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Backend networks are used to interconnect large systems such as mainfraimes, supercomputers and mass storage devices. Bulk data transfer among a limited number of deices in a small area. High reliability is a requirement along with: High data rate (100Mbps or higher), High speed interface (physical link between station and network must be at high speed), Distributed access (some sort of MAC technique to allow number of devices to share the LAN), Limited Distance (typically employed in a computer room or a small number of contiguous rooms), limited number of devices (generally in the 10's). Typically found at sites of large companiesor research installations with large data processing budgets.----------Chapter 9 PG262 -
local area network (LAN)
LAN invovles the use of shared transmission media or shared switching capacity to acheve high data rates over relatively short distances. Choice of transmission medium, design issue of access control and wireless LAN's are issues to be dealt with. LAN is needed to interconnect equipment on the user's premises and to provide a means to efficiently connect to outside services and other corporate sites.----------Chapter 9 PG261 - 265
storage area network(SAN)
A SAN is a separate network to handle storage needs. Generally uses the Fiber Channel protocol. In a SAN, no server sits between the storage devices and the networkl instead the storage device and servers are linked directly to the network. The SAN arrangement improves client to storage access efficiency.----------Chapter 9 PG263 -
Instead of using a single LAN which suffers from: reliability, capacity and cost issues; a better idea is to employ lower cost, lower capacity LANs within buildings and interconnect them with a higher capacity LAN. This is the backbone LAN.----------Chapter 9 PG264 -
Servers, used within a department or shared by users in a number of departments, can perform a variety of functions. Support expensive peripherals such as mass storage devices, provide applications that require large amounts of processor resources and maintaining databases accessible by many users.----------Chapter 9 PG265 -
Tiered LAN is the use of lower capacity and lower cost LANs and hook them together to a backbone LAN. Each tier will support a cluster of computers or shared services which can also be supported off of the backbone LAN. If a mainframe is attached, then a separate high speed LAN supports these devices and may be linked as a whole to the backbone LAN.----------Chapter 9 PG265 - 266
Electromagnetic waves are guided along a solid medium such as copper twisted pair, copper coaxial cable or optical fiber.----------Chapter 9 PG267 -
The physical path between transmitter and receiver. Can be guided or unguided media.----------Chapter 9 PG267 -
The atmosphere and outerspace are examples or unguided media which provides a means of transmitting electromagnetic signals but do not guide them.----------Chapter 9 PG267 -
The transmitting but not guiding of electromagnetic signals----------Chapter 9 PG267 -
Twisted Pair consists of two insulated copper wires arranged in a regular spiral pattern. A wire pair acts as a single communication link. These pairs are bundled together into a cable by wrapping them into a tough protective sheath. Much less expensive than other commonly used guided transmission media and is easier to work with. Limited in distance, bandwidth and data rate. It is susceptible to interference and noise due to its potential for coupling with electromagnetic fields. The use of different twist lengths in adjacent pairs reduces crosstalk.----------Chapter 9 PG269 - 271
shielded twisted pair (STP)
Shielded twisted pair improves upon UTP by covering it with a metallic braid to reduce interference. It provides better performance at lower data rates, but is more expensive and not as easy to work with.----------Chapter 9 PG270 -
unshielded twisted pair (UTP)
Unshielded Twisted Pair is ordinary telephone wire. This is the least expensive and easiest to work with. It is subject to electromagnetic interference including from nearby twisted pair and from noise generated in the environment.----------Chapter 9 PG270 -
Coaxial cable is like twisted pair in that it consists of two conductors but is constructed differently to permit it to operate over a wider range of frequencies. It consists of a hollow outer cylindircal conductor that surrounds a single wire conductor. Coax can be used over longer distances and supports more stations on a shared line than twisted pair. Coax provides good immunity from electromagnetic interference and provides greater capacity than STP but is more expensive.----------Chapter 9 PG272 -
Optical Fiber is a thin flexible medium capable of conducting an optical ray. The lowest losses have been obtained using fibers of ultrapure fused silica. It's difficult to make though, so higher loss multicomponent glass fibers are more economical and still provide good performance. Plastic fibers are less costly still, but have the highest loss (ok for short distances). Optical Fiber is better than coax and twisted pair because of: Greater capacity (data rates up to 100s of Gbps over 10km has been demonstrated), smaller size and weight (easier to use in cramped quarters), Lower attenuation (lower than twisted pair and coax), electromagnetic isolation (optical fiber systems are not affected by external electromagnetic fields - so it's not vulnerable to interference, impulse noise, or crosstalk) Fiber is also difficult to tap and secure from eavesdropping.----------Chapter 9 PG272 -
Standards that specify the cabling types and layout for commercial buildings. The generic wiring scheme has the following characterisitics: refers to the wiring in the building, scope of the system includes cabling to support al types of information transfer including voice, LANs, video and image transmission, and other forms of data transmission, the cabling layout and cable selection is independent of vendor and end-user equipment, the cable layout is designed to encompass distribution to all work areas within the building, so that relocation of equipment does not require rewiring but simply requires plugging the equipment into a preexisting outlet in the new location. The standards for structured cabling are EIA/TIA-568 and ISO 11801. The two standards are quite similar. Strategy is based on the use of a hierarchical, star wired cable layout. External Cable to Equipment Room to Backbone Cable (vertical) to telecommunications closet to Horizontal cable. Standards are vendor and equipment independent.----------Chapter 9 PG274 -
The committee that developed the architecture for the LAN protocol. The IEEE 802 reference model relates the LAN protocols with that of the OSI model. Therefore, the first layer is the physical layer, the second layer is broken up into 2 parts (Medium access control (MAC) and the Logical Link Control (LLC), above that are the higher level protocols----------Chapter 9 PG276 - 281
Logical Link Control (LLC)
The logical Link Control is the upper of the sub layers of layer 2 in the LAN protocol architecture. The LLC provides the interface with the higher level layers and performs flow and error control. LLC specifies the mechanisms for addressing stations across the medium and for controlling the exchange of data between two users. Three services are provided as alternativesfor attached devices using LLC: Unacknowledged connectionless service (a datagram style servce that is simple and involves no flow or error control thus the delivery of data is not guaranteed which is usually ok since most of the time a higher layer software deals with that), Connection-mode service (a logical connection is setup between two users and flow and error control are provided - like the HDLC), Acknowledged connectionless service (cross between the other two - datagrams are to be acknowledged, but no prior logical connection is set up).----------Chapter 9 PG277 - 278
Medium Access Control (MAC)
MAC protocols control access to the transmission medium to facilitate an orderly and efficient use the capacity. The MAC assembles the MAC frame for transmission and upon receipt disassembles the MAC frame for address recognition and error detection. The MAC frame consists of: MAC Control (field contains the protocol control information - priority can be set here), Destination MAC address( the destination physical attachment point), Source MAC Address (the source physical attachment point), LLC PDU (what is passed down from the LLC - user data and the source and destination service access point (SAP (or portals)), and the CRC (cyclic redundancy check (Frame check sequence(FCS)) for error detecting.----------Chapter 9 PG277 - 278
The measure of the ratio between two signal levels. Decible gain = Gdb = 10log10 Pout/Pin. G= Gain, Pin=input power level, Pout = output power level. Loss = Ldb = 10log10 Pin/Pout. Decible can also measure the loss or gain of voltage. Gdb = 20log10 Vout/Vin Loss = Ldb =20log10Vin/Vout.----------Chapter 9 PG285 -
Fibre Channel is used in SANs. Fibre Channel is designed to combine the best features of channel communications and protocol based network communications. Specifically the simplicity and speed of channel comms with the flexibility and interconnectivity of protocol based network comms. ----------Chapter 10 PG289 - onlinw
The extension of CSMA/CD to higher speeds. No one would develop a high speed network from scratch using CSMA/CD, however, it is used for high speed Ethernet LANs since the protocol is well understood by vendors and end users Cost and ease of implementation is more important. Fast Ethernet is a set of specifications devveloped by the IEEE 802.3 committee to provide a lo-cost, Ethernet compatible LAN operating at 100 Mbps.----------Chapter 10 PG289 - 302
Most widely used high speed LAN today and was developed by the IEEE 802.3 Committee. There is a MAC and a physical control layer like all LANs. The original LAN was operated at 10 Mbps but subsequent standards were developed for 100Mbps, 1Gbps, and 10 Gbps. Classic Ethernet operates at 10Mbps over a bus topology using CSMA/CD MAC control protocol.----------Chapter 10 PG290 -
Bus Topology LAN
In a bus topology all stations attach through appropriate hardare interfacing know as a tap, directly to a linear transmission medium or bus. Any transmission is transmitted to all stations along the bus and at each end of the bus is a terminator to end the signal and remove it from the bus. Since it goes through all stations, only one station can transmit at one time or else there will be a collission. To fix this, the stations transmit in frames and use CSMA/CD MAC.----------Chapter 10 PG290 -
The stations on a bus topology transmits data in small chunks called frames. Each frame consists of a portion of the data that a station wishes to transmit plus a frame header that contains control information. Each station along a bus is assigned a unique address, or identifier, and the destination address for a frame is included in the header.----------Chapter 10 PG290 -
carrier sense multiple access with collision detection (CSMA/CD)
Is a medium access control protocol that operates with bus topology in classic LAN ethernet. It works by having each station "sense" whether there is a transmission occuring on the medium. If it is idle, then it transmits, if the medium is busy then the station continues to listen until the medium is idle. If there is a collision, a jamming signal is issued to let every station know that there was a collision and to cease transmission. Each station waits a random amount of time (known as the "backoff") then they attempt to transmit again. The logic is easy to implement and is relatively simplistic so there is little to go wrong in the execution.----------Chapter 10 PG290 - 291
frame check sequence
FCS is a error control mechanism. Which uses an algorithm (usually prime numbers 17 or 33) to run an equation on the frame to ensure that nothing is lost or in error in the delivery. The MAC frame for the 802.3 (LAN ethernet) is a 32 bit Cyclic Reduncancy Check.----------Chapter 10 PG293 -
MAC Frame for 802.3 protocal (Ethernet)
The MAC frame consists of the preamble (7 octet pattern), Start Frame Delimiter (SFD) (Sequence that indicates start of the frame), Destination Address, Source Address, Length (Length of the LLC data field in octets - max frame size excluding preamble and sfd is 1518 octets), Pad (added octets to ensure that the frame is long enough for proprer CD ops), FCS (32 bit CRC).----------Chapter 10 PG293 -
Bridge allows for the expansion of a single LAN to interconnect other LANs and a WAN. Bridge is a simple device that provides the means to interconnect similar LANs. The bridge is designed for use between LANs that use identical protocols for the physical and link layers (all conforming to IEEE 802.3). Because the devices use the same protocols, the amount of processing required at the bridge is minimal. The use of a bridge allows for reliability (the networks can be partitioned into self-contained units), performance (performance declines with the increase of devices on the LAN, so breaking it up into smaller clusters can improve the overall LAN), Security (different types of data can be kept on physically separate media and then connected with the bridge), geography (geographically distant locations can be connected with a bridge). Frequently the bridge function is performed by two half bridges, one on each LAN. The functions are relatively simple: Read all frames transmitted on A and accept those addressed to any station on B. Using the MAC protocol for B retransmit each frame on B. Do the same for B-to-A Traffic. A few aspects to highlight, the bridge makes no modification to the content or format of the frames it receives, nor does it encapsulate them with an additional header. Each frame is simply copied from one LAN and repeadted with the same bit pattern on the other LAN. The bridge should contain enough buffer space to meet peak demands. Bridge must contain addressing and routing intelligence. At a minimum thebridge must know which addresses are on each network to know which frames to pass. A bridge may connect more than two LANs.----------Chapter 10 PG294 -
A hub is an alternative to the bus topology. Each station is connected to the hub by two lines (transmit and receive). The hub acts as a repeater: When a single station transmits, the hub repeats the signal on the outgoing line to each station. Oridnarily, the line line consists of two UTP's. The scheme is physically a star, but logically a bus. If two stations transmit at once a collision will occur. Multiple levels can be cascaded in a hierarchical configuration. There is one header hub and one or more intermediate hubs.----------Chapter 10 PG296 -
layer 2 switch
The layer 2 switch has replaced the hub in popularity for High Speed LANs. The layer 2 switch is sometimes referred to as the switching hub. The central hub acts as a switch, similar to a packet switch. An incoming frame is swtiched to the appropriate output line to be delivered to the intended destination. The other unused lines can be used for switching other traffic. Each individual device is limited to the capacity of the layer 2 switch, but the overall throughput is a multiple of the number of transmitting devices and the device capacity (so 2 devices X 10 Mbps = 20Mbps throughput). Features of the layer 2 switch: No change is required to the software or hardware of the attached devices to convert a bus LAN or a hub LAN to a swtiched LAN. In either case the MAC protocol still uses the same protocol to access the LAN. The layer 2 switch scales easily. Additional devices can be added to the layer 2 switch simply by increasing the capacity of the later 2 switch. There are two types of switches: cut-through switch and store-and-forward switch. The layer 2 switch can be viewed as a full-duplex version of the hub and can function as a multiport bridge. A bridge will do the frame handling in software where as the switch does it in hardware (faster). bridge can only do store and forward whereas switch can do cut through. Bridge can only do one frame at a time where as the switch has multiple paths and can handle multiple frames at once.----------Chapter 10 PG297 - 299
The layer 2 switch as a cut-through takes advantage of the fact that the destination address appears at the beginning of the MAC frame. As soon as it recognizes the intended destination, the layer 2 switch begins repeating the incoming frame onto the appropriate output line. This yields the highest possible throughput but at some risk to propagating bad frames since the switch is not able to check the CRC.----------Chapter 10 PG299 -
cyclic redundancy check (CRC)
CRC is the algorithm performed on frames to ensure that they contain no damanged or incomplete frames.----------Chapter 10 PG299 -
layer 3 switch
Due to potential problems with layer 2 switches - broadcast overload and lack of multiple links, layer 3 switches are being used. A set of devices connected by layer 2 switches is considered to have a flat address space (all users share a common MAC broadcast address). Thus if any device issues a MAC frame with a braodcast address, that frame is delivered to all devices attached to the overall network connected by layer 2 switches. This can create tremendous overhead or a broadcast storm with a malfunctioning device jams the network with numerous broadcast frames. A second performance issue is that there can be no closed loops so there is only one path between any two devices and thus implossible to provide multiple paths through multiple switches (which limits performance and reliability). Therefore you break up a large network into various subnetworks with routers. However routers are limited in the amount of packets that they can accomodate since they do all of the routing processing via software. Layer 3 switches implement the packet forwarding logic of the router but have it in hardware instead. 2 types of level 3 switches - packet by packet and flow based. Packet by packet is identical to a router but with hardware instead of software to accommodate the heavy load. Flow based tries to enhance the performance by identifying flows of IP packets that have the same source and destination so that a predefined route can be established through the network.----------Chapter 10 PG299 -
Store and forward is one of the types of layer 2 switches. It accepts a frame, buffers it briefly and then routes it to the appropriate output line----------Chapter 10 PG299 -
Wireless LAN is one that makes use of a wireless transmission medium. There are 4 applications for wireless LANs: LAN extension, cross building interconnect, nomadic access and ad hoc networks. Wireless was not always used due to issues with high prices, low data rates, occupational safety concerns and licensing requirements - these issues are being addressed and therefore popularity has grown----------Chapter 11 PG317 -
When a wireless LAN is hooked into a wired LAN on the same premises. The wireless is an extension of the wired LAN for areas where wired LAN is either not feasible or practical. There typically is a backbone wired LAN, such as an ethernet, that supports the servers, workstations, and one ormore bridges or routers to link to other networks. In addition there is a control module that acts as an interface to a woreless LAN. The control Module includes either bridge or a router functionality to link the wireless LAN to the backbone. It includes osme sort of access control logic such as polling or token passing scheme to regulate the access from theend systems. Hubs may also be part of the wireless configuration.----------Chapter 11 PG317 - 318
Another use of Wireless LAN is to connect LANs in nearby buildings be they wired or wireless. In this case a point to point wireless link is used between two buildings (two microwave or infrared transmitter/receiver units) The devices, so connected, are typically bridges or routers. This is not a LAN per se but it is usual to include this application under Wireless LAN.----------Chapter 11 PG319 -
Nomadic access provides a wireless link between a LAN hub and a mobile data terminal equipped with an antenna, such as a laptop computer. Users who move around and want to connect to a wired LAN from various locations can use this.----------Chapter 11 PG319 -
ad hoc networking
An ad-hoc network is a peer-to-peer network (no centralized server) set up temporarily to meet some immediate need. For instance, employees meet for a meeting in a conference room and set up their laptops to develop a temporary network just for the duration of the meeting.----------Chapter 11 PG320 -
Service Area is the coverage area for the wireless LAN. Typically 100 to 300 m in diameter.----------Chapter 11 PG321 -
Infrared LAN is an individual cell that is limited to a single room since infrared light does not penetrate opaque walls.----------Chapter 11 PG322 -
For higher speeds, a technology known as Orthogonal Frequency Division Multiplexing (OFDM) is superior to spread spectrum. These products typically operate in either the 2.4 GHZ or the 5GHZ band----------Chapter 11 PG322 -
spread spectrum LAN
This type of LAN makes use of spead spectrum transmission technology. These LANs operate in the ISM (industiral, scientific, and medical) 2.4GHZ microwave bands so that no FCC licensing is required for their use in the US----------Chapter 11 PG322 -
access point (AP)
Any entity that has station functionality and provides access to the distribution system via the wireless medium for associated stations. The AP functions as a bridge and a relay point for the Basic Service Set that is trying to connect to a distribution system.----------Chapter 11 PG322 - 324
The MAC protocol and physical medium specifications devoted to wireless LAN. Common standards are 802.11a, 802.11b, 802.11g and 802.11n.----------Chapter 11 PG322 - 325
Mulitple input Multiple output antenna architecture is the most important of the enhancements provided by 802.11n. In a MIMO scheme the transmitter employs multiple antennas so a better receive signal is possible.----------Chapter 11 PG331 -
narrowband microwave LAN
Narrowband microwave is similar the broadcasting of a radio station. The broadcast range is about 5000sq meters, but cannot go through load-bearing or steel walls. Licensed by the FCC, narrowband microwave uses the 18.82 to 19.205GHz of the radio spectrum.----------Chapter 11 PG??? -
UTP cables and associated connecting hardware whose transmission characteristics are specified up to 16 Mhz----------Chapter 9 PG270 - 271
UTP cables and associated connecting hardware whose transmission characteristics are specified up to 20 Mhz----------Chapter 9 PG270 - 271
UTP cables and associated connecting hardware whose transmission characteristics are specified up to 100 Mhz----------Chapter 9 PG270 - 271
Cat 3 vs Cat 5
Cat 3 and 5 are the cables that are used the most for LAN. Cat 3 is the voice grade cable that is pre-wired into most older construction. Data rates of up to 16 Mbps should be achievable depending on distance and design quality. The main difference between Cat 3 and 5 i the number of twists in the cable per unit distance. Cat 5 is more tightly twisted which is more expensive but allows for better performance. Cat 5 cable can achieve 100Mbps with limited distance and quality design----------Chapter 9 PG270 - 271
How do the key requirements for computer room networks differ from those for personal computer local networks?
Personal Computer networks are used for connecting personal computers and communicating between clients and servers. Therefore the need is to have a access to numerous personal systems throughout the company for collaboration, shared access of resources, and access to the internet. In contrast, the computer room networks (Backend networks) are used to transfer high volume data transfer amongst a small number of devices. A backend network can typically be deployed in one computer room, hence the name computer room network. Other requirements for Backend networks are: High data rate (100Mbps or higher), high speed interface (physical link between station and network must be high speed), and distributed access (using distributed MAC to enable efficient sharing of the LAN).----------Chapter 9 PG9.1 -
What are the differences among backend LANs, SANs and backbone LANs?
The backend LAN is used to interconnect systems such as supercomputers and mainframes in order to transfer and process large amounts of data. The Storage Area Network is used to handle storage needs and connects directly with the network with out the need of a server (which improves client to storage access efficiency). A backbone LAN is the interconnection of lower cost and lower capacity LAN's in departments or buildings with a higher capacity LAN in order to improve upon cost, capacity and reliability of a single LAN.----------Chapter 9 PG9.2 -
Other than large storage capacity, what other advantage does a SAN provide?
In addition to the storage capacity, Storage Area Networks increase the efficiency of client to storage access and allows for backups and replicating through direct storage to storage communications.----------Chapter 9 PG9.3 -
What is the protocol typically used in storage area networks?
Fibre Channel Protocol.----------Chapter 9 PG9.4 -
Differentiate between guided media and unguided media.
Guided media is the guiding of electromagnetic waves down a solid medium - twisted pair, optical fiber, etc.----------Chapter 9 PG9.5 -
Why are the wires twisted in twisted pair copper wire?
The twisting of the wires can reduce low-frequency interference and the use of different twist length in adjacent pairs can reduce cross talk.----------Chapter 9 PG9.6 -
What are some major limitations of twisted pair wire?
There are some major limitations of twisted pair wire: it is highly susceptible to interference and noise due to the potential for coupling with electromagnetic fields, impulse noise can also intrude in and it is limited in bandwidth, range and data rate transfer. These limitations can be reduced by shielding or sheathing the wire to reduce interference, the actual twisting of the wires can reduce low-frequency interference and the use of different twist length in adjacent pairs can reduce cross talk.----------Chapter 9 PG9.7 -
What is the difference between unshielded twisted pair and shielded twisted pair?
Unshielded Twisted Pair (UTP) is telephone wire which is pre-wired into office buildings with much more than is needed for just the phone system. It is the least expensive of all the transmission media for LANs. It is covered in plastic wrap with a plastic covering.----------Chapter 9 PG9.8 -
Describe the components of optical fiber cable.
Optical fiber cable consists of three concentric sections. The outer layer is a protective plastic light absorbing jacket layer that protects the inner two layers of glass. The center layer is the core fiber and the next layer out is the cladding. The core and cladding have different indexes of refraction. The optical fiber is thin and flexible and is capable of conducting an optical ray.----------Chapter 9 PG9.9 -
What are the wavelengths used in fiber optic communications?
There are three types of wavelengths used for fiber optic communications: step-index multimode, single-mode and graded-index multimode. ----------Chapter 9 PG9.1 -
Arrange the following in order from highest to lowest regarding bandwidth: UTP, fiber and coaxial cable.
This depends on which UTP you are referencing and the distances, however according to Stallings p. 273 - practical bandwidth for fiber is 100's of Gbps, Coaxial is 100's of Mbps and twisted pair is up to 100 Mbps. So:----------Chapter 9 PG9.11 -
Arrange the following in order from highest to lowest regarding cost: UTP, fiber and coaxial cable.
Highest Cost to Lowest Cost - Fiber, Coaxial, UTP----------Chapter 9 PG9.12 -
What is the purpose of the IEEE 802 committee?
The purpose of the IEEE 802 committee is to develop, maintain and revise the LAN and MAN standards..----------Chapter 9 PG9.13 -
Why are there multiple LAN standards?
There are multiple LAN standards because there is a wide range of data rates and transmission media that the LAN may need to support. Therefore there are there are a set of standards that are developed by the IEEE 802 committee that are accepted and used by most of the products on the market. Some of the used standards are for the Token Ring, Wireless LAN, Ethernet, etc.----------Chapter 9 PG9.14 -
List and briefly define the services provided by LLC.
The LLC is used as an interface with higher layers and for flow and error control. The services that the LLC provides are:----------Chapter 9 PG9.15 -
List and briefly define the types of operation provided by the LLC protocol.
The LLC defines operation in the areas of connection oriented and connectionless oriented. The connection oriented operation uses the connection-mode service which provides flow control and reliability mechanisms. It's used in simple devices with little software operating above it. For connectionless oriented operations there is unacknowledged connectionless service and acknowledged connectionless service. Unacknowledged connectionless service does not require a lot of logic and is useful if there are higher layers of software such as TCP which provides for the reliability and flow control which the service does not provide and is useful when there is no need for lots of overhead. The Acknowledged connectionless service is useful when a user needs guaranteed delivery for a large number of destinations which makes connection oriented impractical. Also useful in handling time critical signals in a factory.----------Chapter 9 PG9.16 -
List some basic functions performed at the MAC layer.
Some basic functions of the Medium Access Control (MAC) layer are: provide an orderly and efficient use of the transmission medium, detecting errors and discarding frames that contain errors, and assemble the data into frames with error detection fields on transmission.----------Chapter 9 PG9.17 -
What is a server farm?
A server farm is a group of servers that are used by a number of users for traffic or application that is greater than the capacity of a single server. Generally significant traffic is generated and may be used for processing demanding applications or supporting mass storage devices.----------Chapter 10 PG10.1 -
Explain why a data rate of 10 Mbps on all LAN segments is increasingly inadequate for many businesses.
The increase in network data flow, newer applications that demand faster data rates, increase in network storage capabilities and requirements, support for larger organizations and geographic dispersal have required businesses to have LANs with high speed data rates.----------Chapter 10 PG10.2 -
What is CSMA/CD?
CSMA/CD (carrier sense multiple access with collision detection) is the MAC protocol for classic Ethernet. CSMA/CD first uses the carrier sense multiple access by listening to the medium to see if another transmission is currently in progress. If not, the station transmits. If there is another transmission, the station waits until the medium is idle. If there is a collision (two or more stations transmit at same time), the station transmit a jamming signal to let all stations know there has been a collision, then wait a random amount of time (called the backoff) and then transmit if the medium is idle. The advantage to CSMA/CD is its simplicity and ease of implementation.----------Chapter 10 PG10.3 -
What functions are performed by a bridge?
A bridge is a device that is used for the interconnection of LANs that use identical protocols for the physical and link layers. The use of a bridge increases reliability (break network into self-contained units), performance, security and geographic distance. The functions of the bridge are relatively simple: Each frame is transferred by copying it from one LAN and repeated with the same bit pattern to the other, it contains buffer space adequate to handle peak needs, the bridge must contain routing and addressing information and it may handle more than two LANs.----------Chapter 10 PG10.4 -
What is the difference between a hub and a layer 2 switch?
A layer 2 switch has replaced hub as the device of choice for high speed LAN and is sometimes referred to as a "switching hub". The main difference is that in a hub, only one station can transmit at a time, and then the transmission is retransmitted over all outgoing lines. A layer 2 switch, on the other hand, allows for more than one station transmission at a time. Instead of transmitting the output over all lines, it switches the output to the appropriate output line to be delivered. This increases the overall throughput on the LAN over the data rate dedicated to each individual device.----------Chapter 10 PG10.5 -
What is the difference between a store-and-forward switch and a cut-through switch?
A store-and-forward switch is a layer 2 switch which accepts a frame on an input line, buffers it and then sends it to the appropriate output line. There is a delay in the sending but the overall integrity of the network is increased. A cut-through switch, on the other hand, analyzes the destination address at the beginning of the MAC frame and starts repeating the frame onto the appropriate output line as soon as the destination address is recognized. This increases the throughput but also increases the risk of sending bad frames since it doesn't have time to examine the CRC.----------Chapter 10 PG10.6 -
What are the differences between a bridge and a switch?
There are numerous differences between bridges and switches: The frame handling in a switch is performed in hardware where in a bridge it's performed by software. A switch can analyze multiple frames at a time due to its multiple data paths whereas a bridge can only typically handle one frame at a time. The bridge only uses the store and forward operation where the switch can use the store and forward or the cut-through operation instead. These differences allow switches to have much higher performance than the bridge and therefore the bridge has diminished commercially.----------Chapter 10 PG10.7 -
What is meant by the phrase flat address space?
Flat address space refers to the fact that all users for devices connected by a layer 2 switch share a MAC broadcast address. So any device that sends out a MAC frame with a broadcast address, all devices on the network will receive that frame. This can cause a significant overhead or worse a clogging of the network by a malfunctioning device(also called a broadcast storm).----------Chapter 10 PG10.8 -
What are the transmission medium options for Fast Ethernet?
100BASE-TX - 100BASE-FX - 100BASE-T4 - - - The 100 Base-X schemes use two physical links between nodes - one for transmission and one for reception. 100Base-T uses 802.3 MAC protocol and frame format. 100 BASE-X option requires the installation of new cable in most buildings so 100BASE-T4 which can use Cat 3 UTP and CAT5 UTP is sometimes more attractive.----------Chapter 10 PG10.9 -
What are the transmission medium options for Gigabit Ethernet?
1000BASE-LX - long wavelength option supports single-mode and multimode fiber. - 1000BASE-SX - short wavelength option supports multimode fiber - 1000BASE-T - uses 4 pairs of CAT 5 UTP to support devices up to 100 m - 1000BASE-CX - Supports STP cable among devices within a single room using copper jumpers.----------Chapter 10 PG10.1 -
What are the transmission medium options for 10-Gbps Ethernet?
10GBASE-S - (Short) uses multimode fiber for distances up to 300 m. - 10GBASE-L - (Long) uses single-mode fiber for distances up to 10km - 10GBASE-E - (Extended) Single-mode fiber for distances up to 40km. - 10GBASELX4 - Uses wavelength multiplexing to multiplex the bit stream across four light waves. Can be used on single-mode or multi-mode for up to 10km.----------Chapter 10 PG10.11 -
List and briefly define four application areas for wireless LANS
LAN extension - When a wireless LAN is hooked into a wired LAN on the same premises. The wireless is an extension of the wired LAN for areas where wired LAN is either not feasible or practical. There typically is a backbone wired LAN, such as an ethernet, that supports the servers, workstations, and one ormore bridges or routers to link to other networks. In addition there is a control module that acts as an interface to a woreless LAN. The control Module includes either bridge or a router functionality to link the wireless LAN to the backbone. It includes osme sort of access control logic such as polling or token passing scheme to regulate the access from theend systems. Hubs may also be part of the wireless configuration. Cross building interconect - Another use of Wireless LAN is to connect LANs in nearby buildings be they wired or wireless. In this case a point to point wireless link is used between two buildings (two microwave or infrared transmitter/receiver units) The devices, so connected, are typically bridges or routers. This is not a LAN per se but it is usual to include this application under Wireless LAN. Nomadic Access - Nomadic access provides a wireless link between a LAN hub and a mobile data terminal equipped with an antenna, such as a laptop computer. Users who move around and want to connect to a wired LAN from various locations can use this. Ad-Hoc Networking -set up their laptops to develop a temporary network just for the duration of the meeting.----------Chapter 11 PG11.1 -
List and briefly define key requirements for wireless LANs
The wireless LANs must meet the same sort of requirements typical of any LAN, including high capacity, ability to cover short distances, full connectivity among attached stations and bradcast capability. In addition there are a number of requiremenets specific to wireless LANs. They are: Throughput (MAC protocal should make as efficient use as possible of the sireless medium to maximize capacity), number of nodes (Wireless LANs may need to support hundreds of nodes across multiple cells), connection to backbone LAN (In most cases, interconnection with stations on a wired backbone is required. For infrastructure wireless LANs this is easy with the use of control modules that connect both types of LANs), Service Area (A typical coverage area for wireless LAN has a diameter of 100 to 300 m), Battery power consumption (typically wireless LAN implementations have features to reduce power consumption while not using the network. MAC protocols that require mobile nodes to monitor access point constantly or engage in frequent handshakes uses battery life of mobile users and is therefore inappropriate), Transmission robustness and security (wireless LAN may be intereference prone and easily eavesdropped. The design of a wireless LAN must permit reliable transmission even in a noisy environment and shoulb provide some level of security from eavesdropping), Collocated network operations (As wireless LANs become more popular it is likely that 2 or more wireless LANs will operate in the same area and therefore intereference is possible. Such intereference may allow for unauthorized access and may thwart normal operation of MAC algorithm), License Free operations (operate wireless LAN without having to secure a license for a frequency band), Handoff/roaming (MAC protocol used in the wireless LNA should enable mobile stations to move from one cell to another.), Dynamic configuration (the MAC addressing and network management aspects of the LAN should permit dynamic and automated addition, deletion and relocation of end systems without disruption to other users). ----------Chapter 11 PG11.2 -
What is the difference between a single cell and multiple cell wireless LAN?
A single cell wireless LAN is an example of an LAN extension where all of the wreless end systems are within range of a single control module. In a multiple cell wireless LAN, there are multiple control modules interconnected by a wired LAN and each control module supports a number of wireless end systems within its transmission range. An example of multiple cell wireless is infared LAN where transmission is limited to a single room so there is a cell needed for each room in an office building.----------Chapter 11 PG11.3 -
What is the basic building block of an 802.11 WLAN?
The basic service set which consists of some number of stations executing the same MAC protocol and competing for access to the same shared wireless medium. A BSS may be isolated or it may connect to a backbone distribution system (DS) through an Access Point (AP). The AP functions as a bridge and a relay point. In a BSS, the client stations do not communicate directly with one another, rather, if one station in the BSS wants to communicate with another station in the same BSS, the MAC frame is first sent from the originating station to the AP and then from the AP to the destination station. The BSS is generally considered a Cell. When all the stations in a BSS are mobile stations with no connection to other BSSs, the BSS is called an Independent BSS. An IBSS is typically an adhoc network and the stations communicate directly with no AP involvement.----------Chapter 11 PG11.4 -
Define an extended service set
An extended service set (ESS) consists of two or more basic service sets interconnected by a distribution system. Typically the DS is a wired backbone LAN but can be any communications network. The ESS appears as a single logical LAN to the logical link Control (LLC) level.----------Chapter 11 PG11.5 -
List and briefly define IEEE 802.11 services
There are 9 services that need to be provided by a wireless LAN to achieve functionality equivalent to that of a wired LAN: 1) MSDU delivery - the basic service which is responsible for the basic transport of MAC service data units (MSDUs) - MSDU is a block of data passed down from the MAC user to the MAC layer. The MSDU is typically a LLC PDU. If too large, it can be fragmented and passed down as MAC frames. 2) Distribution - is the primary service used by statios to exchange MAC frames when the frames must traverse the DS from one BSS to another BSS. Example STA2 to STA7. Requires information about stations in ESS(info is provided by association-related services). 3) Integration- enables the transfer of data between a station on an IEEE 802.11 LAN and a station on an integrated IEEE 802.x LAN. Takes care of any address translation andmedia conversion log required for exchanging data. 4) Association - Establishes an initial associaten between a station and an AP (an address is required for a station on a wireless LAN). 5) Reassociation - enables an established association to be transferred from one BSS to another (mobility management). 6) Disassociation - Terminates an association. 7) Authentication - Establishes the identity of stations to each other. 8) Deauthentication - Terminates an existing authentication 9) Privacy - Prevents transmission from being read by others.----------Chapter 11 PG11.6 -
What is the difference between an access point and a portal?
The access point is the logic within a wireless station that provides access to the DS by providing DS services in addition to acting as a station. A portal is actually integrated into the bridge or router that is part of the Wired LAN and that is attached to the DS. The portal integrates the IEEE802.11 architecture with the wired LAN.----------Chapter 11 PG11.7 -
Is a distribution system a wireless network?
A DS can be a wired network, a wireless network or a switch.----------Chapter 11 PG11.8 -
How is the concept of an association related to that of mobility?
The primary purpose of the MAC layer is to transfer MSDUs between MAC entities; this purpose is fulfilled by the DS. Before the DS can deliver data to or accept data from a station that station must be associated. The mobility of the station is integral to associatin that station. There are 3 mobility transition types: No transition - A station of this type is stationary or moves only within a single BSS, BSS transition - a station of this type moves from one BSS to another BSS within the same ESS, and ESS transition - a station of this type moves from a BSS in one ESS to a BSS within another ESS.----------Chapter 11 PG11.9 -
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