1.0—Media and Topologies
Understanding media and topologies is important when designing and troubleshooting networks. Therefore, CompTIA has included several questions on the exam that are related to the different media types used with today's networks and their characteristics. In addition, the exam includes questions that test your knowledge of the various network topologies.
Network Types and Physical and Logical Topologies
The following are some of the important aspects of network types:
There are two types of computer networks:
Peer-to-peer networks— Peer-to-peer networks are useful for only relatively small networks. They are often used in small offices or home environments.
Client/server networks— Client/server networks, also called server-centric networks, have clients and servers. Servers provide centralized administration, data storage, and security. The client system requests data from the server and displays the data to the end user.
The following are some of the important aspects of network topologies:
Network topologies can be defined on a physical level or on a logical level.
The bus network topology is also known as a linear bus because the computers in such a network are linked together using a single cable called a trunk, or backbone. The following are important features of the bus topology:
The computers can be connected to the backbone by a cable, known as a drop cable, or, more commonly, directly to the backbone, via T connectors.
At each end of the cable, terminators prevent the signal from bouncing back down the cable.
If a terminator is loose, data communications may be disrupted. Any other break in the cable will cause the entire network to fail.
Table 1 shows the advantages and disadvantages of the bus topology.
| Features | Advantages | Disadvantages |
|---|---|---|
| It uses a single length of cable. | It is inexpensive and easy to implement. | It does not scale well; that is, it cannot be expanded easily. |
| Devices connect directly to the cable. | It doesn't require special equipment. | A break in the cable renders the entire segment unusable. |
| The cable must be terminated at both ends. | It requires less cable than other topologies. | It is difficult to troubleshoot. |
In a star topology, each device on the network connects to a centralized device via a single cable. The following are important features of the star topology:
Computers in a star network can be connected and disconnected from the network without affecting any other systems.
In a star configuration all devices on the network connect to a central device, and this central device creates a single point of failure on the network.
The most common implementation of the physical star topology is the Ethernet 10BaseT standard.
Table 2 lists the features, advantages, and disadvantages of the star topology.
| Features | Advantages | Disadvantages |
|---|---|---|
| Devices connect to a central point. | It can be easily expanded without disruption to existing systems. | It requires additional networking equipment to create the network layout. |
| Each system uses an individual cable to attach. | A cable failure affects only a single system. | It requires considerably more cable than other topologies, such as the linear bus. |
| Multiple stars can be combined to create a hierarchical star. | It is easy to troubleshoot. | Centralized devices create a single point of failure. |
In the ring topology, the network layout forms a complete ring. Computers connect to the network cable directly or, far more commonly, through a specialized network device. Breaking the loop of a ring network disrupts the entire network. Even if network devices are used to create the ring, the ring must still be broken if a fault occurs or the network needs to be expanded. Table 3 lists the features, advantages, and disadvantages of the ring topology.
Table 3. Ring Topology: Features, Advantages, and Disadvantages Features Advantages Disadvantages Devices are connected in a closed loop or ring. It is easy to troubleshoot. A cable break can disrupt the entire network. Dual-ring configuration can be used for fault tolerance. Network expansion creates network disruption. The mesh topology requires each computer on the network to be individually connected to every other device. This configuration provides maximum reliability and redundancy for the network. Table 4 lists the features, advantages, and disadvantages of the mesh topology.
Table 4. Mesh Topology: Features, Advantages, and Disadvantages Features Advantages Disadvantages A true mesh uses point-to-point connectivity between all devices. Multiple links provide fault tolerance and redundancy. It is difficult to implement. A hybrid mesh uses point-to-point connectivity between certain devices, but not all of them. The network can be expanded with minimal or no disruption. It can be expensive because it requires specialized hardware and cable. Wireless networks use a centralized device known as a wireless access point (WAP) that transmits signals to devices with wireless network interface cards (NICs) installed in them. Table 5 lists the features, advantages, and disadvantages of wireless networks.
| Features | Advantages | Disadvantages |
|---|---|---|
| No physical connections are required. | It provides flexible network access. | It is still relatively new and expensive. |
| It can be used in local area network (LAN) or wide area network (WAN) environments. | It can be used in environments where physical access is not possible. | It has potential security issues. Speed is limited in certain implementations. |
Standards and Access Methods
The following are descriptions of the Institute of Electrical and Electronics Engineers (IEEE) 802 standards:
802.1, internetworking— Defines internetwork communications standards between devices and includes specifications for routing and bridging.
802.2, the LLC sublayer— Defines specifications for the Logical Link Control (LLC) sublayer in the 802 standard series.
802.3, CSMA/CD— Defines the carrier-sense multiple-access with collision detection (CSMA/CD) media access method used in Ethernet networks. This is the most popular networking standard used today.
802.4, a token passing bus (rarely used)— Defines the use of a token-passing system on a linear bus topology.
802.6, metropolitan area network (MAN)— Defines a data transmission method called distributed queue dual bus (DQDB), which is designed to carry voice and data on a single link.
802.7, Broadband Technical Advisory Group— Defines the standards and specifications of broadband communications methods.
802.8, Fiber-Optic Technical Advisory Group— Provides assistance to other IEEE 802 committees on subjects related to the use of fiber-optics.
802.9, Integrated Voice and Data Networks Group— Works on the advancement of integrated voice and data networks.
802.10, network security— Defines security standards that make it possible to safely and securely transmit and exchange data.
802.11, wireless networks— Defines standards for wireless LAN communication.
802.12, 100BaseVG-AnyLAN— Defines standards for high-speed LAN technologies.
The Network+ exam focuses on the LAN standards 802.2, 802.3, 802.5, and 802.11.
Access methods govern the way in which systems access the network media and send data. Following are the key aspects of the CSMA/CD access method:
CSMA/CD, which is defined in the IEEE 802.3 standard, is the most popular media access method because it is associated with Ethernet networking, which is by far the most popular networking system.
CSMA/CD is known as a contention media access method because systems contend for access to the media. Table 6 shows the advantages and disadvantages of CSMA/CD.
| Advantages | Disadvantages |
|---|---|
| It has low overhead. | Collisions degrade network performance. |
| It is able to utilize all available bandwidth when possible. | Priorities cannot be assigned to certain nodes. |
| Performance degrades exponentially as devices are added. |
Token passing is an access method that is specified in IEEE 802.5. Following are the important facts about token-passing networks:
On a token-passing network, a packet called a token is passed among the systems on the network. The network has only one token, and a system can send data only when it has possession of the token.
All cards in a token-passing network must operate at the same speed.
Because a system can transmit only when it has the token, there is no contention, as with CSMA/CD.
Ring networks are most commonly wired in a star configuration. In a Token Ring network, a multistation access unit (MSAU) is equivalent to a hub or switch on an Ethernet network.
To connect MSAUs, the ring in (RI) and ring out (RO) configuration must be properly set.
Table 7 shows the advantages and disadvantages of token-passing networks.
| Advantages | Disadvantages |
|---|---|
| No collisions mean more consistent performance in high-load configurations. | The generation of a token creates network overhead. |
| Performance is consistently predictable, making token passing suitable for time-sensitive applications. | Network hardware is more complex and expensive than that used with other access methods. The maximum speed is limited due to the overhead of token passing and regeneration. |
Media Considerations and Limitations
As a data signal travels through a specific media, it may be subjected to a type of interference known as electromagnetic interference (EMI). Following are important EMI facts:
Many different factors cause EMI; common sources include computer monitors and fluorescent lighting fixtures.
Copper-based media are prone to EMI, whereas fiber-optic cable is completely resistant to it.
Data signals may also be subjected to something commonly referred to as crosstalk, which occurs when signals from two cables in close proximity to one another interfere with each other.
Media has maximum lengths because a signal weakens as it travels farther from its point of origin. The weakening of data signals as they traverse the media is referred to as attenuation.
Two types of signaling methods are used to transmit information over network media:
Baseband— Baseband transmissions typically use digital signaling over a single wire; the transmissions themselves take the form of either electrical pulses or light. Ethernet networks use baseband transmissions.
Using baseband transmissions, it is possible to transmit multiple signals on a single cable by using a process known as multiplexing.
Broadband— Broadband signaling uses analog signals in the form of optical or electromagnetic waves over multiple transmission frequencies.
Dialog Modes
There are three main dialog modes:
Simplex— The simplex mode allows only one-way communication through the media. A good example of simplex is a radio or television signal: There is only one transmitting device, and all other devices are receiving devices.
Half-duplex— Half-duplex allows each device to both transmit and receive, but only one of these processes can occur at a time.
Full-duplex— Full-duplex allows devices to receive and transmit simultaneously. A 100Mbps network card in full-duplex mode can operate at 200Mbps.
Network Media
Two types of coax are used in networking:
Thin coax— Thin coax is only .25 inches in diameter and has a maximum cable length of 185 meters (that is, approximately 600 feet).
Thick coax— Thick coax networks use a device called a tap to connect a smaller cable to the thick coax backbone. Thick coax has a 500-meter cable length.
There are two distinct types of twisted-pair cable: unshielded twisted-pair (UTP) and shielded twisted-pair (STP). STP has extra shielding within the casing, so it copes with interference and attenuation better than regular UTP.
The Electronic Industries Association/Telecommunications Industry Association (EIA/TIA) has specified five categories of twisted-pair cable:
Category 1— Voice-grade UTP telephone cable. Due to its susceptibility to interference and attenuation and its low bandwidth capability, Category 1 UTP is not practical for network applications.
Category 2— Data-grade cable that is capable of transmitting data up to 4Mbps. Category 2 cable is, of course, too slow for networks. It is unlikely that you will encounter Category 2 used in any network today.
Category 3— Data-grade cable that is capable of transmitting data up to 10Mbps. A few years ago, Category 3 was the cable of choice for twisted-pair networks. As network speeds pushed the 100Mbps speed limit, Category 3 became ineffective.
Category 4— Data-grade cable that has potential bandwidth of 16Mbps. Category 4 cable was often implemented in the IBM Token Ring networks.
Category 5— Data-grade cable that is capable of transmitting data at 100Mbps. Category 5 is the cable of choice on twisted-pair networks and is associated with Fast Ethernet technologies.
Fiber-optic cables are not susceptible to EMI or crosstalk, giving fiber-optic cable an obvious advantage over copper-based media. In addition, fiber-optic cable is highly resistant to attenuation.
Two types of optical fiber are available: single-mode and multimode.
Attachment unit interface (AUI) ports are network interface ports that are often associated with thick coax (that is, 10Base5) networks. The AUI port is a 15-pin socket to which a transceiver is connected.
SC and ST connectors are associated with fiber cabling, ST connectors offer a twist-type attachment, and SC connectors are push-on connectors.
RJ-45 connectors are used with UTP cable.
Table 8 summarizes the characteristics of the types of network cable.
| Cable Type | Resistance to Attenuation | Resistance to EMI/Crosstalk | Cost of Implementation | Difficulty of Implementation |
|---|---|---|---|---|
| Thin coax | Moderate | Moderate | Low | Low |
| Thick coax | High | High | Moderate | Moderate |
| UTP | Low | Low | Low | Low |
| STP | Moderate | Moderate | Moderate | Low |
| Fiber-optic | Very high | Very high | Very high | Extremely difficult |
10BaseX, 100BaseX, and 1000BaseX Standards
The following are important facts about the 10BaseX standards:
10Base2, sometimes called Thinnet or Thin Ethernet, is the 802.3 specification for a network that uses thin coaxial cable (that is, RG-58 cable).
10Base2 specifies a maximum speed of 10Mbps and uses BNC barrel and BNC T connectors to connect the cable and computers. At the physical ends of each cable segment, a 50-ohm terminator absorbs the signal, thus preventing signal reflection.
Thinnet cable is prone to breaks, and a break anywhere in the cable renders the entire network unusable.
The 10Base2 standard specifies a limit of 185 meters per segment (that is, approximately 600 feet).
In a 10Base2 network, only 30 networked devices can be attached to a single segment. A maximum of 3 segments can have network devices connected.
10Base5 networks use thick coaxial cable (that is, RG-8 cable), also known as Thicknet or Thick Ethernet, and devices attach to it by using external transceivers and AUI ports.
10Base5 uses baseband transmission, has a maximum transfer rate of 10Mbps, and has a cable distance of 500 meters per segment.
Table 9 provides a summary of 100BaseX standards, and Table 10 provides a summary of 1000BaseX standards.
| Characteristic | 100BaseTX | 100BaseT4 | 100BaseFX |
|---|---|---|---|
| Transmission Method | Baseband | Baseband | Baseband |
| Speed | 100Mbps | 100Mbps | 100Mbps |
| Distance | 100 meters | 100 meters | 412 meters (multimode, half-duplex) 10,000 meters (single-mode, full-duplex) |
| Cable Type | Category 5 UTP, STP | Category 3, 4, 5 | Fiber-optic |
| Connector Type | RJ-45 | RJ-45 | SC, ST, MIC |
| Characteristic | 1000BaseSX | 1000BaseLX | 1000BaseCX |
|---|---|---|---|
| Transmission Method | Baseband | Baseband | Baseband |
| Speed | 1000Mbps | 1000Mbps | 1000Mbps |
| Distance | Half-duplex 275 (62.5 micron multimode fiber); half-duplex 316 (50 micron multimode fiber); full-duplex 275 (62.5 micron multimode fiber); full-duplex 550 (50 micron multimode fiber) | Half-duplex 316 (multimode and single-mode fiber); full-duplex 550 (multimode fiber); full-duplex 5000 (single-mode fiber) | 25 meters for both full-duplex and half-duplex operations |
| Cable Type | 62.5/125 and 50/125 multimode fiber | 62.5/125 and 50/125 multimode fiber; two 10-micron single-mode optical fibers | Shielded copper cable |
| Connector Type | SC connector | SC connector | 9-pin shielded connector, 8-pin Fibre Channel type 2 connector |
Network Devices
Both hubs and switches are used in Ethernet networks. The following facts are relevant to hubs:
Token Ring networks, which are few and far between, use special devices called multistation access units (MSAUs) to create the network.
The function of a hub is to take data from one of the connected devices and forward it to all the other ports on the hub.
Most hubs are referred to as active because they regenerate a signal before forwarding it to all the ports on the device. In order to do this, the hub needs a power supply.
Passive hubs do not need power because they don't regenerate signals.
The following facts are relevant to switches:
Rather than forward data to all the connected ports, a switch forwards data only to the port on which the destination system is connected.
A switch makes forwarding decisions based on the Media Access Control (MAC) addresses of the devices connected to it in order to determine the correct port.
In cut-through switching, the switch begins to forward the packet as soon as it is received.
In store-and-forward switching, the switch waits to receive the entire packet before beginning to forward it.
In fragment-free switching, the switch reads only the part of the packet that enables it to identify fragments a transmission.
The following facts are relevant to both hubs and switches:
Hubs and switches have two types of ports: medium-dependent interface (MDI) and medium-dependent interface crossed (MDI-X).
A straight-through cable is used to connect systems to the switch or hub using the MDI-X ports.
In a crossover cable, Wires 1 and 3 and Wires 2 and 6 are crossed.
Both hubs and switches use light-emitting diodes (LEDs) to indicate certain connection conditions. At the very least, a link light on the hub indicates the existence of a live connection.
Both hubs and switches are available in managed and unmanaged versions. A managed device has an interface through which it can be configured to perform certain special functions.
Bridges are used to divide up networks and thus reduce the amount of traffic on each network.
A bridge functions by blocking or forwarding data, based on the destination MAC address written into each frame of data.
Unlike bridges and switches, which use the hardware-configured MAC address to determine the destination of the data, routers use software-configured network addresses to make decisions.
With distance-vector routing protocols, each router communicates all the routes it knows about to all other routers to which it is directly attached.
Routing Information Protocol (RIP) is a distance-vector routing protocol for both Transmission Control Protocol (TCP) and Internetwork Packet Exchange (IPX).
Modems translate digital signals from a computer into analog signals that can travel across conventional phone lines.
Modems are controlled through a series of commands known as the Hayes AT command set:
| Command | Result |
|---|---|
| ATA | Answers an incoming call |
| ATH | Hangs up the current connection |
| ATZ | Resets the modem |
| ATI3 | Displays modem identification information |
Following is a summary of UART chip speeds:
| UART Chip | Speed (bps) |
|---|---|
| 8250 | 9,600 |
| 16450 | 115,200 |
| 16550 | 115,200 |
| 16650 | 430,800 |
| 16750 | 921,600 |
| 16950 | 921,600 |
Table 11 summarizes the various devices used in networks.