Macintosh Computers
Tangle-Free Networks
 

Paquetes Interactivos S.A. of C.V.

Ing. Pedro Baram K.
General Manager

How to avoid growing pains when the network needs to be expanded.

Some Macintosh computers begin to appear in a department; a few more begin to appear on the other side of the hallway. Soon all PC and Macintosh users begin to demand the need to exchange files and email.

Connecting Macintosh computers to a network is an easy and intuitive task since the entire line of Macintosh computers includes as an integral part of the hardware the possibility of connecting them to a network to share both printing and disk storage resources. This standard connectivity between Macintoshes is called AppleTalk. To connect two or more Macintosh computers together, you only need to install an Apple Talk connector in the serial port corresponding to the printer of each of the network nodes. Apple sells its own connectors and cables for the connection or these can be purchased from other manufacturers, who, unlike Apple, use twisted pair cable for the connection, which is much cheaper.

Designing a functional network of microcomputers requires knowledge and intuition. Modifying it later can become a real nightmare. Careful planning based on proper configuration can help avoid user discomfort and even improve network performance.

As more and more users begin to send email and access printers and file servers, networks begin to degrade. Sometimes they just stop working. Frustrated users have to rediscover the phone and floppy disks. Productivity fades. This is when it becomes necessary to identify bottlenecks in the network. In this article we identify some of the most common network problems and offer solutions, many of them at a relatively low cost.

Topological Maps

The topological map of the network has a definitive effect on the performance of the network. This is because the topology of the network affects how the electrical signals representing data are transmitted through the wires, and the speed at which these signals can degrade or even be lost.

There are three basic topologies to design a network: Daisy connection, connection using a back bone and finally, star connection. Each of them is the most appropriate depending on the number of nodes and the total distance of the network cabling, as well as the available budget.

Daisy Chain Connection

In the Daisy Chain configuration, each node on the network has a network connector installed. There is not a single cable but rather a length of cable that connects one connector to the next connector in the network. AppleTalk connectors and cables or Phone Net connectors and twisted pair cable can be used to create the connection. Figure 1 shows this type of connection.

Apple Computer's AppleTalk connectors can handle networks with this configuration of up to 32 nodes in a network whose total cabling does not exceed 1,000 feet in length. PhoneNet connectors when used in this configuration allow maximum lengths of 1,800 feet, as long as the number of nodes does not exceed 24. PhoneNet connectors require the installation of a terminator (resistor) at each end of the network.

The daisy chain connection is extremely simple. Simply install a connector on each node of the network and join these connectors together using the appropriate cable to have the network up and running. However, this topology has the drawback that when an intermediate node is voluntarily or involuntarily disconnected from the network, it is interrupted.

Connection via Back Bone

In this type of connection, a single cable called Back Bone is used throughout the entire network. Each of the network nodes is connected to this cable as shown in figure 2.

It is customary to pass the Back Bone between the walls of the office where the network is installed, connecting the Back Bone to telephone panels installed on the wall to which each of the nodes is connected through a section of cable and a terminated connector. The maximum number of nodes in this topology is 48.

The gauge of the cable used is decisive in the maximum length that the network can have. The most common gauges are 22, 24 and 26. The diameter of the cable is particularly important since the thinner the cable, the greater the resistance encountered by the signal and therefore the less distance it can travel before degrading. . Likewise, the smaller the gauge of a cable, the larger its diameter. Thus, using this topology, the total length that the network can have is 4500, 3000 and 1800 feet using 22, 24 and 26 gauge cables respectively.

Generally, the installation of a Back Bone requires modification of the building since it involves hiding the cable and installing telephone plates in the wall. Therefore, if not planned in advance, its use is frequently restricted to small work groups.

Star Connection

The star topology brings together a group of nodes at a central point, usually the room where the telephone switch is located. Each computer on the network connects to a telephone plate in the wall, taking advantage of using a twisted pair left over from the telephone wiring. At the central point, all the network cables are joined into a star that can be active or passive.

The basic difference between the two types of star is that the passive star is equivalent to a single cable like the Back Bone but in the shape of a star, while the active star is a series of cables connected to the center, but electrically isolated from each other. , allowing each ray of the star to act as an independent network.

Passive stars are limited and often offer more disadvantages than advantages. Since the star's rays are not electrically insulated, the total length of the cable is limited by its gauge. Additionally, each additional ray reduces the signal strength. No more than 16 nodes are recommended in a 3-ray network on a passive star with a total length of 1800 feet, while a network using Back Bone topology of the same length can support up to 48 nodes.

Active stars require the installation of a controller that electronically increases the maximum distance to the network. The star controller is installed in the room where the telephone switch is located and isolates one ray from the other, forming each of them an electronically independent network. The active star topology is more reliable and allows for greater maximum distances. The maximum length of each beam depending on the gauge used is approximately 3000 feet. Figure 3 shows a typical connection using an active star.

Problems with the network

Once the network topology is defined, it will be installed and users will proceed to connect. We will soon start to notice problems.

The first of them will be that some machines do not access network services. This is because they are poorly connected (loose cables or connectors) and it will be necessary to check their physical connection.

Some machines will appear unstable on the network. That is, on occasions they will be able and on other occasions they will not be able to access the services provided by the network. This is generally due to one of two reasons: The first may be a false contact in the connection that will need to be checked to eliminate the problem. The second could be due to having exceeded the recommended limits on the length of the network. In these cases, a signal repeater can be used that will increase the maximum recommended length.

Utilities such as GraceLAN from Technology Works and Inter Poll from Apple Computer's allow you to test if a certain node on the network is having problems. The test consists of sending an echo packet from the machine that is running the utility, to the machine or printer that is having problems. If 2 or 3 percent of the echo packets do not return, the tested node is acceptable. If 10 percent or more of the echo packets do not return you have a problem.

To diagnose the problem, it is necessary to run one of these utilities from a computer at one end of the network. Echo packets are sent by the utility to each of the nodes in the network. The time required to return echo packets increases with distance. As we move further away, we may begin to lose contact with the node. If we exceed the maximum recommended length, it is necessary to use a signal repeater. If this occurs without exceeding the recommended length, the network is incorrectly terminated.

Terminators (electrical resistors) in the network prevent reflections of electrical signals in the network. Too few or too many terminators degrade network performance. When a terminator is missing, interfering signal reflections are created. Many terminators, particularly on a passive star, weaken the signal and make communication with more distant nodes more difficult and require shorter maximum distances to compensate. There is no need to worry about the number of terminators in a network with daisy chain topology (one at each end) or with active star topology (each ray is a separate network). However, the passive star topology considers all terminators connected to it. It is recommended not to use more than four terminators regardless of the number of rays the network contains.

Once we have confirmed that the network cabling is working properly and that the connections are solid, we will begin to notice problems caused by traffic. Some users regularly send large jobs to print or constantly use a file server. Separating this type of users in another area may be the solution. To create a zone it is necessary to have a router such as Apple Talk Internet Router from Apple Computer's or Liaison from Farallon.

Assign heavier users to the same zone as the server and/or printer they use most frequently. Group other less intense users in other areas. Once the router is installed, the traffic generated by the most intense users will not affect other users. The two groups of users can communicate with each other and share resources thanks to the work that the router does by optimizing network traffic.

Farallon's Traffic Watch II utility allows you to analyze traffic patterns in a network and determine which nodes are causing bottlenecks.

When the number of computers on the network is large and/or the volume of information transmitted through it is intense, the transmission capacity of a network based on AppleTalk (230 Kbits/sec) may be insufficient. If you want to improve the operation of the network, it will be necessary to install an EtherNet card (10 Mbits/sec) in each of the computers.

Not all Macintosh computers allow you to install a communication card. If you want to connect the old Macintosh Plus or their current replacements, the Macintosh Classic, to an EtherNet network, it will be necessary to include them using EtherNet emulators connected to their SCSI port.

Some Tips to Improve Network Performance

Keep printers and print spoolers on a separate network so that printing bottlenecks do not affect other users.

Use EtherNet to transfer large files such as images.

Use routers to separate higher-traffic networks from lower-traffic networks.

Try not to use more than one router between users and the services they use regularly. This prevents unnecessary delays.

Speed up file servers by adding high-speed hard drives. Increase server memory only if they will provide additional services such as email.

Do not mount all services on a single server. Use another database server if the databases will be used intensively.

Know your network when it is operating normally. This will help you diagnose problems when they arise.

Install as much memory as your printer allows. This will speed up acceptance of the printer to start users' jobs.

Do not open applications from the server as this takes up too much of the network and significantly reduces its performance.

For more information about the products described here, please contact:
 

INTERSOFT S.A. de C.V.

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Col. Lomas Reforma

México D.F. 11930

Tel. +52 55 5596-2518 Cel. +52 55 2878-7602