Computer Networking Guide
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Thursday, 22 September 2011
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NETWORK ACCESS PROTOCOLS
In a network, some method must be used to determine which node has use of the network’s communications paths, and for how long it can have it. The network’s hardware protocol handles these functions, and is necessary to prevent more than one user from accessing the bus at any given time. If two sets of data are placed on the network at the same time, a data collision occurs and data is lost. Basically, there are two de-facto networking protocols in use: Ethernet and Token Ring.
Ethernet
The standard specification for Ethernet has been published by the International Electrical and Electronic Association (IEEE) as the IEEE-802.3 Ethernet protocol. Its methodology for control is referred to as carrier sense multiple access with collision detection (CSMA/CD). Using this protocol, a node that wants to transfer data over the network first listens to the LAN to determine whether it is in use. If the LAN is not in use, the node begins transmitting its data. If the network is busy, the node waits for the LAN to clear for a redetermined time, and then takes control of the LAN. If two nodes are waiting to use the LAN, they will periodically attempt to access the LAN at the same time. When this happens, a data collision occurs, and the data from both nodes is rendered useless. The receiver portion of the Ethernet controller monitors the transmission to detect collisions. When it senses the data bits overlapping, it halts the transmission, as does the other node. The transmitting controller generates an abort pattern code that is transmitted to all the nodes on the LAN, telling
them that a collision has occurred. This alerts any nodes that might be waiting to access the LAN that there is a problem. The receiving node dumps any data that it might have received before the collision occurred. Other nodes waiting to send data generate a random timing number and go into a holding pattern. The timing number is a waiting time that the node sits out before it tries to transmit. Because the number is randomly generated, the odds against two of the nodes trying to transmit again at the same time are very low. The Ethernet strategy provides for up to 1,024 users to share the LAN. From the description of its collision-recovery technique, however, it should be clear that with more users on an Ethernet LAN, more collisions are likely to occur, and the average time to complete an actual data transfer will be longer.
Ethernet Specifications
Ethernet is classified as a bus topology. The original Ethernet scheme was classified as a 10Mbps transmission protocol. The maximum length specified for Ethernet is 1.55 miles (2.5km), with a maximum segment length between nodes of 500 meters. This type of LAN is referred to as a 10BASE-5 LAN by the IEEE organization.
The XXBaseYY IEEE nomenclature designates that the maximum data rate across the LAN is 10Mbps, that it is a baseband LAN (verses broadband), and that its maximum segment length is 500 meters. One exception to this method is the 10BASE-2 implementation. The maximum segment length for this specification is 185 meters (almost 200). Newer Ethernet implementations produce LAN speeds of up to 100Mbps using UTP copper cabling. For these networks, the IEEE adopted 10BASET, 100BASET, and 100BASETX designations, indicating that they are operating on twisted-pair cabling and depend on its specifications for the maximum segment length. The 100BASE designation is referred to as Fast Ethernet. The TX version of the Fast Ethernet specification employs two pairs of twisted cable to conduct high-speed, full-duplex transmissions. The cables used with the TX version can be Cat 5 UTP or STP. There is also a 100BASEFX Fast Ethernet designation that indicates the network is using fiber-optic cabling. This specification is described later in this chapter.
Network cards capable of supporting both transmission rates are classified as 10/100 Ethernet cards. The recommended maximum length of a 10/100BASET segment is 100 meters. One problem with using 10/100BASE cards in a system is that the presence of a single 10BASET card in the network can slow down the entire network.
Ethernet Connections
Ethernet connections can be made through 50-ohm, coaxial cable (10BASE-5), thinnet coaxial cable 10BASE-2), or UTP cabling (10BASE-T). The original UTP LAN specification had a transmission rate that was stated as 1Mbps. Using UTP cable, a LAN containing up to 64 nodes can be constructed with the maximum distance between nodes set at 250 meters. Figure 18.8 depicts typical coaxial and UTP connections. Coaxial cables are attached to equipment through BNC (British Naval Connectors) connectors. In a 10BASE-2 LAN, the node’s LAN adapter card is usually connected directly to the LAN cabling, using a T-connector (for peer-to-peer networks), or by a BNC connector (in a client/server LAN). UTP LAN connections are made through modular RJ-45 registered jacks and plugs. RJ-45 connectors are very similar in appearance to the RJ-11 connectors used with telephones and modems. However, the RJ-45 connectors are considerably larger than the RJ-11 connectors. Some Ethernet adapters include 15-pin sockets that allow special systems, such as fiber-optic cabling, to be interfaced to them. Other cards provide specialized ST connectors for fiber-optic connections.
Configuration
|
Conductor
|
Maximum segment length
|
Node
|
Maximum length
|
Transfer rate
|
10base-2
|
RG-58
|
185m
|
30/1024
|
250km
|
10mps
|
10base-5
|
RG-8
|
500m
|
100/1024
|
2.5km
|
10mps
|
10base-T
|
UTP/STP
|
100m/200m
|
2/1024
|
2.5km
|
10mps
|
100base-T
|
UTP
|
100m
|
2/1024
|
2.5km
|
100mps
|
100Base-FT
|
FO
|
412m
|
1024
|
5km
|
100mps
|
Token Ring
Token Ring is a token-passing protocol operating on a ring topology. The token is a small frame that all nodes can recognize instantly.This access protocol standard specification is referred to as the IEEE-802.5 Token Ring Protocol. In a token-passing system, contention for use of the LAN between different nodes is handled by passing an electronic enabling code, called a token, from node to node. Only the node possessing the token can have control of the LAN.. The token is passed from node to node along the LAN. Each node is allowed to hold the token a prescribed amount of time. After sending its message, or after its time runs out, the node must transfer the token to the next node. If the next node has no message, it just passes the token along to the next designated node. Nodes do not have to be in numeric sequence; their sequences are programmed in the network management software. All nodes listen to the LAN during the token-passing time. The Token Ring cabling is a two-pair, shielded twisted-pair cable. The main cable is called the trunk cable, and the individual drops are referred to as the interface cable. The cables are grouped together in hardware units called concentrators. Internally, the concentrator’s ports are connected into a ring configuration. In this manner, the concentrator can be placed in a convenient area, and have nodes positioned where they are needed. Some Token Ring adapters provide nine-pin connectors for shielded twisted-pair (STP) cables as well. The data-transfer rate stated for Token Ring systems is between 4- and 16Mbps. Token-passing is less efficient than other protocols when the network load is light. It evenly divides the network’s usage among nodes, however, when traffic is heavy. It can also be extremely vulnerable to node crashes when that node has the token. LAN adapter cards are typically designed to monitor the LAN for such occurrences so that they can be corrected without shutting down the entire network. The IEEE specifications for both Ethernet (802.3) and Token Ring (802.5) make provisions for a high-speed, full-duplex mode (two way simultaneous communication). This mode is normally encountered in large networks that have multiple servers. Its primary use is to perform backup functions between the large system servers where a lot of data must be moved through the network. In full-duplex mode, the standard Ethernet transfer rate of 10Mbps is boosted to 20Mbps; the Token Ring rate is raised to 32Mbps. This mode is rarely encountered on desktop client units. These units tend to operate in half-duplex (two-way communication, but in only one direction at a time) mode.
Fiber-Optic LANs
As indicated earlier in this chapter, fiber-optic cabling offers the prospect of very high performance links for LAN implementation. It can handle much higher data-transfer rates than copper conductors and can use longer distances between stations before signal deterioration becomes a problem. In addition, fiber-optic cable offers a high degree of security for data communications: Because it does not radiate EMI signal information that can be detected outside the conductor, it does not tap easily, and it shows a decided signal loss when it is tapped into.
Fiber Ethernet Standards
The IEEE organization has created several fiber optic variations of the Ethernet protocol. They classify these variations under the IEEE- 803 standard. These standards are referenced as the 10/100BASE-F specification. Variations of this standard include á 10BASE-FP—This specification is used for passive star networks running at 10Mbps. It employs a special hub that uses mirrors to channel the light signals to the desired node. á 10BASE-FL—This specification is used between devices on the network. It operates in full-duplex mode and runs at 10Mbps. Cable lengths under this specification can range up to 2 kilometers. á 100BASE-FX—This protocol is identical to the 10BASE-FL specification with the exception that it runs at 100Mbps. This particular version of the specification is referred to as Fast Ethernet because it can easily run at the 100Mbps rate.
The FDDI Ring Standard
A Token Ring-like network standard has been developed around fiber-optic cabling: the Fiber Distributed Data Interface (FDDI) specification. The FDDI network was design to work almost exactly like a Token Ring network—with the exception that it works on two counter-rotating rings of fiber-optic cable. FDDI employs token passing access control and provides data transfer rates of 100Mbps. Using the second ring, FDDI can easily handle multiple frames of data moving across the network at any given time. Of course, the dual ring implementation provides additional network dependability because it can shift over to a single ring operation if the network controller senses that a break has occurred in one of the rings.
Infrared LANs
The IrDA infrared transmission specification makes provisions for multiple IrDA devices to be attached to a computer so that it can have multiple, simultaneous links to multiple IrDA devices. Figure 18.10 shows how IrDA links can be used to share computers and devices through a normal Ethernet hub. In these scenarios, the IrDA link provides the high-speed transmission media between the Ethernet devices.
 |
| Infared LAN Network |
Wireless LANs
Recently, a variety of wireless local area networking (WLAN or LAWN) specification has been introduced into the market. These networks connect computer nodes together using high-frequency radio waves. The IEEE organization has presented a specification titled IEEE-802.11 to describe its wireless networking standard. A typical wireless LAN, consists of a wireless LAN adapter card with an RF antenna. The WLAN adapter cards in the various systems of the LAN communicate with each other and a host system through an Access Point device. Current wireless LANs operate in the range of 1Mbps transfer rates. Wireless LAN adapters are typically available in the form of PCI and PCMCIA cards.
 |
| Wireless LAN Network |
INSTALLING AND CONFIGURING LANS
A portion of the A+ Core Hardware objective 6.1 states that the test taker should be able to: Identify procedures for swapping and configuring network interface cards. Because PC technicians are typically responsible for maintaining the portion of the network that attaches to the computer, they must be able to install, configure, and service the network adapter card and cable.
LAN Adapter Cards
In a LAN, each computer on the network requires a network adapter card (also referred to as a network interface card or NIC), and every unit is connected to the network by some type of cabling. These cables are typically either twisted-pair wires, thick or thin coaxial cable, or fiber-optic cable. LAN adapter cards must have connectors that are compatible with the type of LAN cabling being used. Many Ethernet LAN cards come with both an RJ-45 and a BNC connector, so the cards can be used in any type of Ethernet configuration. Each adapter must have an adapter driver program loaded in its host computer to handle communications between the system and the adapter. These are the Ethernet and Token Ring drivers loaded to control specific types of LAN adapter cards. In addition to the adapter drivers, the network computer must have a network protocol driver loaded. This program can be referred to as the transport protocol, or just as the protocol. It operates between the adapter and the initial layer of network software to package and un-package data for the LAN. In many cases, the computer can have several different protocol drivers loaded so that the unit can communicate with computers that use other types of protocols. Typical protocol drivers include the Internetworking Packet Exchange/Sequential Packet Exchange (IPX/SPX) model produced by Novell, and the standard Transmission Control Protocol/Internet Protocol (TCP/IP) developed by the U.S. military for its ARPA network.
Installing LANs
Installing a LAN card in a PC follows the basic steps of installing most peripheral cards. Place the adapter card in a vacant expansion slot and secure it to the system unit’s back plate. Then, connect the LAN card to the network as directed by the manufacturer’s installation guide and load the proper software drivers for the installed adapter.
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