El Papel Desempeñado por los QE

In LANs, there are two predominant methods of controlling access to the physical medium: Carrier Sense Multiple Access with Collision Detection (CMSA/CD) and deterministic access. CSMA/CD is the access method for Ethernet.

CSMA/CD is best described as the same set of rules you would follow in a meeting. In a meeting, everyone in the room has the right to speak, but everyone follows the generally accepted rule of “Only one person can talk at one time.” If

Figure 1.4Mesh Topology

File Server Network Printer

you want to speak, you need to listen to see if anyone is else is speaking before you begin. If someone else is speaking, you must wait until they are finished before you can begin. If nobody is speaking, you can speak, but will continue to listen in case someone else decides to speak at the same time. If they do, both speakers must stop talking, wait a random amount of time, and start the process again. If a speaker fails to observe the protocol of only one speaker at a time, the meeting will quickly lose all effective communication. (Sounds too familiar, doesn’t it?)

In Ethernet, the multiple access (MA) is the terminology for many stations connected to the same cable and having the opportunity to transmit. No device or station on the cable has any priority over any other device or station. All devices or stations on the cable do take turns communicating per the access algo- rithm to ensure that one device on the LAN does not monopolize the media.

The CS (carrier sense) refers to the process of listening before speaking in an Ethernet network.The carrier sense operation is performed by every device on the network by looking for energy on the media, the electrical carrier. If a carrier exists, the cable is in use, and the device must wait to transmit. Many Ethernet devices maintain a deferral or back-off counter defining the maximum number of attempts the device will make to transmit on the cable. If the deferral counter is exceeded, typically 15 attempts, the frame is discarded.

The CD (collision detect) in Ethernet refers to the capability of the devices on the wire to know when a collision occurs. Collisions in Ethernet happen when two devices transmit data at the same time on the cable. Collisions may be caused by the cable distance being exceeded, a defective device, or a poorly written driver that does not adhere to Ethernet specifications.When a collision is detected, the participants generate a collision enforcement signal.The enforce- ment signal lasts as long as the smallest Ethernet frame size, 64 bytes.This sizing ensures that all stations know about the collision and do not attempt to transmit during a collision event. After the collision enforcement signal has finished, the medium is again open to communications via the carrier sense protocol.

Deterministic access is the protocol used to control access to the physical medium in a token ring or FDDI network. Deterministic access means that a control system is in place to ensure that each device on the network has an equal opportunity to transmit.

Cabling

The physical infrastructure of a LAN is one of the most important components of a network. If the physical medium that data is traversing is faulty or installed incorrectly, network performance and operation will be impacted. It is analogous

to the foundation of a building. Everything in the building is set upon the foun- dation, typically strong reinforced concrete or other equally durable and reliable building materials. If the foundation is not installed properly, everything built on this foundation is suspect. A LAN is the same, a faulty foundation can be disas- trous to a network.You can install all of the high-end gear, switches, routers, servers, but if they don’t have the physical infrastructure to communicate effec- tively, your network will fail.

There are two primary forms of physical medium a network will utilize: copper and fiber. Between these two forms, there are sometimes many different standards of cable. For example, copper may be shielded, unshielded, twisted, untwisted, solid core, or braided core.We explore copper and fiber cable in more detail to provide a solid understanding of the importance of cabling in your net- work.You may be asking yourself “Why are we covering cabling in a book on wireless?”That is a very good question.Wireless, as its name implies, does not use physical cabling to provide communications to the wireless network. However, it does use copper cabling to connect to your existingLAN. If your existing LAN has out-of-spec or faulty cabling, your WLAN may not meet your expectations. (Or more importantly, your boss’s expectations!)

The most common form of LAN cabling installed today is copper. Copper has been the “preferred” installation since networks starting taking hold in the corporate world in 1980 when Xerox developed Ethernet. Copper is relatively cheap, easy to install, and can meet most distances that LANs were designed to cover.The original Ethernet specification used what is called thick coaxial cable. This cable lived up to its name for sure! Thick coax is much bigger than the tra- ditional copper cable you might be familiar with. After thick coax came thin coax.Thin coax was a cheaper and easier to handle and install cable alternative. Both of these cable types are implemented in a bus topology. As we covered ear- lier, a bus topology is linear LAN architecture. Each device or station on a bus is connected to the same medium. One of the major downsides to thick and thin coax was that it created a single point of failure. If the bus were to experience a failure or cut, the network became nonfunctioning.

With the advances made in copper technology, twisted pair cable became a popular LAN medium.There are two main types of twisted pair cable: shielded and unshielded. Shielded, as its name implies, contains smaller copper cables, twisted among themselves with a shielded jacket around them. Shielded twisted pair allows copper cable to be installed in facilities where there is significant interference to the electrical signals passed along the cable.The shielding—as well as the twisting of the

cables—plays a role in protecting the cable from this interference.Twisted pair cables are less prone to interference than flat, or nontwisted cables.

Among the twisted pair cabling family are a number of different levels of cables.These are commonly referred to as categories, or CAT for short.The pri- mary differences between the categories is the number of twists per foot in the cable. More twists per foot equals less susceptibility to outside interference. Some of the newer, higher categories of cabling also have internal dividers intertwined with the copper cabling to further reduce interference.These higher standards allow faster communications such as Fast Ethernet at 100 Mbps and Gigabit Ethernet at 1000 Mbs over copper cabling.

Understanding How Wireless