Novell Del Mar College
CIS 306 - Managing NOVELL® Networks
Instructor: Michael P. Harris
Networking Primer

3. Connecting Network Devices

Now that you understand what a transmission medium is, we will discuss ways in which devices are connected, both physically and electronically, so that they can communicate with each other.

First, we will look at how guided transmission media are commonly connected physically to form the physical topology of a local area network. Then, we will examine three logical topologies, the electronic schemes used to connect network devices.

Common Physical Topologies: Bus, Star, and Star-Wired Ring

Physical topology is applicable to networks in which the devices are connected with some type of guided transmission medium (or media). Physical topology is the physical layout of the guided transmission media. The most common physical topologies are the bus, the star, and the star-wired ring.

Physical Bus

The simplest form of a physical bus topology consists of a trunk (main) cable with only two end points. When the trunk cable is installed, it is run from area to area and device to device—close enough to each device so that all devices can be connected to it with short drop cables and T-connectors. This simple "one wire, two ends" physical bus topology is illustrated in Figure 5.

Figure 5: Physical bus topology

A more complex form of the physical bus topology is the distributed bus (also called the tree topology). In the distributed bus, the trunk cable starts at what is called a "root," or "head end," and branches at various points along the way. (Thus, unlike the simple bus topology described above, this variation uses a trunk cable with more than two end points.) Where the trunk cable branches, the division is made by means of a simple connector (as opposed to the star physical topology discussed below, where connections are made to a central, somewhat sophisticated connection device). The distributed bus topology is illustrated in Figure 6.

Figure 6: Distributed bus topology

Physical Star

The simplest form of the physical star topology consists of multiple cables—one for each network device—attached to a single, central connection device. For example, 10Base-T Ethernet networks are based on a physical star topology—each network device is attached to a 10Base-T hub by means of twisted-pair cable.

In a real-life implementation of even a simple physical star topology, the actual layout of the transmission media need not form a recognizable star pattern; the only required physical characteristic is that each network device be connected by its own cable to the central connection point.

The simplest form of the physical star topology is illustrated in Figure 7.

Figure 7: Physical star topology

A more complex form of the physical star topology is the distributed star. In this topology, there are multiple central connection points, which are all connected to form a string of stars. This topology is illustrated in Figure 8.

Figure 8: Distributed star topology

Physical Star-Wired Ring

In the star-wired ring physical topology, individual devices are connected to a central hub, as they are in a star or distributed star network. However, within each hub the physical connections form a ring. Where multiple hubs are used, the ring in each hub is opened, leaving two ends. Each open end is connected to an open end in some other hub (each to a different hub) so that the entire network cable forms one physical ring. This physical topology, which is used in IBM's Token-Ring network, is illustrated in Figure 9.

Figure 9: Physical star-wired ring topology

In the star-wired ring physical topology, the hubs are "intelligent." If the physical ring is somehow broken, each hub is able to close the physical circuit at any point in its internal ring so that the ring is restored. Refer to details shown in Figure 9, hub A, to see how this works.

Currently, the star topology and its derivatives are most preferred by network designers and installers because using these topologies makes it simple to add network devices anywhere. In most cases, you can simply install one new cable between the central connection point and the desired location of the new network device, without moving or adding to a trunk cable or making the network unavailable for use by other stations.

Common Logical Topologies: Bus, Ring, and Star (Switching)

While the star, distributed star, and star-wired ring are currently the most commonly used physical topologies, there is considerably more variety (and parity) in the use of logical topologies. A logical topology is the electronic scheme used to enable network devices to transmit and receive data across the transmission media without interfering with each other.

There are three basic logical topologies, each of which has distinct advantages in specific situations. As you study the figures representing these topologies, remember that the figures represent a logical (electronic), not a physical, connection scheme.

Logical Bus

In the logical bus topology, transmissions (called frames) are broadcast simultaneously in every direction to every point on the transmission media. Every network station checks each frame to determine whether the frame is intended for it. When the signal reaches any end point on the transmission media, it is absorbed (removed from the media) by appropriate electronics. Removing the signal prevents it from being reflected back along the transmission media and interfering with subsequent transmissions.

On a logical bus network, the transmission media is shared. To prevent transmission interference, only one station may transmit at a time. Thus, there must be a method for determining when each station is allowed to use the media. This method is called the media access control (MAC).

The media access control method most commonly used for a logical bus network is a contention method called "carrier sense multiple access with collision detection (CSMA/CD)." This media access control method is similar to the access scheme used on a telephone party line. When any station wants to send a transmission, it "listens" (carrier sense) to determine if another station is currently transmitting on the media. If another station is transmitting, the station that wants to transmit waits. When the media become free, the waiting station transmits. If two or more stations determine that the media are free and transmit simultaneously, there is a "collision." All transmitting stations detect the collision, transmit a brief signal to inform all other stations there has been a collision, and all stations then wait a random amount of time before attempting to transmit.

A logical bus network may also use token passing (token bus) for media access control. In this MAC method, each network station is assigned a logical position in an ordered sequence, with the last number of the sequence pointing back to the first (the logical order that the stations are assigned need not correspond with any physical order). A control frame, called a "token," is used to control which station can use the media. A station can transmit only when in possession of the token. Furthermore, a station can have the token only a limited time before it must pass the token to the next station. The token starts at the first station in the predefined logical order. While the first station has the token, it transmits, polls stations, and receives responses (gives other stations permission to use the media) until the allotted time expires; or, it passes the token when it no longer needs control of the media, whichever happens first. The first station passes the token to the second station in the logical sequence. This token passing (in sequence) continues nonstop while the network is running—thus, every station gets equitable access to the transmission media.

The logical bus transmission scheme is used in combination with both the physical bus and physical star topology, and the MAC method can vary in different cases. For example, the cable on thin Ethernet networks is laid out as a physical bus and the transmission scheme is a logical bus, but the cable on 10Base-T Ethernet networks and on ARCnet networks is laid out as a physical star, although both use the logical bus transmission scheme. And thin Ethernet (physical bus) and 10Base-T Ethernet (physical star) both use the CSMA/CD MAC method, but ARCnet (physical star) uses token passing as its MAC method.

Figure 10 shows a thin Ethernet network (physical bus, logical bus), and Figure 11 shows a 10Base-T Ethernet network (physical star, logical bus). In both figures, notice that the network signal (shown by the arrows) emanates from the sending station and travels in all directions, to all parts of the transmission media (the determining criterion for a logical bus topology).

Figure 10: Thin Ethernet network (physical bus, logical bus)

Figure 11: 10Base-T Ethernet network (physical star, logical bus)

Logical Ring

In the logical ring topology, frames are transmitted in one direction, around a physical ring, until they have passed every point on the transmission media. (The logical ring must be used in combination with a physical ring topology, such as the star-wired ring explained earlier.) Each station on the physical ring receives the signal from the station before it and repeats the signal for the next station. When a station transmits data, it gives the data the address of some other station on the ring. The data is circulated around the ring through each station's repeater until it reaches the station to which it is addressed and is copied. The receiving station adds an acknowledgment of receipt to the frame. The frame continues on around the ring until it returns to the station from which it was originally transmitted, which reads the acknowledgment and removes the signal from the ring. Figure 12 shows how data would flow on a logical ring network with a star-wired ring physical topology.

Figure 12: Logical ring topology

Media access control for the logical ring topology is almost always based on a form of token passing, the basics of which are described in the logical bus topology section. (Stations are not necessarily granted media access in the same order in which they receive frames on the physical ring.) IBM's Token-Ring network is a logical ring network based on the star-wired ring physical topology.

Logical Star (Switching)

In the logical star topology, network switches are used to restrict transmissions to a specific part of the transmission media (transmission path restriction is the identifying characteristic of a logical star).

In its pure form, switching provides a dedicated line for each end station. This means that when one station transmits a signal to another station on the same switch, the switch transmits the signal only on the two paths connecting the sending and receiving station. Figure 13 shows how data would be transmitted from one station to another if two stations were directly connected to the same switch.

Figure 13: Switching

Most switching technology adds switching capability to existing connection standards, incorporating the logical connection schemes (including the media access control methods) of the existing standards.

For example, a 10Base-T Ethernet switch supports the Ethernet CSMA/CD media access control method. Some switches are designed to support and combine multiple network standards. For example, a switch might contain both 10Base-T Ethernet ports and Fiber Distributed Data Interface (FDDI) ports. In this case, the switch would support the logical connection scheme for both standards, including the Ethernet CSMA/CD and the FDDI token-ring MAC methods.

Switches have built-in connection logic and significant amounts of fast memory. This enables them to simultaneously service all connected stations at full access speed. Thus, when you connect a station directly to a switch, you can increase the total throughput of your network—a significant performance advantage.

Switching illustrates well that a logical topology consists of the total of the various aspects of the electronic connection scheme, not just the MAC method. By combining new (switching) capabilities with existing logical connection schemes, engineers create a new logical topology.

Switching can be distributed (multiple switches can be connected using one or more physical topologies). Switches can be used not only to connect individual stations, but also to connect network segments (groups of stations). Thus, in many circumstances, switching can be used to improve the performance of your network.

Connecting a Simple Network

Now that we've seen the hardware pieces that make up a network and discussed the difference between physical and logical topology, let's connect some hardware to form a simple network. Figure 14 shows some of the hardware items we have discussed, connected to form a very basic computer network.

Figure 14: Various networking hardware connected to form a simple network

The network in this illustration includes the following components: three computers connected through a 10Base-T concentrator by means of unshielded twisted-pair wiring; three Ethernet 10Base-T network adapters, one installed inside each of the computers; and a laser printer that is connected to one of the computers.

The computer at the bottom center of the illustration is a network server; it controls the network (details will be covered in a following section). The other two computers are workstations. The workstations use the network under the control of the network server. One workstation is an IBM PC and the other is an Apple Macintosh computer.

The 10Base-T concentrator serves as a common connection point for the three computers; it repeats network signals.

The lines between the different components of the network represent the transmission medium, which is twisted-pair wiring. As you may remember from our recent discussion of topologies, this 10Base-T network is connected in a physical star, but it is based on a logical bus that uses a contention scheme as the means for workstations to get access to the transmission medium.

The printer in this network is connected directly to the server by means of a parallel interface cable, which is a standard connection method. The server accepts print jobs from either workstation and sends the jobs through the parallel interface cable to the printer. This is the simplest way to enable both workstations to use the printer. There are other ways to connect printers to a network, including attaching them to a computer set up as a dedicated print server or connecting them to a computer that runs special software enabling it to function as both a workstation and a print server. Many printers are now manufactured with an internal network adapter so that they can be attached directly to the transmission medium at any physical point in the network.


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