Layer 1: Media, Connections, and CollisionsChapter 5

LAN Media function is to transmit data can be electrical pulses, referred

to as voltage, on copper conducting wires, light pulses within optical fibers, or sent on wireless media

transmission process is referred to as encoding

cable types include STP, UTP, coaxial, fiber-optics

LAN Media--STP

•4-pair wiring combines shielding and cancellation via twisting of wires

•normally a 150-Ohm cable

• reduces crosstalk EMI, RFI

•affords greater protection against all types of external interference than UTP

LAN Media--ScTP

•wrapped in a metallic foil shield or screen

•usually 100- or 120-Ohm cable

•Both STP and ScTP prevent incoming electromagnetic waves from causing noise on data wires and also minimizes outgoing radiated electromagnetic waves.

LAN Media—Disadvantages of STP and ScTP• cannot run cable as far as some

other types of networking media without signal being repeated

• insulation adds considerably to size, weight, and cost of cable

• shielding materials make terminations more difficult and susceptible to poor workmanship

LAN Media--UTP

• relies solely on the cancellation effect, produced by twisted wire pairs to limit signal degradation caused by EMI and RFI

•has four pairs of either 22- or 24-gauge copper wire

•has impedance of 100 Ohms

• easy to install and is less expensive than other types of networking media

• costs less per meter than any other type of LAN cabling

• small external diameter does not fill up wiring ducts as rapidly as other types of cable

• is installed using an RJ connector so potential sources of network noise are greatly reduced and a good solid connection is practically guaranteed

LAN Media—Advantages of UTP

LAN Media--Coaxial

• thinnet and thicknet

• longer network runs without using repeaters than twisted-pair

• less expensive than fiber but more expensive than twisted-pair

LAN Media—Disadvantages of UTP

• cable is more prone to electrical noise and interference than other types of networking media

• distance between signal boosts is shorter for twisted-pair than for coaxial and fiber-optics

LAN Media—Fiber-Optic

•conducts modulated light transmission

•not susceptible to EMI or RFI and is capable of higher data rates than other networking media

•electromagnetic waves are guided through optical fiber

Fiber-Optic Mode Types•Single Mode

• also called axial because light travels down the axis of the cable

• faster than multimode (up to 10 Gbps) because of the dispersion in multimode

• typically used for WANS• smaller in diameter than multimode (less

dispersion)• uses ILD most often but also LED

•Multimode• light enters the glass pipe at different angles

and travels nonaxially, which means it bounces back and forth off the walls of the glass tube

• larger than single mode, used most often in LANS

• susceptible to greater dispersion

Wireless Communication•The primary way

of distinguishing between different electromagnetic waves is by their frequency.

•Low frequency electromagnetic waves have a long wavelength whereas high frequency electromagnetic waves have a short wavelength.

Wireless Transmission Methods

•Lasers• output a coherent

electromagnetic field in which all waves are at the same frequency and are aligned in a phase

•Infrared• normally a line-of-sight

technology but can be bounced or redirected

• cannot go through opaque objects•Radio

• carry data signals that can pass through walls

• both terrestrial and satellite radio technologies

WLANS typically use radio waves, microwaves, and infrared waves.

Cable Specification and TerminationYour work in this curriculum focuses on the standards for networking media that were developed and issued by the following groups:

IEEE—Institute of Electrical and Electronics Engineers

UL—Underwriters Laboratories

EIA—Electronic Industries Alliance

TIA—Telecommunications Industry Association

ANSI—American National Standards Institute

Cable Specification and Termination• The IEEE has outlined cabled

requirements in its 802.3 and 802.5 specifications for Ethernet and Token Ring systems and other standards for FDDI.

• UL issues cabling specifications that are concerned primarily with safety standards, but they also rate twisted-pair networking media for performance.

TIA/EIA Standards

TIA/EIA-568 and TIA/EIA-569have been and

continue to be themost widely used

standards for technical

performance ofnetworking media.

TIA/EIA-568-A• The TIA/EIA standards address six elements of the

LAN cabling process:• horizontal cabling• telecommunications closets• backbone cabling• equipment rooms• work areas• entrance facilities

• The TIA/EIA standards specify the minimum require-ments for multi-product and multi-vendor environments. They allow for the planning and installation of LAN systems without dictating the use of specific equipment, thus giving LAN designers the freedom to create options for improvement and expansion.

TIA/EIA-568-A• defines horizontal cabling as cabling that runs from

a telecommunications outlet to a horizontal cross-connect

• includes the networking medium that runs along a horizontal pathway, the telecommunications outlet or connector, the mechanical terminations in the wiring closet, and the patch cords or jumpers in the wiring closet

• includes the networking media that is used in the area that extends from the wiring closet to a workstation

• contains specifications governing cable performance, which calls for running two cables, one for voice and one for data, to each outlet

• Of the two cables, the one for voice must be four-pair UTP.

TIA/EIA-568-A• The TIA/EIA-568-A standard specifies five

categories in the specifications. • These are category 1 (CAT 1), category 2 (CAT 2),

category 3 (CAT 3), category 4 (CAT 4), and category 5  (CAT 5) cabling. Of these, only CAT 3, CAT 4, and CAT 5 are recognized for use in LANs.

• Of these three categories, CAT 5 is the one most frequently recommended and implemented in installations today.

• The networking media that are recognized for these categories are the ones you have studied:

• shielded twisted-pair • unshielded twisted-pair • fiber-optic cable • coaxial cable

TIA/EIA-568-A• For shielded twisted-pair cable, the TIA/EIA-

568-A standard calls for two pair 150-Ohm cable.

• For unshielded-twisted pair, the standard calls for four pair 100-Ohm cable.

• For fiber-optic, the standard calls for two fibers of 62.5/125  multimode cable.

• Although 50-Ohm coaxial cable is a recognized type of networking media in TIA/EIA-568-A, it is not recommended for new installations. Moreover, this type of coaxial cable is expected to be removed from the list of recognized networking media the next time this standard is revised.

• For the horizontal cabling component, TIA/EIA-568-A requires a minimum of two telecommunications outlets or connectors at each work area. This telecommunications outlet/connector is supported by two cables. The first is a four-pair 100-Ohm CAT 3 or higher UTP cable along with its appropriate connector. The second can be any one of the following: • four-pair 100-Ohm unshielded twisted-pair cable

and its appropriate connector • 150-Ohm shielded twisted-pair cable and its

appropriate connector  • coaxial cable and its appropriate connector • two-fiber 62.5/125 µ optical fiber cable and its

appropriate connector

TIA/EIA-568-A

• The maximum distance for cable runs in horizontal cabling is 90 meters (m). This is true for all types of CAT 5 UTP recognized networking media.

• The standard also specifies that patch cords or cross-connect jumpers located at the horizontal cross-connect cannot exceed 6m in length. TIA/EIA-568-A also allows 3m for patch cords that are used to connect equipment at the work area. The total length of the patch cords and cross-connect jumpers used in the horizontal cabling cannot exceed 10m.

• A final specification for horizontal cabling contained in TIA/EIA-568-A requires that all grounding and bonding must conform to TIA/EIA-607 as well as to any other applicable codes.

TIA/EIA-568-A

• The latest industry standards being developed are for Cat 5e, Cat 6, and Cat 7 cabling, all of which offer improvements over Cat 5.

TIA/EIA-568-A

Testing Your Network1. Break the system in logically conceived functional elements2. Note any symptoms.3. Based on the symptoms you observe, determine what is the

most likely dysfunctional element.4. Use substitution or additional testing to discover whether the

likely element is, in fact, dysfunctional.5. If the element suspected of

being dysfunctional proves not to be the problem, proceed to the next most likely element you suspect.

6. When you find the dysfunctional element, repair it if possible.

7. If it is not possible to repair the dysfunctional element, replace it.

Testing Your Network

• can perform tests that measure overall capability of a cable run• determining cable

distance• locating bad

connections

• providing wire maps for detecting crossed pairs

• measuring signal attenuation

• detecting near-end crosstalk

• detecting split pairs• performing noise

level tests• tracing cable behind

walls

(also called TDRs)

Straight-Through Cable• maintains the pin connection all the way

through the cable, thus the wire connected to pin 1 is the same on both ends of the cable

• wired to TIA/EIA 568-B or A standards for 10Base-T Ethernet, which determines what color wire is on each pin

• T568-B (also called AT&T specification) is more common, but many installations are also wired to T568-A (also called ISDN).

• conforms to the structured cabling standards and is considered to be part of the “horizontal” cabling, which is limited to 99m total between workstation and hub or switch

Straight-Through Cable

• used to connect such devices as PCs or routers to other devices such as hubs or switches

• can be used in a workstation area to connect the workstation NIC to the wall plate data jack or it can be used in the wiring closet to connect the patch panel (horizontal cross connect) to an Ethernet hub or switch

• are wired straight through because the cable from the workstation to the hub or switch is normally crossed over automatically at the switch or hub

• The ports on most hubs have an X next to them, which means the send and receive pairs will be crossed when the cabling reaches the switch.

1

Steps for Creating a Straight-Through Cable

2 3 4

5 6 7 8

9 10 11 12

W-G G W-O Bl W-Bl O W-Br Br W-O O W-G Bl W-Bl G W-Br Br

This cable is used to connect a PC to the router for purposes of accessing the router.

Console Cable

Rollover Cable• can be used to connect a workstation or dumb

terminal to the console port on the back of a router or Ethernet switch in order to be able to configure the router or switch

• uses an asynchronous serial interface to the router or switch

• both ends have RJ-45 connectors—one plugs directly into the RJ-45 console management port on the back of the router or switch and the other end plugs into an RJ-45-to-DB9 terminal adapter

• The terminal adapter converts the RJ-45 to a 9-pin female D connector that plugs into the DB9 serial port male adapter on the back of a PC running terminal emulation software.

Rollover Cable• uses 8 pins but is different from a straight-

through cable or crossover cable• Pin 1 on one end connects to Pin 8 on the other

end; then Pin 2 connects to Pin 7, Pin 3 to Pin 6 and so on.

Crossover Cable• pairs 2 and 3 on one end of the cable will be

reversed on the other end• will be wired to EIA/TIA 568-B and A standards

for 10Base-T Ethernet• pinouts will be T568-A on one end and T568-B on

the other• used between hubs or switches and is

considered to be part of the “vertical” cabling also known as backbone

• can be used as a backbone cable to connect two or more hubs or switches in a LAN or to connect two isolated workstations to create a mini-LAN

• allows the connection of two workstations or a server and workstation without needing a hub in between

Where are we going?

LAN Technologies• In this curriculum you will be introduced to three

LAN technologies: Ethernet, Token Ring, and FDDI.

• All three have a wide variety of Layer 1 components and devices.

• The focus of this chapter will be Ethernet 10BASE-T technologies.

• When it was developed, Ethernet was designed to fill the middle ground between long distance, low-speed networks, and specialized computer room networks that carried data at high speeds for very limited distances. Ethernet is well-suited to applications in which a local communication medium must carry sporadic, occasionally heavy, traffic at high-peak data rates.

LAN Technologies

• The Ethernet 10BASE-T technologies carry Ethernet frames on inexpensive twisted-pair wiring.

• You will study four components and three devices that are related to these technologies.

• The first four components are passive, meaning they require no energy to operate. They are:

• patch panels, plugs, cabling, and jacks • The last three are active. They require energy to do

their jobs. They are: • transceivers, repeaters, and hubs

All of these devices - passive and active - create or act on the bits. They recognize no information patterns in the bits, no addresses, and no data. Their function is simply to move bits around. Layer 1 is fundamental to troubleshooting networks, and should never be underestimated. Many network problems are traceable to bad RJ-45 terminations, jacks, punch-downs, repeaters, hubs, or transceivers.

Type of Networks• shared media environment - occurs when

multiple hosts have access to the same medium. For example, if several PCs are attached to the same physical wire, optical fiber, or share the same airspace, they all share the same media environment. Occasionally you may hear someone say "all the computers are on the same wire." It means that they all share the same media - even though the "wire" might be CAT 5 UTP, which has four pairs of wire.

Type of Networks• extended shared media environment - is

a special type of shared media environment in which networking devices can extend the environment so that it can accommodate multiple-access, or more users. There are, however, negative aspects to this as well as positive aspects.

Type of Networks

• point-to-point network environment - is most widely used in dial-up network connections, and is the one with which you are most likely familiar. It is a shared networking environment in which one device is connected to only one other device via a link, such as you connecting to internet service provider by phone line.

Type of Networks

• circuit-switched - an indirectly-connected network in which actual electrical circuits are maintained for the duration of the communication. The current telephone system is still, in part, circuit-switched, although the telephone systems in many countries is now concentrating less on circuit-switched technologies.

Type of Networks

• packet-switched - rather than dedicating a link as an exclusive circuit connection between two communicating hosts, the source sends messages in packets. Each packet contains enough information for it to be routed to the proper destination host. The advantage is that many hosts can share the same link; the disadvantage is that conflicts can occur.

Collisions and Collision Domains• A collision occurs when two bits propagate at

the same time on the same network.

• Collision domains are extended by adding repeaters and hubs.

• Networks can be segmented by adding intelligent devices such as bridges, switches, and routers.

Collisions and Collision Domains

• The four repeater rule in Ethernet states, that no more than four repeaters or repeating hubs can be between any two computers on the network.

• To assure that a repeated 10BASE-T network will function properly, the following condition must be true: (repeater delays + cabledelays + NIC delays) x 2< maximum round-trip delay.

Collisions and Collision Domains• Repeater delays for 10BASE-T are usually less

than 2 microseconds per repeater; cable delays are near 0.55 microseconds per 100m trip; NIC delays are about 1 microsecond per NIC; and the maximum round-trip delay (the 10BASE-T bit time of 0.1 microseconds times the minimum frame size of 512 bits) is 51.2 microseconds.

• For a 500m length of UTP connected by 4 repeaters (hubs) and 2 NIC delays the total delay would be well below the maximum round-trip delay. Repeater latency, propagation delay, and NIC latency all contribute to the 4-repeater rule. Exceeding the four repeater rule can lead to violating the maximum delay limit.

Collisions and Collision Domains• When this delay limit is exceeded, the number of

late collisions dramatically increase.• A late collision is when a collision happens after the

first 64 bytes of the frame are transmitted. The chipsets in NICs are not required to retransmit automatically when a late collision occurs. These late collision frames add delay referred to as consumption delay.

• As consumption delay and latency increase, network performance decreases. This Ethernet rule of thumb is also known as the 5-4-3-2-1 rule. Five sections of the network, four repeaters or hubs, three sections of the network are "mixing" sections (with hosts), two sections are link sections (for link purposes), and one large collision domain.

Segmenting Collision Domains

Network Topologies•A physical

topology describes the plan for wiring the physical devices.

•A logical topology describes how information flows through a network.

Network Topologies--Bus• Mathematical Perspective

The bus topology has all of its nodes connected directly to one link, and has no other connections between nodes.

• Physical Perspective Each host is wired to a common wire. In this topology, the key devices are those that allow the host to join or tap into the single shared medium. One advantage of this topology is that all hosts are connected to each other, and thus, can communicate directly. One disadvantage of this topology is that a break in the cable disconnects hosts from each other.

• Logical Perspective A bus topology enables every networking device to see all signals from all other devices. This can be an advantage if you want all information to go to every device.

Network Topologies--Ring• Mathematical Perspective

A ring topology is a single closed ring consisting of nodes and links, with each node connected to only two adjacent nodes.

• Physical Perspective  The topology shows all devices wired directly to each other in what is called a daisy-chain. This is similar to the manner in which a mouse on an Apple PC plugs into the keyboard and then into the PC.

• Logical PerspectiveIn order for information to flow, each station must pass the information to its adjacent station.

Network Topologies—Dual Ring• Mathematical Perspective

A dual ring topology consists of two concentric rings, each of which is linked only to its adjacent ring neighbor. The two rings are not connected.

• Physical Perspective A dual ring topology is the same as a ring topology, except that there is a second, redundant ring, that connects the same devices. In other words, in order to provide reliability and flexibility in the network, each networking device is part of two independent ring topologies.

• Logical Perspective A dual ring topology acts like two independent rings, of which, only one at a time is used.

Network Topologies—Star• Mathematical Perspective

A star topology has a central node with all links to other nodes radiating from it and allows no other links.

• Physical PerspectiveA star topology has a central node with all links radiating from it. Its primary advantage is that it allows all other nodes to com-municate with each other, conveniently. Its primary disadvantage is that if the central node fails, the whole network becomes disconnected. Depending on the type of networking device used at the center of the star network, collisions can be a problem.

• Logical PerspectiveThe flow of all information would go through one device. This might be desirable for security or restricted access reasons, but it would be very susceptible to any problems in the star's central node.

Network Topologies—Extended Star• Mathematical Perspective

An extended star topology repeats a star topology, except that each node that links to the center node is, also, the center of another star.

• Physical Perspective An extended star topology has a core star topology, with each of the end nodes of the core topology acting as the center of its own star topology. The advantage of this is that it keeps wiring runs shorter, and limits the number of devices that need to interconnect to any one central node.

• Logical Perspective An extended star topology is very hierarchical, and information is encouraged to stay local. This is how the phone system is currently structured.

Network Topologies—Tree

• Mathematical PerspectiveThe tree topology is similar to the extended star topology, the primary difference is that it does not use one central node. Instead, it uses a trunk node from which it branches to other nodes. There are two types of tree topologies: the binary tree (each node splits into two links); and the backbone tree (a backbone trunk has branch nodes with links hanging from it).

• Physical PerspectiveThe trunk is a wire that has several layers of branches.

• Logical PerspectiveThe flow of  information is hierarchical.

Network Topologies—Irregular•Mathematical Perspective

In the irregular network topology there is no obvious pattern to the links and nodes.

•Physical PerspectiveThe wiring is inconsistent; the nodes have varying numbers of wires leading from them. This is how networks that are in the early stages of construction, or poorly planned, are often wired.

•Logical PerspectiveThere is no obvious pattern to the links and nodes.

Network Topologies—Complete (Mesh)• Mathematical Perspective

In a complete, or mesh topology, every node is linked directly to every other node.

• Physical Perspective This wiring has very distinct advantages and disadvantages. One advantage is every node is physically connected to every other node (creating a redundant connection). Should any link fail to function, information can flow through any number of other links to reach its destination. Another advantage of this topology is that it allows information to flow along many paths on its way back through the network. The primary physical disadvantage is that for anything more than a small number of nodes, the amount of media for the links, and the amount of connections to the links becomes overwhelming.

• Logical  Perspective The behavior of a complete, or mesh topology depends greatly on the devices used.

Network Topologies—Cellular

• Mathematical PerspectiveThe cellular topology consists of circular or hexagonal areas, each of which has an individual node at its center.

• Physical PerspectiveThe cellular topology is a geographic area that is divided into regions (cells) for the purposes of wireless technology – a technology that becomes increasingly more important each day. There are no physical links in a cellular topology, only electromagnetic waves. Sometimes the receiving nodes move (e.g. car cell phone), and sometimes the sending nodes move (e.g. satellite communication links).

Network Topologies—Cellular

• Physical Perspective • The obvious advantage of a cellular (wireless) topology is

that there are no tangible media other than the earth's atmosphere or the vacuum of off-planet space (and satellites). The disadvantages are that signals are present everywhere in a cell and, thus, are susceptible to disruptions (man-made and environmental) and to security violations (i.e. electronic monitoring and theft of service).

• Logical PerspectiveCellular technologies communicate with each other directly (though distance limitations and interference sometimes make it extremely difficult), or communicate only with their adjacent cells, which is  extremely inefficient. As a rule, cellular-based topologies are integrated with other topologies, whether they use the atmosphere or satellites.