1. Function of Access Methods

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Chapter 3, Understanding Network Architecture

Chapter 3, Lesson 1

Access Methods

1. Function of Access Methods

A. Overview

1. Rules that define how a computer puts data on and takes it from the network cable is known as access method.

2. Access methods regulate the flow of network traffic.

B. Traffic Control on the Cable

1. Access the cable without running into other data.

2. Receiving computer must access cable with reasonable assurance that data has not been destroyed in a data collision during transmission.

3. Prevents simultaneous access to the cable.

2. Major Access Methods

A. Three methods to prevent simultaneous use of the cable:

1. Carrier-sense multiple access methods (with collision detection and with collision avoidance)

2. Token-passing method

3. Demand-priority method

B. Carrier-Sense Multiple Access with Collision Detection (CSMA/CD) Access Method

1. Each computer on the network, including clients and servers, checks the cable for network traffic.

a. Only when there is no traffic, can data be sent.

b. After data is sent, no other computer can transmit until original data reaches its destination.

c. A data collision occurs when two or more computers send data at the same time.

d. When this happens, each computer stops data transmission and waits to resend when it “senses” the cable is free.

2. Contention Method.

a. Computers on the network contend, or compete, for an opportunity to send data.

b. If there is data on the cable, no other computer may transmit until the data reaches its destination and cable is free.

c. Current implementations of CSMA/CD are so fast that users are not even aware they are using a contention access method.

3. CSMA/CD Considerations.

a. More computers means more network traffic.

b. Can be a slow access method depending on the number of users and the applications being used.

C. Carrier-Sense Multiple Access with Collision Avoidance (CSMA/CA) Access Method

1. Least popular of the three major access methods.

2. Each computer signals its intent to transmit before it actually transmits data to avoid transmission collisions.

3. Broadcasting the intent to transmit data increases the amount of traffic on the cable and slows down network performance.

D. Token-Passing Access Method

1. A special type of packet, called a token, circulates around a cable ring from computer to computer.

2. Computer wanting to send data must wait for the free token.

3. When computer obtains free token, it can transmit its data.

4. No other computer can transmit while this computer is transmitting.

5. No contention and no collisions take place.

6. No time is spent waiting to resend data due to traffic.

E. Demand Priority Access Method

1. Overview

a. Designed for 100 Mbps Ethernet, known as 100VG-AnyLAN.

b. Sanctioned and standardized by the Institute of Electrical and Electronic Engineers (IEEE) in its 802.12 specification.

c. Repeaters and end nodes are the two components that make up all 100VG-AnyLANs.

d. Repeaters are responsible for searching for requests to send.

e. Repeaters are responsible for noting all addresses, links, and end nodes, and verifying that they are all functioning.

f. End node can be a computer, bridge, router, or switch.

2. Demand Priority Contention

a. Computers contend (transmit at the same time).

b. Hub determines priority (could be based on data type).

c. Higher priority is transmitted first; if same, then they are alternated.

3. Demand Priority Considerations

a. Uses four pairs of wires, allowing computers to transmit and receive at the same time.

b. Transmissions through the hub.

c. No network-wide broadcasts, only computer-to-hub-to-computer.

Chapter 3, Lesson 2

How Networks Send Data

1. The Function of Packets in Network Communications

A. Data sent in small, manageable packets, each wrapped with the essential information needed to get it from its source to the correct destination.

1. Large files flood and tie up the network.

2. Impact of retransmitting large units of data multiply network traffic.

3. Small packets provide timely error recovery, and the network only has to resend small amount of data.

B. Sender breaks data into packets and adds special control information to each packet making it possible to:

1. Send original, disassembled data in small chunks.

2. Reassemble data in proper order at destination.

3. Check data for errors after it has been reassembled.

2. Packet Structure

A. Packets contain several types of data, including:

1. Information, such as messages or files.

2. Computer control data and commands, such as service requests.

3. Session control codes (such as error correction) that indicate the need for retransmission.

B. Packet Components

1. Common packet components:

a. Source address that identifies sending computer

b. Data intended for transmission

c. Destination address that identifies the recipient

d. Instructions telling network components how to pass data

e. Information to receiving computer about how to reassemble the data

f. Error-checking information to ensure that packet arrives intact

2. Header

a. Alert signal indicating packet is being transmitted

b. Source address

c. Destination address

d. Clock information to synchronize transmission

3. Data

a. Actual data being sent: 0.5 KB to 4 KB in size

b. Larger files broken into smaller multiple packets

4. Trailer

a. Exact content of the trailer varies depending on the communication method, or protocol.

b. Contains error-check component called cyclical redundancy check (CRC), a number produced by mathematical computation at source.

c If CRC number is calculated to be the same when it arrives, the data is assumed good; if not, it is retransmitted.

C. Example: Packets in Printing

1. Sending computer establishes a connection with the print server.

2. Computer breaks the large print job into packets containing:

a. Source and destination address.

b. Data.

c. Control information.

3. NIC in each computer examines the receiver’s address on all packets sent on its segment of the network.

a. NIC compares the incoming packets destination address with its own address.

b. NIC interrupts the computer only when it detects a packet addressed specifically to itself.

4. Packets enter the destination computer (print server) through the cable into the NIC.

5. No computer resources are used until the NIC identifies a packet addressed to itself.

6. NOS in the receiving computer reassembles the packets back into the original text file and moves the file into the computer’s memory.

Chapter 3, Lesson 3


1. Origin of Ethernet

A. Late 1960’s, the University of Hawaii developed a WAN called ALOHA using the CSMA/CD access method.

B. In 1972, a cabling and signaling scheme was invented at Xerox Palo Alto Research Center (PARC).

C. In 1975, the first Ethernet product was introduced; a system of 2.94 megabits per second (Mbps) to connect over 100 computers connected on one kilometer (.62 miles) of cable.

D. Xerox, Intel Corporation and Digital Equipment Corporation drew up standard for 10-Mbps Ethernet.

2. Ethernet Specifications

A. International Organization for Standardization (ISO)

1. In 1978, ISO released a set of specifications for connecting dissimilar devices.

2. This set of specifications is referred to as the Open System Interconnect (OSI) model.

B. Ethernet specification performs the same functions as the OSI physical and data-link layers of this model.

1. These specifications affect how hardware links or passes information back and forth.

2. Project 802 published by IEEE in 1980s.

3. The IEEE 802.3 specification.

3. Ethernet Features

A. Most Popular Network Architecture

1. Ethernet media is passive, which means it requires no power source.

2. Will not fail unless the media is physically cut or improperly terminated.

B. Ethernet Basics

1. Topology: Linear bus or star bus

2. Architecture: Baseband (digital: bidirectional)

3. Access method: CSMA/CD

4. Specifications: IEEE 802.3

5. Transfer speed: 10 or 100 Mbps

6. Cable type: ThickNet, ThinNet, UTP

C. The Ethernet Frame Format

1. Ethernet breaks data into frames.

a. Package of information transmitted in a single unit (between 64 and 1518 bytes long).

b. Ethernet uses 18 bytes for itself; therefore, data is between 46 and 1500 bytes long.

2. Parts of the frame:

a. Preamble, the start of the frame

b. Destination and source address

c. Type of frame, usually IP or IPX (Novell’s Internetwork Packet Exchange)

d. Error checking field (Cyclical redundancy check (CRC)

4. 10-Mbps IEEE Standards

A. 10BaseT Standard

1. 10-Mbps, baseband, over twisted-pair cable.

2. Usually unshielded twisted-pair, but STP will work.

3. Uses hubs (multiport repeaters) from wiring closets.

4. Configured as star, but internally it is a bus.

5. Each computer has two pairs of wire: one pair receives data and one pair transmits.

6. Cable type is Category 3, 4, or 5 UTP. Maximum length of 100 meters (328 feet); minimum of 2.5 meters (8 feet).

7. 1024 computers maximum.

B. 10Base2 Standard

1. 10 Mbps, baseband, over thin coaxial cable.

2. ThinNet with BNC connectors and terminators.

3. BNC barrels to extend sections.

4. Maximum length of 185 meters (607 feet); minimum of 0.5 meters (20 inches).

5. ThinNet can support maximum of 30 nodes per populated segment.

6. Repeaters used to extend to total of 925 meters (3035 feet).

7. Relatively inexpensive, easy to install, easy to configure.

8. Uses 5-4-3 rule.

C. 10Base5 Standard

1. 10 Mbps, baseband, over 500-meter (five 100-meter) segments.

2. Called ThickNet or standard Ethernet.

3. ThickNet uses bus topology and can support up to one hundred nodes per populated segment.

4. ThickNet segment can be 500 meters (1640 feet) long for a total network length of 2500 meters (8200 feet).

5. Transceivers are devices that transmit and receive, and also provide communications between the computer and the main LAN cable.

6. Transceiver cable uses a DIX or AUI 15-port to connect the transceiver to the NIC.

7. N-series connectors, including N-series barrel connectors, and N-series terminators.

8. Uses 5-4-3 rule.

9. Maximum length of 50 meters (164 feet); minimum of 2.5 meters (about 8 feet).

D. Combining ThickNet and ThinNet Cable

1. ThickNet cable is good for backbones.

2. ThinNet cable is used for branch segments.

3. Branch segments connect the computer to the backbone.

E. 10BaseFL Standard

1. Ethernet using fiber-optic cable to connect computers and repeaters.

2. Accommodates long runs between buildings.

3. Maximum segment length of 2000 meters (about 6800 feet).

5. 100-Mbps IEEE Standards

A. Able to support high-bandwidth applications:

1. Computer-aided design (CAD)

2. Computer-aided manufacturing (CAM)

3. Video

4. Imaging and document storage

B. 100BaseVG AnyLAN Standard

1. 100BaseVG AnyLAN Ethernet is IEEE 802.12 standard.

a. Combines Ethernet and Token Ring technology

b. Originally developed by Hewlett Packard

2. Specifications:

a. Minimum data rate of 100 Mbps

b. Cascading star topology (parent/child) over Category 3, 4, and 5 twisted-pair and fiber-optic cable

c. Demand priority access method

d. Ability to filter individually addressed frames at the hub

e. Support Ethernet frames and Token Ring packets

3. Topology.

a. Star topology in which all computers are attached to a hub.

b. Add child hubs to the central hub to expand the network.

4. Limitations of 100BaseVG:

a. Requires own hubs and cards

b. Two longest cables from hub to computer less than or equal to 250 meters (810 feet)

c. Requires more wiring closets than 10BaseT

C. 100BaseX Ethernet Standard

1. Fast Ethernet

2. Uses UTP Category 5 wire

3. Star bus topology like 10BaseT

4. Incorporates three media specifications:

a. 100BaseT4 (4-pair Category 3, 4, 5 UTP)

b. 100BaseTX (2-pair Category 5 UTP or STP)

c. 100BaseFX (2-strand fiber optic)

6. Performance Considerations

A. Ethernet architecture can use multiple communication protocols:

1. NetWare


3. Windows

4. Macintosh

B. Segmentation

1. Divides crowded segment into two less crowded segments using bridge or router.

2. Reduces traffic and improves access time.

C. NOSs on Ethernet

1. Microsoft Windows 95, Windows 98, and Windows 2000

2. Microsoft Windows NT Workstation and Windows NT Server

3. Microsoft Windows 2000 Professional and Windows 2000 Server

4. Microsoft LAN Manager

5. Microsoft Windows for Workgroups

6. Novell NetWare

7. IBM LAN Server

8. AppleShare


Chapter 3, Lesson 4

Token Ring

1. Overview

A. General Information

1. Token Ring: 802.5 specifications

2. Introduced by IBM in 1984 for all its computer environments, from PCs to mainframes (SNA)

3. Adopted by ANSI/IEEE in 1985

B. Token Ring Features

1. Architecture

a. Logical ring is within the hub.

b. Computers are wired in a star from the hub (star-wired ring topology).

c. Token still passes in a ring from computer to computer.

2. Token Ring Basics

a. Star-wired ring topology

b. Token passing access method

c. IBM Type 1, 2, 3 cabling (STP or UTP)

d. Transfer rates of 4 Mbps or 16 Mbps

e. Baseband transmission

f. IEEE 802.5 specifications

C. Frame Formats

1. Start delimiter indicates the start of the frame.

2. Access Control and Frame Control.

3. Source and Destination Addresses.

4. Information or Data.

5. CRC Error Checking.

6. End delimiter indicates the end of frame.

7. Frame status tells whether the frame was recognized, copied, or whether the destination address was available.

2. How Token Ring Network Works

A. Overview

1. First computer on the network generates a token.

2. Token travels around the ring polling each computer.

3. Computer signals that it wants to transmit data and takes control of the token.

4. Computer cannot transmit unless it has possession of the token.

5. No computer can transmit data while the token is in use.

6. Each computer acts as a unidirectional repeater, regenerates the token, and passes it along.

7. After taking control of the token, it sends a data frame out on the network.

8. Frame proceeds around the ring until it reaches the destination computer.

9. Destination computer copies the frame into its receive buffer.

10. Destination computer marks the status field as received.

11. Frame continues around the ring until sending computer receives it.

12. Sending computer removes the frame and generates a new token.

13. Only one token at a time can be active on the network.

14. Token travels in only one direction.

B. Monitoring the System

1. First computer online is assigned to monitor network activity by the Token Ring system and is responsible for:

a. Frames being delivered and received correctly.

b. Frames that have circulated the ring more than once.

c. Ensuring only one token is on the network.

2. Process of monitoring is called beaconing.

a. Active monitor sends out a beacon announcement every seven seconds.

b. Beacon passed from computer to computer throughout the ring.

c. If station does not receive an expected announcement from its upstream neighbor it sends a message including:

(1) Stations address
(2) Address of neighbor that did not announce
(3) Type of beacon

d. Ring attempts to diagnose and repair without disrupting the entire network.

e. If unable to reconfigure automatically, manual intervention is required.

3. Recognizing a Computer.

a. When a new computer comes online the initialization includes:

(1) Checking for duplicate addresses.
(2) Notifying others of its existence.

3. Hardware Components

A. The Hub

1. Overview

a. Token ring hub is known by several names:

(1) MAU (Multistation Access Unit)
(2) MSAU (MultiStation Access Unit)
(3) SMAU (Smart Multistation Access Unit)

b. The hub’s internal ring automatically converts to an external ring at each connection point when a computer is connected.

2. Capacity

a. IBM MSAU has 10 connection ports creating a logical ring of eight stations (two other ports extend the ring).

b. Up to 33 hubs on a ring supporting 72 computers that use UTP or 260 that use STP on a MSAU-based network.

c. Hubs must be connected in a ring; ring-out on one hub must connect to ring-in on the adjacent hub.

d. Patch cables connect many MSAUs on top of each other while still forming a continuous ring.

3. Built-in Fault Tolerance

a. NIC failure should bring down ring.

b. MSAU will sense a failed NIC and disconnect from it.

c. Faulty computer or connection will not affect rest of network.

B. Cabling

1. Overview

a. IBM Type 1, 2, 3; usually Type 3 UTP cabling.

b. IBM states maximum cabling distance MSAU to computer is 46 meters (150 feet), although some vendors say 152 meters (500 feet).

c. Maximum distance from one MSAU to another is 152 meters (500 feet).

d. Single Token Ring can accommodate only 260 computers with STP cable and 72 computers with UTP cable.

2. Patch Cables

a. Extend connection between computer and MSAU or between two MSAUs.

b. IBM Type 6, up to a maximum of 46 meters (150 feet).

3. Connectors

a. Media interface connectors (MICs), also known as universal data connectors, connect Types 1 and 2 cable.

b. Neither male nor female so they can be connected together.

c. RJ-45 for 8-pin connection for Type 3 cable.

d. RJ-11 for 4-pin connection for Type 3 cable.

e. Media filter required to make connection between Token Ring interface card and standard RJ-11/RJ-45 telephone jacks.

4. Media Filters

a. Required with Type 3 telephone twisted-pair cable.

b. Convert cable connectors and reduce line noise.

5. Patch Panels

a. Organize cable that runs between a MSAU and a telephone punchdown block.

b. Punchdown block is a kind of hardware that provides terminal connections for bare network cable ends.

6. Repeaters

a. Increase Token Ring cable distances.

b. Actively regenerates and retimes the Token Ring signal.

7. NICs

a. Either 4 Mbps or 16 Mbps.

b. If the network is a 4 Mbps network, the 16 Mbps cards can be used, but will revert to 4 Mbps mode (16 Mbps networks will not accept slower 4 Mbps cards because they cannot speed up).

8. Fiber-Optic Cable

a. Well suited for unidirectional, high-speed fiber-optic cable.

b. Although more expensive, it greatly increases the range (up to 10 times that of copper).

4. Future of Token Ring Networks

A. Overview

1. Token ring is losing market share to Ethernet.

2. Large companies are selecting Token Ring to support mission-critical applications.

B. Users of token ring face the following challenges:

1. Complexity, manageability, cost and space requirements

2. Bridge and segment congestion

3. Upgrading to high-speed technologies

Chapter 3, Lesson 5

AppleTalk and ArcNet

1. AppleTalk Environment

A. AppleTalk

1. Overview

a. Apple Computer introduced AppleTalk in 1983 as proprietary network architecture for small groups.

b. Built into Macintosh computers.

c. AppleTalk Phase 2 is the current release of AppleTalk.

2. When a device attached to a LocalTalk network comes online, the following occurs in order:

a. Device checks to see if it has stored an address from a previous networking session. If not, the new device assigns itself a random address from a range of allowable addresses.

b. The device broadcasts the address to see if any other device is using it.

c. If no other device is using the address, the device stores address for the next time it comes online.

B. LocalTalk

1. Overview

a. CSMA/CA access method in STP bus or tree topology.

b. Cabling components include cables, connector modules, and cable extenders.

c. Supports a maximum of 32 devices.

d. Other vendors offer more robust solutions.

2. Not widely used by Ethernet or Token Ring because:

a. Maximum communication data rate is 230.4 Kbps.

b. LocalTalk NICs for PC-compatible computers are obsolete.

C. AppleShare

1. AppleShare is the file server on AppleTalk network.

2. LocalTalk networks can be joined using zones.

3. Each zone is a workgroup and is given a name.

D. EtherTalk

1. EtherTalk allows AppleTalk to use Ethernet coaxial cable (ThinNet or ThickNet).

2. EtherTalk card allows a Macintosh computer to connect and 802.3 Ethernet network.

E. TokenTalk

1. TokenTalk connects Macintosh II to an 802.5 Token Ring.

2. Compatible with AppleTalk Phase 2.

F. AppleTalk considerations

1. PCs and mainframes can use AppleTalk.

2. Apple encourages third-party use of network.

2. ArcNet Environment

A. Overview

1. Datapoint Corporation developed the Attached Resource Computer Network (ArcNet) in 1977.

2. Simple, inexpensive, flexible network architecture designed for workgroup-sized networks.

3. Token-passing bus networks using broadband cable.

4. Star bus or bus topology.

B. How ArcNet works:

1. Token-passing access method in a star bus.

2. 2.5 Mbps, although ArcNet Plus is 20 Mbps.

3. Token moves in numerical order of card address, regardless of where it is on the network.

4. ArcNet packet includes source and destination addresses, and up to 508 bytes of data.

C. Hardware

1. Hubs can be active, passive, or smart.

2. Uses 93-ohm RG-62 A/U coaxial cable.

3. Maximum cable length between active hub and workstation is 610 meters (2000 feet).

4. Maximum distance between drops on a linear coaxial bus is 305 meters (1000 feet).

5. Using UTP, maximum cable distance is 244 meters (800 feet) between devices on both star and bus topologies.

Outline, Chapter 3

Networking Essentials Plus, Third Edition

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