4.1 Introduction to Media and Devices

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1 Chapter 4 Network Hardware 4.1 Introduction to Media and Devices Many of the issues discussed in this course, such as topology, scalability, and speed, depend on hardware. Unlike many of your computer science classes, networking uses a lot of hardware that is outside of computers: cables or other connections, peripheral devices, and some autonomous devices not directly connected to any client or server machines. However, since many of the tasks performed by these devices are information processing tasks, many of the same tasks may also be performed by ordinary computers. 4.2 Transmission Media The most common method of digital communication is by electricity through copper or other metallic wires. Twisted pair means that two copper wires are twisted around each other. Coaxial cables (or simply coax ), on the other hand, share a single center. All electrical wires carry, in addition to the electrical signal inside them, an electromagnetic field that propagates around them. This field can cause interference with the signal of other wires or radio signals. Coaxial cables can limit some of this electromagnetic field interference because the two wires have the same axis. Ethernet cables are typically four twisted pairs for eight signals, with eight pins on the connectors, which are the same at each end of the cable. Optical fibers have become increasingly important for digital communications. Optical fibers are typically made from very pure glass. The distance 27

2 28 CHAPTER 4. NETWORK HARDWARE alightsignalcantravelthroughanyglassisdeterminedbythepropertiesof the glass so the maximum length of an optical fiber is limited. The center of the fiber and the glass surrounding it have different properties to keep the light going down the center by total internal reflection. 4.3 Hubs, Repeaters The job of a hub or repeater is to simply take a message in and pass it on to very device connected. Since hubs do not limit access or perform switching, the devices communicating on a hub must have a method of handling collisions, as described in the previous chapter. A hub is one of the most common ways to implement an Ethernet local area network (LAN). Recently, many devices sold as hubs are actually switches (or switching hubs), which are described in the following section. 4.4 Bridges and Switches As a local network grows, non-switching devices such as hubs are less able to handle new devices. One reason is simply the physical number of connections possible on each device. Another reason is that more devices mean more collisions, which may mean more time for each packet to successfully be sent. A bridge splits the network into two halves and allows communication between the two halves but only when the communication is relevant to the other half. A table lists the addresses on each side. (Usually, switching is by MAC, Media Access Control, address, described in more detail in Chapters 6and9.) Ifthedestinationaddressonapacketisonthesamesideasthe packet is coming from, there is no need to allow it to pass through to the other side. If the packet s destination address is on the other side then the bridge allows it to pass. A switch is the same as a bridge except that instead of splitting into two halves, the switch breaks the network into multiple segments. As in the bridge, packets can be sent only to the segment on which the destination address is tabled and packets addressed to destinations not on the table can simply be dropped. As switches become faster and less expensive, they are gradually replacing hubs because the reduction of collisions allows more traffic and more speed.

3 4.5. ROUTERS AND GATEWAYS Routers and Gateways Routers are the basic element of the Internet that allows it to function. When apacketissentoverthenetwork,eachrouteritencountersdeterminesthe best path to send the packet. Because of this important role, we will go into more detail about how a router determines the route of a given packet in later lessons. For now, we should understand that the router uses tables similar to switches and bridges. Additionally, a router may use more sophisticated algorithms to determine the shortest or least congested routes, to update tables, etc., in order to dynamically determine the best route. A gateway connects two networks with different protocols. Since these conversions and translations from one protocol to another can be done by software, often a device sold as a router can be used as a gateway. For example, the protocol for your home may be IPX/SPX (discussed in a later class) and the protocol for the network may be TCP/IP. In this case, the home router acts as a gateway between the two protocols. NAT routers implement Network Address Translation protocol, which allows the local area network to use local IP addresses that are not normally visible to the outside network. As you can see, routers have many tasks but their basic function is to send each packet received on to the best route or, sometimes, to discard the packet (for reasons discussed in Chapter 9). 4.6 Wireless Access Points Wireless access points (WAPs) act similarly to hubs and switches but with added security and access control. The main difference is that with a hub or switch, the administrators may assume (perhaps wrongly) that physical access to the device can be controlled. Once physical access to a device is gained, the problems of access and security become much more difficult. However, in the case of wireless access, it is not practical to block access physically because any device within range may send or receive radio signals. Signals from low power wireless devices can still be picked up from hundreds of meters away using easily constructed directional antennae. Similar antennae can be used to direct wireless traffic to an access point. In an urban setting, there may be hundreds or thousands of machines that have radio access to and from a given wireless access point. These vulnerabilities are the main reasons that wireless access points require much more information-

4 30 CHAPTER 4. NETWORK HARDWARE based access control than wired devices. 4.7 Power and Storage Redundancy Redundancy is a general engineering concept which means having more than one resource available to perform a task so that if there is a failure, at least one system may be able to carry out the required task. In networks, many of the media and devices discussed above may be redundant in practice. For example, over a Wide Area Network (WAN) or over the Internet, multiple routes may be available, which allows for failure in a few routers because there will almost always be enough routers to achieve the task of transmitting a given packet. In this section we will learn about two types of system that have the sole purpose of providing redundancy. Redundant power for devices on a network is often provided by UPS (Uninterruptible Power Supplies). Storage redundancy is often provided by RAID (Redundant Array of Independent Disks). Without these kinds of devices, services would be very unreliable. The time during which a service is not available is called downtime. The network can often route around failures on the network but it is up to the service administrators reduce downtime by making sure that services stay available whenever they are needed even when the power goes out or disk drives break. UPS devices are intended to provide a uniform and unbroken power supply in emergencies and during power signal irregularities. The power signal from electrical mains is AC, alternating current. Electronic devices convert this to DC, direct current, but it is convenient for the UPS devices to be connected directly between the power supply outlet and the device so consumer UPS devices provide AC power. A complete loss of power signal is called a blackout. The UPS provides power from a battery during that time so that services are maintained and no devices need to be rebooted. Other power irregularities may include brownout (also called sag), during which the power falls to 80% or less, and surge, which is when the electrical signal rises to 110% or more of the expected voltage for more than 2.5 seconds. A spike is a very brief but intense increase in power sometimes caused by lightning during electrical storms. A spike can destroy network devices. It is important to note, however, that while many UPS devices can handle a spike in the power signal including some intense spikes from lightning, electrical network connections can also carry a spike that the UPS device will not block.

5 4.8. GLOSSARY 31 RAID Minimum Level disks Description 0 2 Striping only 1 2 Mirroring only 4 3 Three mirrored data disks, one parity disk 5 3 Similar to 4 but with parity distributed across all disks Table 4.1: Several Standard RAID Levels Storage redundancy is essential for all servers. (A server is any hardware or software system that provides a network service, which may include storage, communication, or processing.) In addition to performing routine backup to external storage, many servers are configured to store the same data on multiple disks internally. RAID (usually expanded as Redundant Array of Independent Disks) is a set of various technologies for providing faster and more reliable storage. RAID can be implemented in hardware or software and is classified as a standard level based on the type of redundancy used. Some RAID levels are listed in table 4.1. As you can see, not all RAID implementations provide data redundancy or error-checking so the correct level of RAID must be implemented based on the server needs. Mirroring, as we learned in Chapter 3, is maintaining multiple copies of the same data. Striping means splitting contiguous sets of data between two disks, which can improve read/write speeds. Parity in this case may be simple parity (using mirroring for recovery), block parity, or cyclic error checking but these operations may slow down read/write cycles as the parity is computed every time. 4.8 Glossary alternating current (AC) anelectricalpowerdistributionschemein which the voltage alternates between positive and negative. Used to distribute power over long distances from a generating station to the enclosure of a device (where it is converted to DC for internal power distribution).

6 32 CHAPTER 4. NETWORK HARDWARE blackout acompletelossofelectricalpower. bridge adevicethatlogicallyseparatestwolocalareanetworksegments and only forwards relevant packets from one segment to the other. Packets exchanged between two hosts on the same segment is not forwarded to the other segment. brownout a partial loss of electrical power where the power level drops to 80% or less. client anetworkdevicethatconnectstoaserverinordertomakeuseof aservice. coaxial cables (coax) electrical cables in which a central copper conductor is surrounded by an electrical shield designed to prevent interference. copper a metal with very high electrical conductivity used to manufacture most wires and cables. direct current (DC) an electrical power distribution scheme in which the voltage remains constant. Usually used to distribute power over short distances inside a device enclosure. electromagnetic field acombinationofelectricalandmagneticfields that propagate through space at the speed of light and which can be modulated to carry information. An electromagnetic field will cause an electrical disturbance in any conductor that it crosses. Ethernet a wired, electrical physical layer technology for Local Area Networking. gateway a router that connects networks using two different protocols and which performs any necessary conversions in packet format. hub arepeaterthatbroadcastsinformationtransmittedbyoneconnected device to all connected devices. Local Area Network (LAN) a network on which hosts are physically connected to the same medium. mirroring maintainingmultiplecopiesofthesamedataforreliability (retrieval or retransmission in case of data corruption). NAT router arouterthatperformsnetworkaddresstranslation.

7 4.8. GLOSSARY 33 Network Address Translation (NAT) conversionoflocalipaddresses on a LAN to global IP addresses for communication with remote hosts. Destination addresses in replies from remote hosts are translated back to a local IP address and forwarded to the the local machine involved in the communication. optical fiber wire made of glass that carries visible light signals rather than electrical signals. RAID level anumbercharacterisingthekind(s)ofredundancyusedin a RAID array, which might including mirroring and parity. redundancy keepingadditionalidenticalcopiesofdata,potentiallyon different hardware, for reliability. Redundant Array of Independent Disks (RAID) asetofdiskson which data is replicated and/or distributed and/or verified by parity. repeater adevicethatretransmitsasignal,eithertorepairsignaldegradation or to provide broadcast. router anetworknodethatforwardspacketsfromonelantoanother, moving packets closer to their destination with each hop. sag synonymforbrownout. segment (in physical networking) one part of a LAN to which all devices are physically connected. A bridge or switch might connect the segment to others belonging to the same LAN. server amachinethatprovidesaremoteservicetonetworkhostsor devices. spike atransient,potentiallydamaging,increaseinthevoltagelevelon an electrical power distribution line. striping splittingdataacrossmanydiskstoallowparallelreadsand writes during sequential access. surge apotentiallydamagingincreaseinthevoltagelevelonanelectrical power distribution line that lasts for more than a few seconds. switch anetworkdevicethatconnectsseverallansegmentstogether, forwarding traffic between pairs of segments based on the physical destination address.

8 34 CHAPTER 4. NETWORK HARDWARE total internal reflection aphenomenonofwavepropagationthatprevents the wave from passing through a boundary, instead causing it to be entirely reflected. Exploited in fiber optic cable to guide the light signal precisely from source to destination with minimal degradation. twisted pair an electrical signal distribution scheme in which two conductors are wound tightly around each other and a differential (voltage difference) signal transmitted along them. Interference impressed on the conductors causes the same voltage change on both of them, which does not alter the voltage difference between them; the differential signal is therefore largely immune to interference. Uninterruptible Power Supply (UPS) abattery-poweredunitthat provides AC power to a device for a limited time in case of AC power distribution failure. Wide Area Network (WAN) anetworkcomposedofmorethanone LAN, with routers forwarding traffic between the individual LANs. wireless access point (WAP) adevicethatfunctionslikearouteror switch between a wireless network and a wired network. Additional security methods (such as authentication and encryption) are usually provided to protect the wireless part of the network from unauthorised access or eavesdropping. Copyright 2014 E.W. Cooper and I.K. Piumarta All rights reserved. Permission is granted for you to download, save and then view or print one copy of this document for personal study purposes. No other form of publishing, duplication or redistribution is permitted without explicit prior consent.

Computer Networks. Week 04 Media and Devices. College of Information Science and Engineering Ritsumeikan University

Computer Networks. Week 04 Media and Devices. College of Information Science and Engineering Ritsumeikan University Computer Networks Week 04 Media and Devices College of Information Science and Engineering Ritsumeikan University Network Media and Devices l When you build a network, the network hardware may be used