Chapter 4 Lecture Notes - Microsoft Networking Essentials

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The precursor of Ethernet was called ALOHA, a WAN developed at the University of Hawaii. This network used a CSMA/CD as its access method. In 1972, Robert Metcalfe and David Boggs invented a cabling and signaling scheme at Xerox PARC and in 1975 introduced the first Ethernet products. Xerox, Intel, and Digital drew up a standard for a 10 Mbps Ethernet, which is now a specification for computers and data systems to share cabling. This design is the basis for the IEEE 802.3 standard and conforms to the OSI data link and physical layers.

Ethernet is a baseband architecture that uses a bus topology, usually transmits at 10 Mbps, and relies on CSMA/CD to regulate traffic on the main cable segment. Ethernet is a passive media, which means that it draws power from the computer and thus will not fail unless the cable is cut or improperly terminated.

Ethernet Basics (Page 253)

Ethernet breaks data down into frames, which are transmitted as a single unit. An Ethernet frame can be between 64 and 1,518 bytes in length, but the frame itself uses at least 18 bytes. The data in an Ethernet frame is therefore 46—1,500 bytes long. Every frame contains control information and follows the same basic organization. (Page 254)

The Ethernet II frame, used for TCP/IP, consists of a preamble, which marks the start of the frame, source and destination addresses, a type which is used to identify the Network layer protocol (IP or IPX), and a cyclic redundancy check (CRC) for error-checking.

The 10 Mbps IEEE Standards (Page 256)

10BaseT describes a specification in which the computer transmits at 10 Mbps, over a UTP cable using baseband transmission. STP may be used in place of UTP with no change to the 10BaseT parameters (except cost).

Ethernet is a bus topology. 10BaseT networks appear to be configured in a star pattern, but are internally (inside the hub or multiport repeater) configured as a bus network.

The maximum length of a 10BaseT segment is 100 meters (328 feet). Repeaters are used to extend this cable length. The minimum cable length between computers is 2.5 meters (8 feet). A 10BaseT LAN will serve up to 1,024 computers. (Discuss figure 4.4 on page 257.)

Discuss 10BaseT Summary (Page 258)

10Base2 (Page 258)

10Base2 can transmit data at 10 Mbps over a thinnet cable and can carry the signal 185 meters. This method uses baseband transmission. The maximum segment length is 185 meters, with a maximum of 30 computers per 185 meter segment. Thinnet is relatively inexpensive, easy to install, and easy to configure.


The 5-4-3 Rule (Page 259)

A 10Base2 network can combine as many as five cable segments, tied together with four repeaters, but only three segments can have computers on them. The other two segments are untapped and are known as inter-repeater links. Repeaters can be used to bring the total length of a 10Base2 network to 925 meters (3,035 feet).

10Base5 (Page 260)

The IEEE 10Base5 specification is also known as standard Ethernet. This specification allows for 10 Mbps transmissions over thick coaxial cable (Thicknet) with up to 500 meter (1640 feet) segments. Thicknet can support up to 100 nodes per backbone segment. The total network length may be up to 2,500 meters (five 500 meter segments).

Thicknet cabling components include transceivers, transceiver cables, DIX/AUI connectors, and N-series connectors.

The 5-4-3 rule applies to 10Base5 networks, too. Each 10Base5 network may have a maximum of 5 backbone segments connected using repeaters; three segments may have computers connected. The minimum distance between computers on the backbone is 2.5 meters (8 feet), not counting the length of the transceiver cables. 10Base5 supports a maximum of 100 computers per segment.

10Base5 Summary (Page 262)

Combining Thicknet and Thinnet (Page 263)

Thicknet is used for the backbone and thinnet is used for the branch segments. The transceiver attaches to the thicknet cable and the transceiver’s AUI/DIX connector plugs into a repeater. The branching segments of thinnet plug into the repeater and connect the computers to the network.

10BaseFL (Page 263)

Fiber-optic cabling is used to connect computers and repeaters. The primary reason for using 10BaseFL is for long cable runs between repeaters. The maximum distance for a 10BaseFL segment is 2,000 meters.

The 100 Mbps IEEE Standard (Page 265)

The two emerging standards for 100 Mbps Ethernet are 100BaseVG-AnyLAN Ethernet and 100BaseX Ethernet (Fast Ethernet).

The 100VG-AnyLan specification is currently being refined and ratified by the IEEE 802.12 committee. This is a specification for transmitting 802.3 frames and 802.5 Token Ring packets.

Token Ring Overview (Page 273)

IBM introduced Token Ring in 1984 as part of its connectivity solution (SNA) for the entire range of IBM computers. Token Ring became an ANSI/IEEE standard in 1985. The token-passing access, more than the cabling, method distinguishes Token Ring networks from other networks

Architecture (Page 274)

The Token Ring architecture is a circle that forms a continuous ring. This may be implemented as either a physical ring where all of the computers are connected in a ring together, or as a logical ring where the hub contains the ring and each computer is connected to the hub in a star pattern.

Token Ring Basics (Page 274)

Frame Formats (Page 275)

How a Token Ring Works (Page 277)

The first Token Ring computer to come online generates a token. The token travels around the ring until a computer takes control of the token to transmit. A computer may not transmit unless it has control of the token. No other computer may transmit. Because of this, Token Ring networks are referred to as deterministic, meaning that a computer cannot force its way onto the network but must wait for the token, which determines which computer may transmit.

The computer transmits a data frame. The frame travels around the network until it reaches the computer that it is addressed to. The destination computer copies the frame into it's receive buffer and marks the frame in the frame status field to show that the frame was received.

The frame continues around the ring until it reaches the sending computer where the transmission is acknowledged as successful. The sending computer removes the frame from the ring, generates a new token, and puts it on the ring

The token may travel in only one direction around the ring and only one token may be active at any time.

The first computer to come online is assigned the task of monitoring the system. This monitor will check to see if the frames are being sent and received correctly, by checking for frames that have circulated the ring more than once and making sure that only one token is on the network at any one time.

The Hub (Page 280)

The hub in a Token Ring network houses the actual ring. It is known by several names, all of which mean the same thing: MAU -- Multistation Access Unit, MSAU -- MultiStation Access Unit, or SMAU -- Smart Multistation Access Unit. The individual clients and servers are all attached to the MSAU by cables. The MSAU looks like a hub, but is wired as a ring internally. The ring continues with the line into and out of each connected computer.


Hub Capacity (page 280)

The MSAU has 10 connection ports. It can connect up to 8 computers and uses a ring-in and ring-out connector to expand the network. A Token Ring network is not limited to one ring (hub). Each ring can have up to 33 hubs. Each MSAU-based network can support up to 72 computers using unshielded wire or up to 260 computers using shielded wire. Each MSAU must be connected so that it becomes part of the ring.

Built-in Fault Tolerance (Page 281)

MSAUs are designed so that if one computer, NIC, or MSAU port fails, that computer or port is removed from the ring and the ring is maintained. A pure Token Ring network, with no MSAUs, would cease to function because the computer could not pass the token.

Cabling (Page 281)

Computers on a Token Ring network are connected using IBM type 1, 2, or 3 cabling. Most networks use IBM type 3 UTP cabling. The maximum distance from the hub when using type 1 cabling is 101 meters (330 feet). Each computer can be up to 100 meters (328 feet) when using STP, or 45 meters (148 feet) using UTP cabling. The minimum cable length is 2.5 meters (8 feet) using either UTP or STP cabling. The distance form one MSAU to another can be up to 500 feet.

Patch Cables -- Connectors (Page 283)

Repeaters (Page 284)

Repeaters actively regenerate and retime the Token Ring signal to extend the distances between MSAUs on the network. Using one pair of repeaters, MSAUs can be located up to 365 meters (1,200 feet) apart using Type 1 cabling. They may be located up to 730 meters (2,400 feet) using Type 1 or Type 2 cabling.

Network Adapter Cards (Page 284)

Token Ring NICs are available in two different types, 4 Mbps and 16 Mbps. You may use a 16 Mbps card on a 4 Mbps network (because it will slow itself down), but you may not use a 4 Mbps NIC on a 16 Mbps network (because it can't speed itself up).

Fiber optic cabling can increase the range of a Token Ring network up to 10 times the distance that copper cabling is capable of.

AppleTalk (Page 288)

AppleTalk is a network architecture that is built into Macintosh computers. The architecture is a collection of protocols that correspond to the OSI model. When an AppleTalk device comes online, there are three things that it does:

  1. The device assigns itself an address. Chosen at random from a range of allowable addresses.
  2. The device broadcasts the address to see if any othe device is already using it.
  3. If no other device is using the address. it stores the address for later use the next time the device comes online.

LocalTalk (Page 289)

LocalTalk uses CSMA/CA in a bus or tree topology using STP cabling. However, it can also use either fiber-optic or UTP cabling. LocalTalk is inexpensive because it is built into the Macintosh, but it is a low-performance network and it not used very much in large business networks.

LocalTalk also refers to the physical cabling components, including cables, connector modules, and cable extenders. A LocalTalk network supports a maximum of 32 devices.

AppleShare (Page 290)

AppleShare is the server on an AppleTalk network. The client-side software is bundled with each copy of the Mac OS. Single LocalTalk networks can be joined together into one larger network using zones. Each subnetwork is identified by a zone name. Users of one AppleTalk network can access resources in other connected networks by selecting the zone. Also, working groups on a single LocalTalk network can be divided into zones to relieve congestion on the entire network.

EtherTalk and TokenTalk (Page 291)

The ArcNet Environment (Page 293)

ArcNet loosely maps to the IEEE 802,4 specification. It uses a token-passing scheme in a star bus topology, passing data at 2.5 Mbps. The ArcNet Plus specification allows data transmission at 20 Mbps.

The token moves from one computer to another based on the numerical order regardless of the actual location of the computer. The ArcNet packet contains a source and destination address and 508 byte4s of data, 4,096 bytes for ArcNet Plus.

Hardware (Page 293)

ArcNet uses 93 ohm RG-62 A/U cable.