LAN Topolologies and Security Issues By Alex Gett

Basic Overview of LAN Topologies • A network’s basic design plays a big part in its operation and performance. The network topology dictates the media used, the type of channel access, and the speed at which the network operates.

Topologies that will be covered • Physical bus • Logical bus • Physical ring • Logical ring • Physical star • Wireless • Variations of physical topologies: extended star, • mesh and combination star bus topologies. Hubs and Switches

Definitions • Topology- A network’s topology refers to the physical layout of it’s computers, cables and other resources and how these components communicate with each other.

Definitions cont. • Physical topology- The arrangement of cabling in a network. • Logical topology- The path that data travels between two computers on a network

Basic Topologies • Today, all network designs are based on three simple physical topologies: bus, star and ring. Star Topology Ring Topology

Physical Bus Topology • A bus consists of a series of computers • • • connected along a single cable segment. A bus topology is the simplest method for connecting computers; at one time it was also the most popular method. However, these topologies have a weakness. A single break in the backbone, the cable segment that interconnects all the computers in a straight line, can halt the entire network. Two properties involved with a physical bus topology: Signal propagation and signal bounce.

Physical Bus cont. • Signal propagation- The traveling of data as • • electrical pulses across the length of the cable. These signals continue to travel along the cable and through any connecting devices until they weaken enough where they are no longer detectable or until they come across a device that absorbs them. Even if a signal reaches the end of a cable, it will bounce back and travel the other direction.

Physical Bus cont. • Signal Bounce- A signal traveling across a • • network continuously, bouncing back and forth and preventing other computers from sending data. To prevent this, a terminator is attached to each end of a cable to prevent signals from bouncing. The terminator absorbs all signals that reach it, clearing the network for new communications.

Physical Bus cont. • A cable break in a bus network occurs when the cable is physically cut or one end becomes disconnected. • When this occurs, the cable is no longer terminated and signals are now able to bounce, which halts network activity.

Logical Bus Topology • When a computer has data to send, it addresses • that data, breaks it into manageable chunks and sends it across the network as electronic signals. On a logical bus, all connected computers receive these chunks of data. However, because of the address included in the data, only computers for which the signals are destined receive the data.

Logical Bus cont. • In a logical bus environment, only one computer • • can send information successfully at a time. Because of this, all network users must share the available amount of transmission time. The more computers that are ready to send data at the same time, the longer some computers must wait to send data; which slows the overall network performance.

Logical Bus cont. • A bus topology is a “passive topology” • Passive topology- Computers on the bus only • listen for data being sent; they aren’t responsible for moving data from one computer to the next. The other form of this concept is called an active topology; which is when computers and other devices attached to the network regenerate signals and are responsible for moving data through the network.

Physical Ring Topology • A physical ring network is when each computer connects directly to the next computer in line, ending at the starting computer. • Because the circle has no end, signals travel in one direction around the ring; this eliminates the need for termination.

Physical Ring cont. • Every computer in a ring is responsible for retransmitting the data, making it an active topology. • A typical single-ring network can fail if one computer in the ring fails, but a dual-ring network can still operate is such a failure occurs.

Logical Ring Topology • The data in a logical ring topology travels from one • • • device to the next until it reaches its destination. One method of this data passing is called token passing; when small packet called a token passes around the ring to each computer in turn. If a computer has information to send, it modifies the token, adds address information and the data, then sends it around the ring. The information travels around the ring until it reaches its destination or returns to the sender. When the intended destination receives the information, it returns a message to the sender to acknowledge its safe arrival.

Logical Ring cont. • Most logical ring topologies use smart hubs that • • recognize a computer’s failure and remove the computer from the ring automatically. The technology of smart hubs is one of the advantages of using a ring topology. Another advantage is its capability to share network resources fairly; this feature is useful in high-traffic and time-critical applications.

Physical Star Topology • A star topology describes computers connected • by cable segments to a central device. Because all computers connect at one location, the star topology requires a more involved cable installation: through conduits, under floors, and in ceilings to get the cable from the computer work area to the connecting device.

Physical Star cont. • If the central connecting device (hub) fails, all other computers and devices attached to that device lose network access, on the flip side, if one computer or a cable fails, it has no effect on the rest of the network (unlike the bus or ring topologies).

Physical Star cont. • Another advantage of the star topology is the ease of trouble shooting. • Since all computers connect at a central location, an administrator can quickly isolate network problems involving a single device or cable segment without affecting other devices.

Logical Bus Implemented as a Physical Star • When the arrangement cables forms a physical • • star, but data traveling to the computers follows a logical bus, the result is referred to as a “star bus. ” A hub is the central device in this topology. When a computer sends a signal, the hub receives it and retransmits it down every other cable segment to all other computers attached to it.

Logical Ring Implemented as a Physical Star • A “star ring” topology also uses a star’s physical cable arrangement, but data travel within the central device is in a ring configuration. • The central device in this topology is called a concentrator, or multistation access unit (MSAU).

Wireless Topologies • Wireless networks eliminate the need for a • • visible physical topology, which is the main reason for their growing popularity. There are no cables to run, terminate or test. The simplest wireless configuration is when two computers can communicate directly with one another. This is called an ad hoc topology, or a peer-to-peer topology. This arrangement is usually used in small or temporary installations.

Wireless cont. • There is no central device in a wireless topology. • In most situations, their central device is called an • • access point, which controls communications, much like a hub. When wireless devices are configured to use this central access point, it is called infrastructure mode. Wireless LAN’s use a physical star topology because all signals travel through one central device, but since only one wireless device can communicate with an access point at a time, it could also be called a logical bus topology.

Extended Star Topology • This topology is the most widely used in • • networks containing more than just a few computers. This topology revolves around a central device called a switch. Other switches, or hubs, that connect the computers which make up the network are connected into the central switch’s ports. This topology is also referred to as a hierarchical star because there are two or more layers of stars, all connecting back to the central star.

Mesh Topology • A mesh network topology is the most fault tolerant, but also the most expensive. • Any single cable or device failure in a mesh configuration barely affects network performance because of the multiple connections to each device; but costs increase because more cable and hardware required.

Mesh cont. • In most cases, a mesh topology is used in a WAN to ensure that all sites remain able to communicate, even with one or more cable failures. • One of the best examples of a mesh topology is the internet.

Hubs • Hubs act as the center of a star topology. • A hub is defined as the center of activity. • There are different variations to a hub: active hub, passive hub, repeating hub and switching hub.

Hubs cont. • The majority of hubs installed in networks today are • active hubs. These hubs regenerate/repeat the signals as they receive them and send them along. Usually active hubs have eight or more ports and are sometimes referred to as multiport repeaters or repeating hubs. A multiport repeater: – Takes a signal coming in on one port – Cleans the signal by filtering out noise and other undesired information – Strengthens the signal by regenerating it to original signal levels. – Sends the regenerated signal out to all other ports.

Hubs cont. • In Passive hubs (patch pannel, punchdown • • block) the signal passes through the hub without any amplification or regeneration. A passive hub is simply a central connection point and requires no power because no electronic signal modification occurs. These hubs are used as junction points between long and short runs of cable.

Switches • A switch is also a central connecting point in a • • star topology network. Unlike a hub, a switch does more than just generate signals. A switch looks just like a hub, but a switch determines to which port the destination device is connected and forwards the message to only that port. This procedure allows a switch to handle several conversations at one time.

Switches cont. • The performance advantage of switches has • • made them the device of choice in corporate networks. Most networks today are designed to use switches for connecting computers, servers, and peripheral devices. Although multiport repeaters (hubs) can still be found in the workplace, few networks are being designed to include these devices.

Selecting a Topology • When selecting a topology, you should choose one that best fits • • • your environment, here are some things to consider: A ring topology should be considered when equipment associated with ring architecture would be too difficult to replace or too expensive A hub-based bus topology should be considered if a small network is being constructed; this topology can also be considered if a few computers are being added to an existing network. A wireless topology can be considered as an add-on to an existing wired network. This can provide connectivity to mobile users and reach locations that might be difficult to connect with wires; this topology is also best suited for environments that are not suitable for running cables.

Security Issues • There are some security issues to be aware of when working with certain topologies. • Here a few examples:

Security cont. • When sending a data from one user to another, the star topology sends it directly through the hub to the receiver. In the ring and bus topologies, the message is routed past other users. As a result, sensitive data messages can be intercepted by these other uses in these types of topologies.

Security cont. • When setting up a wireless topology, many • administrators fail to remember basic security precautions. Since all 802. 11 b networking is done using high-band radio-frequency (RF) signals, the data transmitted is easily accessible to any user with a 802. 11 b NIC, or other sniffing tools (dsniff, snort). -802. 11 is the wireless networking standard.

Conclusion • All computers, no matter what their topology, communicate in the very same way. They address data to one or more computers and then transmit that data across the cable in the form of electronic signals.