Network Plus 2005 In Depth

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Token Ring has traditionally been more expensive to implement than Ethernet, but because of its token-passing routine, does not suffer collisions and offers high reliability and fault tolerance. Few Token Ring networks remain, as Ethernet can achieve higher throughput at lower costs.

FDDI (Fiber Distributed Data Interface) is a networking standard originally specified by ANSI in the mid-1980s and later refined by ISO. It uses a dual fiber-optic ring to transmit data at speeds of 100 Mbps. FDDI’s fiber-optic cable and dual fiber rings offer greater reliability and security than twisted-pair copper wire. It is much more expensive than Fast Ethernet.

ATM (Asynchronous Transfer Mode) is a Data Link layer standard that relies on fixed packets, called cells, consisting of 48 bytes of data plus a 5-byte header.

ATM is a connection-oriented technology. Its switches establish virtual circuits, or logical point-to-point connections between sender and receiver, and then transmit data. Having a reliable connection enables ATM to guarantee QoS (quality of service) levels for designated transmissions.

Wireless standards vary by frequency, methods of signal, and geographic range. The IEEE 802.11 committee has specified three notable wireless standards: 802.11b, 802.11a, and 802.11g. All three share characteristics at the MAC sublayer level, including the CSMA/CA access method, frame formats, and methods of association between access points and stations.

Currently, 802.11b is the most popular standard used on wireless networks. Its maximum throughput is 11 Mbps (though actual throughput is typically half of that). Home networks might use Bluetooth or Infrared (IR) technology, whose ranges are shorter and throughputs are lower than those of 802.11 networks.

Key Terms

802.11a—The IEEE standard for a wireless networking technique that uses multiple frequency bands in the 5-GHz frequency range and provides a theoretical maximum throughput of 54 Mbps. 802.11a’s high throughput, compared with 802.11b, is attributable to its use of higher frequencies, its unique method of encoding data, and more available bandwidth.

802.11b—The IEEE standard for a wireless networking technique that uses DSSS (direct sequence spread spectrum) signaling in the 2.4–2.4835-GHz frequency range (also called the 2.4-GHz band). 802.11b separates the 2.4-GHz band into 14 overlapping 22-MHz channels and provides a theoretical maximum of 11-Mbps throughput. 802.11b is also known as Wi-Fi.

802.11g—The IEEE standard for a wireless networking technique designed to be compatible with 802.11b while using different encoding techniques that allow it to reach a theoretical maximum capacity of 54 Mbps. 802.11g, like 802.11b, uses the 2.4-GHz frequency band.

802.15.1—The IEEE standard for wireless personal area network (WPAN) technology, including Bluetooth.

802.3af—The IEEE standard that specifies a way of supplying electrical power over Ethernet (PoE). 802.3af requires CAT 5 or better UTP or STP cabling and uses power sourcing equipment to supply current over a wire pair to powered devices. PoE is compatible with existing 10BASE-T, 100BASE-TX, and 1000BASE-T implementations.

access method—A network’s method of controlling how nodes access the communications channel. CSMA/CD (Carrier Sense Multiple Access with Collision Detection) is the access method specified in the IEEE 802.3 (Ethernet) standard. CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) is the access method specified by IEEE 802.11 (wireless LAN) standards.

active monitor—On a Token Ring network, the workstation that maintains timing for token passing, monitors token and frame transmission, detects lost tokens, and corrects problems when a timing error or other disruption occurs. Only one workstation on the ring can act as the active monitor at any given time.

active scanning—A method used by wireless stations to detect the presence of an access point. In active scanning, the station issues a probe to each channel in its frequency range and waits for the access point to respond.

active topology—A topology in which each workstation participates in transmitting data over the network.

association—In the context of wireless networking, the communication that occurs between a station and an access point to enable the station to connect to the network via that access point.

Asynchronous Transfer Mode—See ATM.

ATM (Asynchronous Transfer Mode)—A Data Link layer technology originally conceived in 1983 at Bell Labs, and standardized by the ITU in the mid-1990s. It relies on fixed packets, called cells, that each consist of 48 bytes of data plus a 5-byte header. ATM relies on virtual circuits and establishes a connection before sending data. Having a reliable connection therefore allows network managers to specify QoS levels for certain types of traffic.

beacon frame—In the context of wireless networking, a frame issued by an access point to alert other nodes of its existence.

Bluetooth—A wireless networking standard that uses FHSS (frequency hopping spread spectrum) signaling in the 2.4-GHz band to achieve a maximum throughput of either 723 Kbps or 2.1 Mbps, depending on the version. Bluetooth was designed for use primarily with small office or home networks in which multiple devices (including cordless phones, computers, and pagers) are connected.

Bluetooth Special Interest Group (SIG)—A consortium of companies, including Sony Ericsson, Intel, Nokia, Toshiba, and IBM, that formally banded together in 1998 to refine and standardize Bluetooth technology.

bus—The single cable connecting all devices in a bus topology.

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bus topology—A topology in which a single cable connects all nodes on a network without intervening connectivity devices.

Carrier Sense Multiple Access with Collision Avoidance—See CSMA/CA.

Carrier Sense Multiple Access with Collision Detection—See CSMA/CD.

cell—A packet of a fixed size. In ATM technology, a cell consists of 48 bytes of data plus a 5- byte header.

circuit switching—A type of switching in which a connection is established between two network nodes before they begin transmitting data. Bandwidth is dedicated to this connection and remains available until users terminate the communication between the two nodes.

collapsed backbone—A type of backbone that uses a router or switch as the single central connection point for multiple subnetworks.

collision—In Ethernet networks, the interference of one network node’s data transmission with another network node’s data transmission.

collision domain—The portion of an Ethernet network in which collisions could occur if two nodes transmit data at the same time.

CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance)—A network access method used on 802.11 wireless networks. In CSMA/CA, before a node begins to send data it checks the medium. If it detects no transmission activity, it waits a brief, random amount of time, and then sends its transmission. If the node does detect activity, it waits a brief period of time before checking the channel again. CSMA/CA does not eliminate, but minimizes, the potential for collisions.

CSMA/CD (Carrier Sense Multiple Access with Collision Detection)—A network access method specified for use by IEEE 802.3 (Ethernet) networks. In CSMA/CD, each node waits its turn before transmitting data, to avoid interfering with other nodes’ transmissions. If a node’s NIC determines that its data has been involved in a collision, it immediately stops transmitting. Next, in a process called jamming, the NIC issues a special 32-bit sequence that indicates to the rest of the network nodes that its previous transmission was faulty and that those data frames are invalid. After waiting, the NIC determines if the line is again available; if it is available, the NIC retransmits its data.

daisy chain—A group of connectivity devices linked together in a serial fashion.

data propagation delay—The length of time data takes to travel from one point on the segment to another point. On Ethernet networks, CSMA/CD’s collision detection routine cannot operate accurately if the data propagation delay is too long.

DB-9 connector—A connector containing nine pins that is used on STP-based Token Ring networks.

distributed backbone—A type of backbone in which a number of connectivity devices (usually hubs) are connected to a series of central connectivity devices, such as hubs, switches, or routers, in a hierarchy.

enterprise—An entire organization, including local and remote offices, a mixture of computer systems, and a number of departments. Enterprise-wide computing takes into account the breadth and diversity of a large organization’s computer needs.

Ethernet_II—The original Ethernet frame type developed by Digital, Intel, and Xerox, before the IEEE began to standardize Ethernet. Ethernet_II contains a 2-byte type field to identify the upper-layer protocol contained in the frame. It supports TCP/IP, AppleTalk, IPX/SPX, and other higher-layer protocols.

FDDI (Fiber Distributed Data Interface)—A networking standard originally specified by ANSI in the mid-1980s and later refined by ISO. FDDI uses a dual fiber-optic ring to transmit data at speeds of 100 Mbps. It was commonly used as a backbone technology in the 1980s and early 1990s, but lost favor as Fast Ethernet technologies emerged in the mid-1990s. FDDI provides excellent reliability and security.

Fiber Distributed Data Interface—See FDDI.

High-Speed Token Ring—See HSTR.

HSTR (High-Speed Token Ring)—A standard for Token Ring networks that operate at 100 Mbps.

hybrid topology—A physical topology that combines characteristics of more than one simple physical topology.

Infrared Data Association—See IrDA.

IrDA (Infrared Data Association)—A nonprofit organization founded in 1994 to develop and promote standards for wireless communication using infrared signals. IrDA is also used to denote the type of wireless technology this group has developed.

jamming—A part of CSMA/CD in which, upon detecting a collision, a station issues a special 32-bit sequence to indicate to all nodes on an Ethernet segment that its previously transmitted frame has suffered a collision and should be considered faulty.

LAN Emulation—See LANE.

LANE (LAN Emulation)—A method for transporting Token Ring or Ethernet frames over ATM networks. LANE encapsulates incoming Ethernet or Token Ring frames, then converts them into ATM cells for transmission over an ATM network.

logical topology—A characteristic of network transmission that reflects the way in which data is transmitted between nodes (which may differ from the physical layout of the paths that data takes). The most common logical topologies are bus and ring.

message switching—A type of switching in which a connection is established between two devices in the connection path; one device transfers data to the second device, then breaks the connection. The information is stored and forwarded from the second device after a connection between that device and a third device on the path is established.

network access method—See access method.

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packet switching—A type of switching in which data is broken into packets before it is transported. In packet switching, packets can travel any path on the network to their destination, because each packet contains a destination address and sequencing information.

padding—The bytes added to the data (or information) portion of an Ethernet frame to ensure this field is at least 46 bytes in size. Padding has no effect on the data carried by the frame.

PAN (personal area network)—A small (usually home) network composed of personal communications devices.

parallel backbone—A type of backbone that consists of more than one connection from the central router or switch to each network segment.

passive scanning—In the context of wireless networking, the process in which a station listens to several channels within a frequency range for a beacon issued by an access point.

PD (powered device)—On a network using Power over Ethernet, a node that receives power from power sourcing equipment.

personal area network—See PAN.

physical topology—The physical layout of a network. A physical topology depicts a network in broad scope; it does not specify devices, connectivity methods, or addresses on the network. Physical topologies are categorized into three fundamental geometric shapes: bus, ring, and star. These shapes can be mixed to create hybrid topologies.

piconet—A PAN (personal area network) that relies on Bluetooth transmission technology.

PoE (Power over Ethernet)—A method of delivering current to devices using Ethernet connection cables.

Power over Ethernet—See PoE.

power sourcing equipment—See PSE.

powered device—See PD.

preamble—The field in an Ethernet frame that signals to the receiving node that data is incoming and indicates when the data flow is about to begin.

probe—In 802.11 wireless networking, a type of frame issued by a station during active scanning to find nearby access points.

PSE (power sourcing equipment)—On a network using Power over Ethernet, the device that supplies power to end nodes.

quality of service (QoS)—The result of standards for delivering data within a certain period of time after their transmission. For example, ATM networks can supply four QoS levels, from a “best effort” attempt for noncritical data to a guaranteed, real-time transmission for timesensitive data.

reassociation—In the context of wireless networking, the process of a station establishing a connection (or associating) with a different access point.

Request to Send/Clear to Send—See RTS/CTS.

ring topology—A network layout in which each node is connected to the two nearest nodes so that the entire network forms a circle. Data is transmitted unidirectionally around the ring. Each workstation accepts and responds to packets addressed to it, then forwards the other packets to the next workstation in the ring.

RTS/CTS (Request to Send/Clear to Send)—An exchange in which a wireless station requests the exclusive right to communicate with an access point and the access point confirms that it has granted that request.

scanning—The process a wireless station undergoes to find an access point. See also active scanning and passive scanning.

scatternet—A network composed of multiple piconets using Bluetooth transmission technology.

serial backbone—A type of backbone that consists of two or more internetworking devices connected to each other by a single cable in a daisy-chain fashion. Hubs are often connected in this way to extend a network.

Service Set Identifier—See SSID.

SFD (start-of-frame delimiter)—A 1-byte field that indicates where the data field begins in an Ethernet frame.

shared Ethernet—A version of Ethernet in which all the nodes share a common channel and a fixed amount of bandwidth.

signal bounce—A phenomenon, caused by improper termination on a bus-topology network, in which signals travel endlessly between the two ends of the network, preventing new signals from getting through.

SSID (Service Set Identifier)—A unique character string used to identify an access point on an 802.11 network.

star topology—A physical topology in which every node on the network is connected through a central device, such as a hub. Any single physical wire on a star network connects only two devices, so a cabling problem will affect only two nodes. Nodes transmit data to the hub, which then retransmits the data to the rest of the network segment where the destination node can pick it up.

star-wired bus topology—A hybrid topology in which groups of workstations are connected in a star fashion to hubs that are networked via a single bus.

star-wired ring topology—A hybrid topology that uses the physical layout of a star and the token-passing data transmission method.

start-of-frame delimiter (SFD)—See SFD.

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switched Ethernet—An Ethernet model that enables multiple nodes to simultaneously transmit and receive data and individually take advantage of more bandwidth because they are assigned separate logical network segments through switching.

switching—A component of a network’s logical topology that manages how packets are filtered and forwarded between nodes on the network.

terminator—A resistor that is attached to each end of a bus-topology network and that causes the signal to stop rather than reflect back toward its source.

token passing—A means of data transmission in which a 3-byte packet, called a token, is passed around the network in a round-robin fashion.

type 1 IBM connector—A type of Token Ring connector that uses interlocking tabs that snap into an identical connector when one is flipped upside-down, making for a secure connection. Type 1 IBM connectors are used on STP-based Token Ring networks.

virtual circuit—A connection between network nodes that, although based on potentially disparate physical links, logically appears to be a direct, dedicated link between those nodes.

Wi-Fi—See 802.11b.

wireless personal area network—See WPAN.

WPAN (wireless personal area network)—A small office or home network in which devices such as mobile telephones, PDAs, laptops, and computers are connected via wireless transmission.

Review Questions

1.A _________________________ topology does not specify device types, connectivity methods, or addressing schemes for the network.

a.logical

b.ring

c.physical

d.bus

2.The term _________________________ topology refers to the way in which data is transmitted between nodes, rather than the physical layout of the paths that data takes.

a.logical

b.ring

c.physical

d.bus

3.In _________________________, a connection is established between two network nodes before they begin transmitting data.

a.modular routing

b.static routing

c.packet switching

d.circuit switching

4._________________________ is a network technology whose standards were originally specified by ANSI in the mid-1980s and later refined by ISO.

a.IEEE

b.FDDI

c.ISA

d.IRQ

5._________________________ is an ITU networking standard describing Data Link layer protocols for both network access and signal multiplexing.

a.Cut-Through Mode

b.Open Shortest Path First

c.Industry Standard Architecture

d.Asynchronous Transfer Mode

6.True or false? In a bus topology, every node on the network is connected through a central device, such as a hub or a switch.

7.True or false? Packets need not follow each other along the same path, nor must they arrive at their destination in the same sequence as when they left.

8.True or false? In active scanning, the station transmits a special frame, known as a probe, on all available channels within its frequency range.

9.True or false? 802.11g is a mobile wireless networking standard that uses FHSS RF signaling in the 2.4 GHz band.

10.True or false? Quality of Service is a standard that specifies that data will be delivered within a certain period of time after it is sent.

11.A(n) _________________________ topology consists of a single cable connecting all nodes on a network without intervening connectivity devices.

12.A(n) _________________________ consists of a number of connectivity devices connected to a series of central connectivity devices, such as hubs, switches, or routers, in a hierarchy.

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13.A network’s _________________________ is its method of controlling how network nodes access the communications channel.

14._________________________ enables multiple nodes to simultaneously transmit and receive data over different logical network segments.

15.In _________________________, a 3-byte packet, called a token, is transmitted from one node to another in a circular fashion around the ring.

Chapter 7

WANs, Internet

Access, and Remote

Connectivity

After reading this chapter and completing the exercises, you will be able to:

■Identify a variety of uses for WANs

■Explain different WAN topologies, including their advantages and disadvantages

■Describe several WAN transmission and connection methods, including PSTN, ISDN, T-carriers, DSL, broadband cable, SONET, and wireless Internet access technologies

■Compare the characteristics of WAN technologies, including throughput, security, and reliability

■Describe the hardware and software requirements for remotely connecting to a network