1. Compare and contrast the four basic network topologies.

The four network topologies follow four basic geometric arrangements. (Donnellan, 1997). Topology can be defined as the way in which computers on the network are interconnected. (Fitzgerald & Dennis, 1998, p. 205). The term derives from a mathematics field that is concerned with points and surfaces in space, or, the layout of objects in space. So, LAN topology is the physical layout of the network – the model for how one configures the medium and attaches the nodes to that medium. (Stamper, 1998, p. 81). Finally, a topology describes pictorially the configuration or arrangement of a (usually conceptual) network, including its nodes and connecting lines. (Whatis.com, 1998). The following list shows the four network topologies:

1. Ring Topology: This connects all of the computers on the LAN in one closed loop circuit, with each computer linked to the next one. Messages are passed around the ring in ONLY one direction, to each computer, in turn. Each device is connected directly to two other devices, one on either side of it. (PC Webopædia, 1998). Each computer receives messages for all of the connected computers, but processes only the ones intended for itself. (p. 209). It does this by checking the addresses, and only keeping the ones intended for itself. It transmits messages to the next computer in the ring. (Donnellan, 1997). Nodes in the ring can be active or inactive, such as when a worker "powers down" their workstation. This inactive state, however, does not cause the network to fail. (Stamper, 1998, p. 82). Ring topologies are relatively expensive and difficult to install, but they offer high bandwidth and can span large distances. (PC Webopædia, 1998).
2. Bus Topology: This topology is where all devices are connected to a central cable, called the bus or backbone. This is a network in which all nodes are connected to a single wire (the bus) that has two endpoints. Bus networks are relatively inexpensive and easy to install for small networks. Ethernet systems use a bus topology. (PC Webopædia, 1998). Every computer on the bus receives ALL messages sent, even the ones intended for other computers. The Ethernet software checks the data link layer and only processes the messages intended for that particular computer. The term BUS implies a high-speed circuit with a limited distance between the computers. Most LANs connected in this way require several hubs, as they span appreciable distance. (Fitzgerald & Dennis, 1998, p. 205).
3. Star Topology: This is where all devices are connected to a central hub. Star networks are relatively easy to install and manage, but bottlenecks can occur because all data must pass through the hub. (PC Webopædia, 1998). The central station routes messages to their proper destination. If the central station, or node, fails, the entire network fails. (Donnellan, 1997). The main advantage of a star network is that one malfunctioning node doesn’t affect the rest of the network, and it’s easy to add and remove nodes. The primary disadvantage of star networks is that they require more cabling than the other topologies, such as a bus or ring networks. In addition, if the central computer fails, the whole network becomes unusable. Standard twisted-pair Ethernet uses a star topology. (PC Webopædia, 1998).
4. Mesh Topology: In this example, any network node can communicate with any other network node. (Donnellan, 1997). There are direct point-to-point connections among all computers. (Fitzgerald & Dennis, 1998, p. 463). A mesh is one possible topology for a network backbone. For redundancy in the case of a circuit outage, a backbone is usually connected in a circular fashion so that if data can’t get to the next node because of an interruption, it can get there by flowing in the opposite direction. A mesh architecture is formed by adding lines that go directly from one node to a second node that is three or four nodes around the "circle" from the initial node. (Cook Report, 1998).

2.a) Describe three types of decentralized routing.

In decentralized routing, all computers in the network are able to make their own routing decisions that follow a formal routing protocol. Most are self-adjusting, where they automatically adapt to changes in network configurations (Donnellan, 1997), which is the ability to add and delete computers and circuits. (Fitzgerald & Dennis, 1998, p. 178). There are three primary types of decentralized routing: static, dynamic, and broadcast.

...b) What are the advantages and disadvantages of each?

• Static Routing: A routing method in which the network manager permanently configures the routing table. The only time that changes are made is when computers are added to or removed from the network. This routing method is commonly used in networks that have few routing options, such as in LANs (and this includes most of them) where there is only one route from the LAN to the backbone. All computers "know" which of the computers is the connection or "gateway" to the outside world. When computers are removed, or if a circuit is broken, each computer recognizes this and updates the routing table accordingly. If there is an alternate route available, the computer will use that, and keep using it until the failed circuit is fixed. If computers are added, they "announce" their presence, and are automatically added by the others, into the routing table. The fact that static routing is used by LANs with few routing options, already has the main disadvantage built in: there are few choices to be made. After the best possible configuration is made, the process is non-changing. If a link in a particular path is down, then there is no communication possible, even though there may be an alternative. This type of system has largely disappeared, except in some fully interconnected networks. (Stamper, 1998, p. 363).

• Dynamic Routing: This is also known as adaptive routing, and it attempts to select the quickest (fastest!) or best current route for the message. This is commonly used in MESH networks. In very simple systems, the receiving node looks for the outbound link with the least activity. This can be very inefficient and cause network congestion. More intelligent adaptive/dynamic routing techniques attempt to select the BEST route, not just the one that is least busy. Knowing things like the speed of links, or the number of required hops, or the congestion, allows this type of routing to avoid difficulties and choose the best possible route. In heavy congestion, sometimes the shortest route is not the quickest! (Stamper, 1998, p. 364) The main drawbacks are that this method requires more processing by each computer, and it uses the network capacity for the transmission of status information. (Donnellan, 1997).

• Broadcast Routing: This form is quite easy. The message is broadcast to all stations, and only the one that the message is intended for, accepts it. (Stamper, 1998, p. 365). Broadcast routing is typically used on bus networks. (Donnellan, 1997). In networking, a distinction is made between broadcasting and multicasting. Broadcasting sends a message to everyone on the network whereas multicasting sends a message to a select list of recipients (PC Webopædia, 1998) or group of computers. This type of broadcast message is usually only transmitted within the same LAN or subnet, so it wouldn’t work if the message needed to be sent outside the subnet.

3. Describe the three stages of standardization.

First of all, a standard is a definition or format that has been approved by a recognized standards organization or is accepted as a de facto standard by the industry. Standards exist for programming languages, operating systems, data formats, communications protocols, and electrical interfaces. From a user’s standpoint, standards are extremely important in the computer industry because they allow the combination of products from different manufacturers to create a customized system. Without standards, only hardware and software from the same company could be used together. In addition, standard user interfaces can make it much easier to learn how to use new applications. (PC Webopædia, 1998).

1. Specification: In a formal standardization process (as opposed to de facto standards, or those that emerge in the marketplace and have no official standing), this stage is where the problems to be addressed are identified, and a nomenclature is developed. One of the most important standards-making organizations is the ISO (International Organization for Standardization). Others are the ANSI (American National Standards Institute), the IEEE (Institute of Electrical and Electronics Engineers, and others. (Fitzgerald & Dennis, 1998, pp. 16-17).
2. Identification of Choices: In this stage, those who are working on the identity of the standard, distinguish various solutions and choose the optimum, topmost interpretation from among all the choices.
3. Acceptance: This is the most difficult stage, consisting of defining the solution and then getting industry leaders to agree on a single and uniform solution. This often is accompanied by corporate politics and governmental influences.

4. What is TCP/IP? How does it work?

TCP/IP is the acronym for Transmission Control Protocol/Internet Protocol, the suite of communications protocols used to connect hosts on the Internet. TCP/IP uses several protocols, the two main ones being TCP and IP. TCP/IP is built into the UNIX operating system and is used by the Internet, making it the de facto standard for transmitting data over networks. The sending computer is the one that creates the TCP/IP packet. (Fitzgerald & Dennis, 1998, pp. 188). Even network operating systems that have their own protocols, such as Netware, also support TCP/IP. (PC Webopædia, 1998).

There are four pieces of network layer addressing and routing information needed by a computer on a TCP/IP network (or one who dials into it): The IP address, a subnet mask (to determine what addresses are part of its subnet), the IP address of a DNS server (so it can translate application layer addresses into IP addresses), and the IP address of a gateway computer (so it can route messages outside of its subnet). (Fitzgerald & Dennis, 1998, pp. 182-183). TCP/IP was originally developed for the Defense Department’s ARPANET, and can send large files across relatively unreliable networks. TCP performs the functions of the transport layer, while IP performs the functions of the network layer. TCP is connection-oriented, while IP is connectionless. (Donnellan, 1997).

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1. What are the distinguishing features of a LAN, and list at least two reasons for developing a LAN.

Some of the distinguishing features of Local Area Networks (LANs) are:

• This is a group of workstations or microcomputers that are connected in the same general area for the exchange of information or resources.
• Usually, one or more computers act as a server for the purposes of sharing hard disks, printers, tape backup, and other peripherals. (Donnellan, 1997).
• LANs are capable of transmitting data at very fast rates, much faster than data can be transmitted over a telephone line; but the distances are limited, and there is also a limit on the number of computers that can be attached to a single LAN. (PC Webopædia, 1998).

Two reasons for developing a LAN are:

1. Resource Sharing: This refers to one computer sharing a hardware device such as a printer, or a software package with other computers on a network, in order to save on costs. If the software is made available on the network, via a program of "licensing" the software, it eliminates the need to load the software on each computer.
2. Information Sharing: This is where users are accessing the same data files, exchange information via E-mail, or search the Internet for information. All users need, and have access to, the same digital information. The main benefit is in its aid to decision-making processes, thus making information sharing more important than resource sharing. (Fitzgerald & Dennis, 1998, p. 193).

2.a)  Discuss the legal issue of using single-computer license software on networks.

Since a software is licensed for ONE user on ONE computer, there are many legal issues that arise when talking about using single-computer license software on networks. Therefore, if 10 users need to use a certain software, the company must purchase 10 individual software licenses. The fees and penalties concerning software copyright violation are quite high, and most companies are very strict about software violations by employees. In fact, most companies and government agencies have stern policies that result in termination for employees that disregard software legal issues. (p. 193). Recently, the SPA (Software Publishers Association in Washington D.C. has begun an aggressive software audit program, to check the number of illegal software copies on LANs. (pp. 193-194). It must be noted that this is indeed a very widespread effort. Even this writer’s small studio received a package from the SPA, in 1998, concerning their project. Along with explanatory materials, there also was a form for the "whistle blowers" mentioned in the textbook, where individuals could report software abuse to the SPA. It was quite a shock to receive the package of materials, until I realized that the sources used by the SPA must have been local, county and state business listings. Any business with a recorded business name probably received the SPA packet.

Again, this kind of use is illegal, and I cannot think of a single instance where this should be allowed to happen. Our text mentions the use of LAN-metering software. I would surmise this, along with LAN-specific software, would be a single license software, and would be installed in one computer for use in surveying the entire LAN. But it wouldn’t be available to all on the network, just the user of that specific computer. This would be true of the software used in running the LAN.

...c)  Also discuss why it is important for organizations to enforce policies restricting use of employee-owned      hardware and software and unauthorized (unlicensed) copies of software.

The biggest reason for enforcing policies governing employee-owned hardware and software would be the element of control. There would BE no way of controlling the use of such devices and software. This would cause concerns over safety from viruses, as well as compatibility factors with different hardware items. The company and their LAN manager would always want to be informed about every aspect of the network. Once non-approved software or hardware are introduced, this could compromise the safety and viability of the network. As for the issue of illegal software copies – the company itself is legally responsible for all actions made in its name. If persons are using illegal software, a lawsuit could be brought against the company as well as the individual.

Many companies even have stringent rules about shareware or freeware, and have rules that severely contain an individual’s ability to download these items from the Internet. Again, one main issue is the introduction of viruses to the company network. Also, there is a compatibility factor with company-owned and –used software. Finally, in the situation of shareware, the question arises as to who is responsible for either paying the shareware fee, or for guarding when the shareware will be removed from individual computers if the fee isn’t paid. Lucent Technologies, in my recent position, had extremely strong rules against the downloading of public-accessible shareware. The rule was: "Don’t." In weekly checks by the network group, if these items were found on individual computers in the backup of company departments, they were immediately destroyed. Downloads of programs from the company Intranet were allowed, as, obviously, they had been approved by the company.

3. a)  List and define the commonly used topologies of a local area network.

• Bus Topology: Our text states that "topology is the basic geometric layout of the network – the way in which the computers on the network are interconnected. Ethernet uses a bus topology." The term implies that it is a high-speed circuit and has a limited distance between computers, such as in one building. (Fitzgerald & Dennis, 1998, p. 205). It further points out that approximately 70% of the world’s LANs use Ethernet, which is a standard that was formalized by the IEEE, as IEEE 802.3. There are now a few slightly different "flavors" of this standard, which was originally developed by DEC, Xerox and Intel. A bus topology can be identified with several things, such as the use of hubs to increase the distance of the bus, ordering the chaos by use of the CSMA/CD (carrier sense multiple access with collision detection) protocol, various types of baseband and broadband, and all the issues of installation and cabling.
• Ring Topology: The second most popular type of LAN is a form of ring topology – the token-ring network. (Fitzgerald & Dennis, 1998, p. 209). About 25% of world LANs are token ring. IBM developed token ring, but it has been standardized as IEEE 802.5. The media access control in token ring is called token passing, where the token moves between the network computers, in sequence, much like a relay race. The main problem with this is, if the computer with the token crashes, the token is "lost" before an acknowledgement is received, it could cause the entire network to go down. The solution is the assignment of a token monitor, and a backup to the monitor! There are two types of Token Ring: Token Ring 4 and Token Ring 16.
• Other topologies and types of LANs: Our text mentions ARCNET (Attached Resource Computing Network), which has a bus-like Ethernet topology, but uses a token-passing media access control-like token ring. (p. 212). This is a low-cost method that the ANSI is expected to designate it as ANSI standard 878.1. Macintosh computers have the built-in hardware/software called AppleTalk, with the Apple LocalTalk cabling system. A nonstandard set of protocols, this is also similar to Ethernet. (p. 212). Finally, the text reviews the Wireless LANs, which are standard IEEE 802.11, and is an Ethernet-like form. The percentage of use is currently small, but growing rapidly.

Currently, I am not involved with a company. I would like to point out that every company I’ve ever worked in the past few years (which is the only time I’ve been aware and somewhat knowledgeable about networking) for used one of two topologies. The most frequent was some form of Ethernet. In several isolated contract instances, I’ve worked with Macintosh-based firms (mostly graphics-related) that used AppleTalk. I personally have used AppleTalk in my home-based studio, albeit only to make my own small network of computer, printer, and other peripherals. It really is a quite simple system that works very well, at least in a small setting. I’ve never seen it in action in a large company.

4. List at least seven ways that baseband and broadband differ from each other.

.

5. List several reasons why twisted pair wire, fiber optic cable, and/or coaxial is/are used in LANs.

• Twisted pair wire is one of the most commonly used types of guided media. It is insulated pairs of wires that can be packed very closely together, and are twisted to minimize the electromagnetic interference between any pairs in the bundle. This type is placed in bundles in city streets and under apartments and large buildings, and of course, homes. The first of the pair is for a telephone, the second is for a second telephone line. It is being replaced by more efficient media such as coaxial cable, microwave, satellite, and optical fibers.
• Coaxial cable has a copper core – the inner conductor – and an outer cylindrical shell for insulation. There is an outer shield just beneath the shell that is a second conductor. These have very little distortion, are less prone to interference, and have a low error rate.
• Fiber optic cable is relatively new, but is becoming more widely used. This technology utilized high-speed streams of light pulses from lasers or LEDs, which carry information inside hair-thin strands of glass or plastic. Glass can be made into a purer product, so plastic is being used less. Glass fiber optics cable can transmit signals over greater distances. Early cable was multi-mode, but is being replaced with single-mode fiber optics, which concentrates the light beam and produces higher speed of transmission. (Fitzgerald & Dennis, 1998, pp. 83-85).

  It is interesting to follow the current state of affairs with cable and transmission methods. CMPnet’s article pretty much describes the excitement and chaos in the world of network transmission:

  "Wired today, mired tomorrow: That’s the sticky situation for net managers planning faster LANs. Much of the copper cabling in use now can’t handle the high-speed technologies that are on the way. The good news is that there’s a way

  out of the cabling conundrum. Net managers have four choices when it comes to wiring for speed--but those who don’t want to get caught by surprise should start looking at the alternatives now. One place to start is with so-called enhanced Category 5 copper, which carries data at higher speeds than ordinary Cat 5. There’s also "Category 6" cabling--it moves traffic much faster than today’s copper. Fiber is another alternative, with the undeniable benefit of virtually unlimited capacity. Finally, net managers could go with new signaling schemes that standards bodies are developing, which will permit big-bandwidth apps to be run over traditional Category 5." (Cray, 1997).

• Installing the cable seems to be the first important concern when installing a LAN. WRONG. This writer believes that the first important step MUST be the creation of a network cable PLAN. This will help in all phases of construction, implementation, testing, and actual use of the system.
• Hardware and software must be installed. Each workstation must be configured into the whole, and system software must be installed on the server (if there is one) and all client computers.
• Testing is a vital phase, whereby it is immediately known if all the LAN features and equipment are working properly.
• Administrative plans will include operating, security, and training programs, and these should be instituted as quickly as possible. Actually, the training process can be effectively used as part of the testing process, with actual use by employees becoming an integral part of testing the new LAN’s abilities.

7. a) What types of LANs use servers?

EVERY LAN uses a server. The first two main components of a LAN are the client computer and the server. Even in the case of peer-to-peer networks, all computers run special network software that lets them be both client and server (they don’t require a dedicated server). The other three components are: network interface cards, network cables and hubs, and the network operating system.

The name itself indicates its main function: to serve. The server processes requests from client computers on the network, hopefully in a timely manner, and provides a particular service to the clients. In a larger LAN, the server is usually a dedicated server.

.. c) Name at least three types of servers.

There are several types of servers: file, database, network, access, modem, facsimile, printer, and gateway.

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Chapter 9: Metropolitan and Wide Area Networks

• A packet’s key feature:
  One of the key features of a packet is that it contains the destination address in addition to the data. In IP networks, packets are often called datagrams. (PC Webopædia, 1998).
• A packet is a unit of data:
  "Packet" and "datagram" mean the same thing. A packet is the unit of data that is routed between an origin and a destination on the Internet or any other packet-switched network.
• A packet is an efficiently sized unit of data:
  When any file (such as an Email message, HTML file, GIF file, URL request, etc.) is sent from one place to another on the Internet, the Transport Control Protocol (TCP) layer of TCP/IP divides the file into ‘chunks’ of an efficient size for routing. Each of these packets includes the Internet address of the destination. The process is called "packetizing."
• A packet may be routed in many different directions by TCP:
  The individual packets for a given file may travel different routes through the Internet; when they have all arrived, they are reassembled into the original file (by the TCP layer at the receiving end). The packet service may be provided by a different provider than the user’s telephone or data services provider.
• Packets are efficient in a connectionless system such as the Internet:
  A packet-switching scheme is an efficient way to handle transmissions on a connectionless network such as the Internet. (Whatis.com, 1998).

...b) Where does the packetizing take place?

A user’s connection into the network is called a PAD (packet assembly/disassembly device). The PAD converts the user’s information into the network layer and data link layer packets that will be used by the packet network, to send through this packet-switched network. The PAD at the other end reassembled these into the network layer and data link layer protocols, and delivers these to the proper computer. This is called packetizing. It is almost instantaneous. The PAD can translate between different data from either sender or receiver. All of this takes place within the PAD (device).

...c) What does a packet contain?

Foremost, the packet contains both the destination address, and the data (up to 1024 bits). It also contains an error check of 16 bits. A note about the type of data: "Packet switching is more efficient and robust for data that can withstand some delays in transmission, such as e-mail messages and Web pages." (PC Webopædia, 1998). In other words, packets will not contain data that is best sent via a circuit-switching technology, such as live audio and video, and other real time data.

...d) Why is packet switching popular?

Most data communications consist of short bursts of data, with intervening spaces that last longer than the bursts of data. Packet switching is popular because it takes advantage of this characteristic by maximizing the use of the shared communication network. It does this by interleaving the bursts of data from many users. This way, the packets can travel in any given direction, and in spite of the "data burst" syndrome, still effectively arrive at their proper destination. Its popularity is a result of its efficiency.

2. a) Define ATM.

ATM (asynchronous transfer mode) is one of the fastest growing new technologies. It is a dedicated-connection switching technology, similar to frame relay, that organizes digital data into 53-byte cells or packets, and transmits them over a medium using digital signal technology. Individually, a cell is processed asynchronously in relationship to other related cells. It is queued before being multiplexed over the line. This allows for very precise settings. Because ATM is designed to be easily implemented by hardware (rather than software), faster processing speeds are possible. The pre-specified bit rates are either 155.520 Mbps or 622.080 Mbps. IEEE Spectrum reports that speeds on ATM networks are expected to reach 10 Gbps.

ATM is scalable; it is also able to prioritize, such as distinguishing between high bandwidth audio and visual files, and low bandwidth items such as Email. Along with several other technologies, ATM is a key component of broadband ISDN (BISDN). (Whatis.com, 1998) and (Fitzgerald & Dennis, 1998, p. 294). The cell used with ATM is relatively small compared to units used with older technologies. The small, constant cell size allows ATM equipment to transmit video, audio, and computer data over the same network, and assure that no single type of data hogs the line. (PC Webopædia, 1998).

...b) Discuss why it may replace existing protocols.

ATM may eventually replace other protocols because of its scalability. It is easy to multiplex basic ATM circuits into much faster ATM circuits. Because speed is currently one of the highest priorities in new technologies, I see this as being the most in its favor. Also, since Quality of Service (QoS) is becoming more of a priority, ATM’s ability to provide very precise settings and therefore higher QoS, is very vital. The smallness of the cell and the ability to more readily send high bandwidth files will make it increasingly popular. A new technology, ATM is attempting to combine the best of both worlds – the guaranteed delivery of circuit-switched networks and the robustness and efficiency of packet-switching networks.

3. a) What is ISDN?

Integrated Services Digital Network (ISDN) is an international set of standards for digital transmission over ordinary telephone copper wire, as well as over other media. Home and business users who install ISDN adapters in place of their modems, will experience the arrival of highly graphic Web pages (up to 128 Kbps). ISDN requires adapters at both ends of the transmission so the user’s provider also needs an ISDN adapter. ISDN is generally available from the local phone company in most urban areas in the United States and Europe. (Whatis.com, 1998). ISDN supports data transfer rates of 64 Kbps (64,000 bits per second). Most ISDN lines offered by phone companies give the user two lines at once, called B channels. One line can be used for voice and the other for data, or both lines can be used for data, which will give data rates of 128 Kbps – three times the data rate provided by today’s fastest modems! The original version of ISDN operates with baseband transmission. Another version, called B-ISDN, uses broadband transmission and is capable of supporting transmission rates of 1.5 Mbps. B-ISDN requires fiber optic cables and is not widely available. (PC Webopædia, 1998).

...b) What distinguishes it from other services?

ISDN is the most common example of a circuit-switched service. As such, the primary difference would be that the circuits are entirely digital, and therefore less prone to noise. They are also able to offer higher data transmission rates. Another distinguishing feature is that ISDN can, in many instances, be installed without adding any new cable, if the end user’s phone is less than 3.5 miles from the common carrier’s end office. Finally, ISDN, in concept, is the integration of both analog or voice data together with digital data over the same network. This concept is very appealing to computer users! Yet, ISDN is without a set of standards, which is part of the problem in implementing widespread use.

4. Describe a basic-rate and primary-rate interface. How do they differ?

There are two levels of service: the Basic Rate Interface (BRI – also called basic access service), intended for the home and small enterprise, and the Primary Rate Interface (PRI – also called primary access service), for larger users. Both rates include a number of B (bearer) channels and a D (delta) channel. The B channels carry data, voice, and other services. The D channel carries control and signaling information. The Basic Rate Interface consists of two 64 Kbps B channels and one 16 Kbps D channel. Thus, a Basic Rate user can have up to 128 Kbps service. The Primary Rate consists of 23 B channels and one 64 Kbps D channel in the United States or 30 B channels and 1 D channel in Europe. This difference in channels makes interconnection with Europe difficult

5. Define T-1, T-2, T-3, T-4, OC-1, and OC-3.

• T-1 – In way of definition, T-carrier circuits are dedicated digital circuits, and are the most common form of dedicated services in North America, currently. T-Carrier is defined as: "An AT&T hierarchy of digital systems designed to carry speech and other signals in digital form, designated T1, T2, and T4." (WorldCom Communications Library, 1998).
• T-1 Carrier – This is a dedicated phone connection supporting data rates of 1.544Mbits per second. A T-1 line actually consists of 24 individual channels, and each supports 64Kbits per second. Each 64Kbit/second channel can be profiled to carry voice or data traffic. Most telephone companies allow the user to buy just some of these individual channels, which is known as fractional T-1 access. T-1 lines are a popular leased line option for businesses connecting to the Internet and for Internet Service Providers (ISPs) connecting to the Internet backbone. The Internet backbone itself consists of faster T-3 connections. T-1 lines are sometimes referred to as DS1 lines. (PC Webopædia, 1998).
• T-2 – This transmits data at a rate of 6.312 Mbps. It is basically a multiplexed bundle of four T-1 circuits.
• T-3 – This circuit allows transmission at a rate of 44.376 Mbps, or approximately 45 Mbps, which is equal to the capacity of 28 T-1 circuits. It is a dedicated phone connection that actually consists of 672 individual channels, each of which supports 64 Kbps. T-3 lines are used mainly by Internet Service Providers (ISPs) connecting to the Internet backbone and for the backbone itself. T-3 lines are sometimes referred to as DS3 lines. (PC Webopædia, 1998).
• T-4 – This transmits at 274.176 Mbps, which is equal to the capacity of 178 T-1 circuits.
• OC-1 – In manner of definition, this term is related to the SONET, or Synchronous Optical Network, which has recently been accepted by the ANSI as a standard for optical transmission at gigabyte-per-second speeds. The OC-1 level is 51.84 Mbps, with each succeeding rate being a multiple thereof.
• SONET defines interface standards at the physical layer of the OSI seven-layer model. The standard defines a hierarchy of interface rates that allow data streams at different rates to be multiplexed. SONET establishes Optical Carrier (OC) levels from 51.8 Mbps (about the same as a T-3 line) to 2.48 Gbps. Prior rate standards used by different countries specified rates that were not compatible for multiplexing. With the implementation of SONET, communication carriers throughout the world can interconnect their existing digital carrier and fiber optic systems. The international equivalent of SONET, standardized by the ITU (International Telecommunication Union), is called SDH (Synchronous Digital Hierarchy). (PC Webopædia, 1998).
• OC-3 – This transmits at 155.52 Mbps. It should be noted that SONET/SDH is available in most large cities worldwide. Having just upgraded to a 56Kbps modem from a 14.4 – this writer can understand SONET numbers about as well as conceptualizing billions of billions, as in miles in the universe!

2. What happens when you replace a hub with a switch?

Hubs are simple devices that operate at the physical layer, and pass all traffic in both directions between the LAN sections that they link. They forward all messages, even if there is an area that they don’t need to go. They generally are just repeaters or amplifiers. The immediate difference will be a drastic improvement in performance of the network.

3. a) Describe FDDI and switched Ethernet. How does each work?

Our text shows that there are real limitations with a typical Switched Ethernet system. However, if a 10/100 Switched Ethernet configuration is adopted, it combines 10Base-T Ethernet and 100Base-T Ethernet. Each switch in the network has ports that can support either 10Base-T or 100Base-T, and some switches can support both. 10/100 Switched Ethernet is often used to provide traditional 10 Mbps Ethernet connections to client computers using traditional 10Base-T, with 100Base-T used in connecting to the server or to other switches. Compared to 10Base-T switched Ethernet, the 10/100 switched Ethernet dramatically reduces congestion at the network server and therefore provides better network performance. Depending on the application and the number of computers on the network, this may become as fast as Fast Ethernet. This would make the throughput fall at around 100 Mbps. (Fitzgerald & Dennis, 1998, pp. 244-45, 247-49, 249-51). FDDI also can provide throughput of 100 Mbps (p. 249). One note to be aware of is that FDDI (and also ATM) require the translation or condensing of Ethernet and token ring packets before they can flow through the backbone. This slows processing at the devices connecting the backbone network to the attached LANs. (p. 261). Comparably, the 10/100 switched Ethernet is bound to become more popular in the future, as it provides several advantages over other types of Ethernet choices. It doesn’t require new cable so is relatively cheaper to install, it helps relieve server congestion, and it improves network performance. FDDI provides a very efficient token-based transmission procedure that allows for many messages to be on the network at the same time, which significantly increases the amount of data it can convey.

Therefore, for FDDI and for Switched Ethernet, the throughput is comparable, as far as this review by this writer. Methods and means – and sometimes results – differ. Personally, I find it exciting that future plans for FDDI include melding of technologies and services with SONET, and the worldwide implications that carries. Only time will tell, for which technology will prevail. Or, perhaps, we will have great choices in the future, as each of the technologies experience growth, according to differing needs of companies and individuals. As time proceeds, and standards are ever achieved or improved, each of the choices will become stronger. At any rate, it seems that the flow of data is bound to increase – in amount, and in speed of delivery.

J

.Back to T.O.C.

References

Cook Report. (1998). The Cook Report: Glossary by Subject: MESH. Indexed Oct. 16, 1998. Modified by author: Sept. 26, 1998. Created by: pobox.com (online reference: http://www.pobox.com/) [Online]. Available: http://cookreport.com/glossary.html

Cray, Andrew. (1997). Wiring for speed, playing for time. CMPnet: The technology newsletter: CMP Media Inc. Issue: April 1997. [Online]. Available: http://www.data.com/tutorials/wiring_speed.html

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_____(1997). Chapter One: The revolution yet to happen; Authors: Bell, Gordon, and Gray, James N..

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Fitzgerald, Jerry, and Alan, Dennis. (1998). Business data communications and networking. New York, NY: John Wiley & Sons, Inc.

_____(1998). Business data communications and networking, Edition 5. [Online]. Available: http://tcbworks.cba.uga.edu/~adennis/fifth.htm
_____(1998). Business data communications and networking, Edition 6. [Online]. Available: http://www.cba.uga.edu/~adennis/telecom

Lantronix. (1997, 1998). Ethernet tutorial covering Ethernet and Fast Ethernet. Modified May 15, 1998. [Online]. Available: http://www.lantronix.com/htmfiles/mrktg/catalog/et.htm

PC Webopaedia Dictionary. (1998). Home page. [Online]. Available: http://webopedia.internet.com/; may be referenced from OneLook Dictionaries: The Faster Finder. [Online]. Available: http://www.onelook.com/

Stamper, David A. (1998). Local Area Networks. 2^nd Edition. Reading, Mass.: Addison-Wesley Longman, Inc.

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©1998 Leanne C. Boyd

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