5.1.1 LAN physical layer

Token Ring is represented by a circle.

FDDI is represented by two concentric circles.

Ethernet symbol is represented by a straight line.

Serial connections are represented by a lightning bolt.

Each computer network can be built with many different media types. Networking media are considered Layer 1, or physical layer, components of LANs.

Each type of media has advantages and disadvantages. These are based on the following factors:

• Cable length
• Cost
• Ease of installation
• Susceptibility to interference

This module will focus on Category 5 UTP, which includes the Category 5e family of cables.

5.1.2 Ethernet in the campus

Ethernet is the most widely used LAN technology. Ethernet was first implemented by the Digital, Intel, and Xerox group (DIX).

DIX created and implemented the first Ethernet LAN specification, which was used as the basis for the Institute of Electrical and Electronics Engineers (IEEE) 802.3 specification, released in 1980.

IEEE extended 802.3 to three new committees known as 802.3u for Fast Ethernet, 802.3z for Gigabit Ethernet over fiber, and 802.3ab for Gigabit Ethernet over UTP.

5.1.3 Ethernet media and connector requirements

The cables and connector specifications used to support Ethernet implementations are derived from the EIA/TIA standards.

The categories of cabling defined for Ethernet are derived from the EIA/TIA-568 SP-2840 Commercial Building Telecommunications Wiring Standards.

Networks with a combination of 10- and 100-Mbps traffic use Category 5 UTP to support Fast Ethernet.

The RJ-45 connector and jack are the most common. RJ-45 connectors are discussed in more detail in the next section.

The AUI connector allows different media to connect when used with the appropriate transceiver. A transceiver is an adapter that converts one type of connection to another. A transceiver will usually convert an AUI to an RJ-45, a coax, or a fiber optic connector.

5.1.5 UTP implementation

EIA/TIA specifies an RJ-45 connector for UTP cable. The letters RJ stand for registered jack and the number 45 refers to a specific wiring sequence. The RJ-45 transparent end connector shows eight colored wires.

The RJ-45 connector is the male component. .

The jack is the female component in a network device, wall outlet, or patch panel.

For electricity to run between the connector and the jack, the order of the wires must follow T568A or T568B color code.

Use straight-through cables for the following connections:

• Switch to router
• Switch to PC or server
• Hub to PC or server

Use crossover cables for the following connections:

• Switch to switch
• Switch to hub
• Hub to hub
• Router to router
• PC to PC
• Router to PC

5.1.6 Repeaters

A repeater receives a signal, regenerates it, and passes it on. It can regenerate and retime network signals at the bit level to allow them to travel a longer distance on the media.

Ethernet and IEEE 802.3 implement a rule, known as the 5-4-3 rule. The rule mandates that between any two nodes on the network, there can only be a maximum of five segments, connected through four repeaters, or concentrators, and only three of the five segments may contain user connections.

The 5-4-3 rule divides the network into two types of physical segments: populated (user) segments, and unpopulated (link) segments. User segments have users' systems connected to them. Link segments are used to connect the network repeaters together.

The Ethernet protocol requires that a signal sent out over the LAN reach every part of the network within a specified length of time. The 5-4-3 rule ensures this. Each repeater that a signal goes through adds a small amount of time to the process, so the rule is designed to minimize transmission times of the signals.

Hubs are actually multiport repeaters. The difference between hubs and repeaters is usually the number of ports that each device provides. A typical repeater usually has two ports. A hub generally has from 4 to 24 ports.

The use of a hub changes the network from a linear bus with each device plugged directly into the wire to a star topology. Data that arrives over the cables to a hub port is electrically repeated on all the other ports connected to the network segment.

Hubs come in three basic types:

• Passive – A passive hub serves as a physical connection point only.  It does not boost or clean the signal. A passive hub is used only to share the physical media. A passive hub does not need electrical power.
• Active – An active hub must be plugged into an electrical outlet because it needs power to amplify a signal before it is sent to the other ports.
• Intelligent – Intelligent hubs are sometimes called smart hubs. They function like active hubs with microprocessor chips and diagnostic capabilities. Intelligent hubs are more expensive than active hubs.

Devices attached to a hub receive all traffic that travels through the hub. If many devices are attached to the hub, collisions are more likely to occur.

A collision occurs when two or more workstations send data over the network wire at the same time. All data is corrupted when this occurs. All devices that are connected to the same network segment are members of the same collision domain.

Sometimes hubs are called concentrators since they are central connection points for Ethernet LANs.

Wireless signals are electromagnetic waves that travel through the air. Wireless networks use radio frequency (RF), laser, infrared (IR), satellite, or microwaves to carry signals between computers without a permanent cable connection.

A common application of wireless data communication is for mobile use. 

The two most common wireless technologies used for networking are IR and RF.

 IR technology:

Workstations and digital devices must be in the line of sight of the transmitter to work correctly.

An infrared-based network can be used when all the digital devices that require network connectivity are in one room.

IR networking technology can be installed quickly.

The data signals can be weakened or obstructed by people who walk across the room. New IR technologies will be able to work out of sight.

RF technology:

RF technology allows devices to be in different rooms or buildings.

The limited range of radio signals restricts the use of this kind of network.

RF technology can be on single or multiple frequencies. A single radio frequency is subject to outside interference and geographic obstructions. It is also easily monitored by others, which makes the transmissions of data insecure.

Spread spectrum uses multiple frequencies to increase the immunity to noise and to make it difficult for outsiders to intercept data transmissions.

5.1.9 Bridges

The devices that are used to connect network segments together include bridges, switches, routers, and gateways.

Switches and bridges operate at the data link layer of the OSI model. The function of the bridge is to make intelligent decisions about whether or not to pass signals on to the next segment of a network.

When a bridge receives a frame on the network, the destination MAC address is looked up in the bridge table to determine whether to filter, flood, or copy the frame onto another segment. This decision process occurs as follows:

• If the destination device is on the same segment as the frame, the bridge will not send the frame onto other segments. This process is known as filtering.
• If the destination device is on a different segment, the bridge forwards the frame to the appropriate segment.
• If the destination address is unknown to the bridge, the bridge forwards the frame to all segments except the one on which it was received. This process is known as flooding.

If placed strategically, a bridge can greatly improve network performance.

A switch is sometimes described as a multiport bridge. A typical bridge may have only two ports that link two network segments. A switch can have multiple ports based on the number of network segments that need to be linked.

Switches reduce traffic and increase bandwidth.

Switches can easily replace hubs because switches work with the cable infrastructures that are already in place.

All switching equipment perform two basic operations. The first operation is called switching data frames. This is the process by which a frame is received on an input medium and then transmitted to an output medium. The second is the maintenance of switching operations where switches build and maintain switching tables and search for loops.

Switches operate at much higher speeds than bridges and can support new functionality, such as virtual LANs.

An Ethernet switch has many benefits. One benefit is that it allows many users to communicate at the same time through the use of virtual circuits and dedicated network segments in a virtually collision-free environment. This maximizes the bandwidth available on the shared medium.

NICs are considered Layer 2 devices because each NIC carries a unique code called a MAC address. This address is used to control data communication for the host on the network.

A transceiver converts one type of signal or connector to another. For example, a transceiver can connect a 15-pin AUI interface to an RJ-45 jack. It is considered a Layer 1 device because it only works with bits and not with any address information or higher-level protocols.

5.1.12 Peer-to-peer

In a peer-to-peer network, networked computers act as equal partners, or peers. As peers, each computer can take on the client function or the server function.

In a peer-to-peer network, individual users control their own resources. The users may decide to share certain files with other users. There is no central point of control or administration in the network.

As networks grow, peer-to-peer relationships become increasingly difficult to coordinate. A peer-to-peer network works well with ten or fewer computers. Since peer-to-peer networks do not scale well, their efficiency decreases rapidly as the number of computers on the network increases.

In a client/server arrangement, network services are located on a dedicated computer called a server. The server responds to the requests of clients.

Typically, desktop computers function as clients and one or more computers with additional processing power, memory, and specialized software function as servers.

Servers are designed to handle requests from many clients simultaneously.

The physical layer implementations vary based on the distance of the equipment from each service, the speed, and the type of service.

Serial connections are used to support WAN services such as dedicated leased lines that run PPP or Frame Relay. The speed of these connections ranges from 2400 bps to T1 service at 1.544 Mbps and E1 service at 2.048 Mbps.

ISDN offers dial-on-demand connections or dial backup services. An ISDN Basic Rate Interface (BRI) is composed of two 64 kbps bearer channels (B channels) for data, and one delta channel (D channel) at 16 kbps used for signaling and other link-management tasks. PPP is typically used to carry data over the B channels.

DSL and cable modem connections have become more popular.  Cable services use the coaxial cable TV line.

For long distance communication, WANs use serial transmission. This is a process by which bits of data are sent over a single channel.

For a Cisco router, physical connectivity at the customer site is provided by one of two types of serial connections. The first type is a 60-pin connector. The second is a more compact ‘smart serial’ connector. The provider connector will vary depending on the type of service equipment.

Routers are responsible for routing data packets from source to destination within the LAN, and for providing connectivity to the WAN.

The DTE is the endpoint of the user’s device on the WAN link. The DCE is typically the point where responsibility for delivering data passes into the hands of the service provider.

When connecting directly to a service device such as a CSU/DSU, the router is a DTE and needs a DTE serial cable. This is typically the case for routers.

However, there are cases when the router will need to be the DCE. When performing a back-to-back router scenario in a test environment, one of the routers will be a DTE and the other will be a DCE.

When cabling routers for serial connectivity, the routers will either have fixed or modular ports.

Interfaces on routers with fixed serial ports are labeled for port type and port number.

Interfaces on routers with modular serial ports are labeled for port type, slot, and port number. The slot is the location of the module. To configure a port on a modular card, using the syntax “port type slot number/port number”.

Example:

Use the label “serial 1/0”, when the interface is serial, the slot number where the module is installed is slot 1, and the port that is being referenced is port 0.

With ISDN BRI, two types of interfaces may be used, BRI S/T and BRI U.

An NT1 is an intermediate device located between the router and the service provider ISDN switch.

The NT1 is used to connect four-wire subscriber wiring to the conventional two-wire local loop.

In North America, the customer typically provides the NT1, while in the rest of the world the service provider provides the NT1 device.

It may be necessary to provide an external NT1 if the device is not already integrated into the router.

A BRI interface with an integrated NT1 is labeled BRI U.

A BRI interface without an integrated NT1 is labeled BRI S/T.

The type of BRI interface may be determined by looking at the port label.

To interconnect the ISDN BRI port to the service-provider device, use a UTP Category 5 straight-through cable.

The Cisco 827 ADSL router has one asymmetric digital subscriber line (ADSL) interface. To connect an ADSL line to the ADSL port on a router, do the following:

• Connect the phone cable to the ADSL port on the router.
• Connect the other end of the phone cable to the phone jack.

To connect a router for DSL service, use a phone cable with RJ-11 connectors. DSL works over standard telephone lines using pins 3 and 4 on a standard RJ-11 connector.

The Cisco uBR905 cable access router provides high-speed network access on the cable television system to residential and small office subscribers. The uBR905 router has an F-connector that connects directly to the cable system.

To initially configure the Cisco device, a management connection must be directly connected to the device. For Cisco equipment this management attachment is called a console port. The console port allows monitoring and configuration of a Cisco hub, switch, or router.

The cable used between a terminal and a console port is a rollover cable, with RJ-45 connectors. The rollover cable, also known as a console cable.

To set up a connection between the terminal and the Cisco console port:

      First, connect the devices using a rollover cable from the router console port to the workstation serial port. An RJ-45-to-DB-9 or an RJ-45-to-DB-25 adapter may be required for the PC or terminal.

      Second, configure the terminal emulation application with the following common equipment (COM) port settings: 9600 bps, 8 data bits, no parity, 1 stop bit, and no flow control.

The AUX port is used to provide out-of-band management through a modem. The AUX port must be configured by way of the console port before it can be used. The AUX port also uses the settings of 9600 bps, 8 data bits, no parity, 1 stop bit, and no flow control.