file. At this point the user has two options. To save a copy of the file, the user can save it on a hard disk or floppy disk local to the workstation that they are using. The other option is to save it on the file server's hard disk. In the latter case, the file could be made available to all other users on the LAN, or kept for private use by using file security attributes. If the user decides to print the file, it can be sent to a printer attached to the server, or printed locally if the workstation has a printer attached to it, or to a network printer.

  • LAN Communication System --

  • When two or more computers are connected on a network, special cable and a network interface board or card (NIC) are required in each computer and server. The cable is used to connect the network interface board to the LAN transmission medium. Most microcomputers are not equipped with an interface port that can be connected to a second microcomputer for networking purposes (except the Macintosh series of computers that has a built-in LocalTalk port). As a result, a network interface card or network adapter must be installed in the microcomputer. There are many different types and brands of NICs, but each performs the same function. It transmits data between computers at a high speed.

    The speed of transmission will depend on the type of medium, the capabilities of the computers network interface, and the computer that it's attached to. Typical speed ranges for LANs are from 230Kbps to 16 Mbps and a few are even higher. However, since the workstations in the LAN are connected by a cable, the geographic range the LAN can cover is limited to buildings or campuses where the cable can be laid.

    Data is transmitted from a workstation to a file server and vice-versa by packetizing it. When a file is requested from the file server, the NIC translates this file into data packets. Normally, the data packets are of fixed size, although they could be different sizes. Most adapters use packets of 500 to 2,000 bytes. AppleTalk packets very in length from a few bytes to several hundred. The file server's NIC places the data packets on the network bus (cable), where they are transmitted to the workstation NIC. Here the data packets are assembled back into the original data files and given to the workstation.

    Each data packet contains the address of the workstation on the network that is to receive the data packet. The address of each node in the LAN is provided by the NIC. This address can be set with switches on the NIC when it is installed. Some NICs already have the address set at the factory before they are shipped to a customer (such as ethernet cards).

    The PC NIC address's uses a combination 8 bits, and therefore can have a value of from 1 to 256. This limits the number of users on the this type of LAN to 256. AppleTalk (Phase 2) internet supports up to 65,279 network addresses, or over 16 million possible node addresses. A node is a device such as a computer or printer on the LAN.

LAN (Typical installation):

First step is to install the NIC card for PC's or Mac's(usually an ethernet card) in each microcomputer that is going to be part of the LAN. The NIC is installed in one of the expansion slots inside the machine. If the NIC's address was set at the factory, the NIC can be installed as is (all ethernet cards/devices have an address given to theme). Otherwise, a set of dip switches on the NIC must be set to a combination that has not already been used on the LAN. It is suggested that each NIC on the network follow a sequence. Then if something goes wrong during the operation of the network it will be easier to identify problems.

After the NIC is installed, each microcomputer must be connected to other microcomputers on the LAN.

Lempel-Ziv Compression:

Lempel-Ziv compression methods compress data by taking advantage of the repetition of strings in data sources. Lempel-Ziv methods are adaptive, universal compression methods frequently implemented in disk and tape drive systems due to their simplicity and versatility. Typically, Lempel-Ziv compression can compress text, executable, and similar data files to approximately half of their original size. Lempel-Ziv performs very well when presented with extremely redundant data files, such as tabulated numbers and computer source code.

The Lempel-Ziv algorithm is a variable-to-fixed length code. Basically, there are two versions of the algorithm presented in the literature: the theoretical version and the practical version. Theoretically, both versions perform essentially the same. However, the proof of the asymptotic optimality of the theoretical version is easier. In practice, the practical version is easier to implement and is slightly more efficient. We explain the practical version of the algorithm as explained in the book by Gersho and Gray and in the paper by Welch.

Lempel-Ziv Coding
(zip codes, etc)

Want to encode a string:   aaabcababcaaa  into 0's and 1's.

Step 1:  Convert to 0's and 1's by any prefix substitution.



Step 2:  Parse the string into "never before seen" strings.



Step 3:  Assign binary number labels to each.



Step 4:  To each string, assign label of its substring plus last bit.


Step 5:  Store this string plus length of labels in bits.



This may be inefficient for small examples but for very

long inputs, it achieves the entropy for the best model.
Local Schemas: (DBMS & DDBMS)

Each local DBMS has its own set of schemas. The local conceptual and local internal schemas correspond to the equivalent levels of the ANSI-SPARC architecture. The local mapping schema maps fragments in the allocation schema into external objects in the local database. It is DBMS independent and is the basis for supporting heterogeneous DBMSs.


The name for Apple Computer's low-cost connectivity product (hardware) consisting of cables, connector modules, cable extenders, and other cabling equipment for connecting computer and other devices to a network. LocalTalk was formerly called the AppleTalk Personal Network Cabling System.

Sometimes the term LocalTalk environment is used to describe the capabilities of an AppleTalk network. A LocalTalk environment is merely an AppleTalk network connected with LocalTalk cabling and connectors. Calling the system a LocalTalk environment differentiates if from an AppleTalk network connected with some other type of media and having different capabilities, such as EtherTalk.

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This page was last updated on 09/18/2005
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