CCNA 1 and 2 Companion Guide, Revised (Cisco Networking Academy Program) part 12 ppt

Host devices are physically connected to the network media using a network interface card NIC.. Network devices extend cable connections, concentrate connections, convert data formats,

These devices allow users to share, create, and obtain information Host devices can

exist without a network, but without a network, host capabilities are greatly reduced

Host devices are physically connected to the network media using a network interface

card (NIC) They use this connection to perform the tasks of sending e-mails, printing

reports, scanning pictures, or accessing databases A NIC is a printed circuit board

that fits into the expansion slot of a bus on a computer motherboard, or it can be a

peripheral device It is also called a network adapter Laptop or notebook computer

NICs are usually the size of a PCMCIA card

Each NIC carries a unique code called a MAC address MAC addresses are covered

more in a moment As the name implies, the NIC controls host access to the medium

There are no standardized symbols for end-user devices in the networking industry

They bear a resemblance to the real device to allow for quick recognition

Network devices provide transport for the data that needs to be transferred between

end-user devices Network devices extend cable connections, concentrate connections,

convert data formats, and manage data transfers Examples of devices that perform

these functions are repeaters, hubs, bridges, switches, and routers The following

sec-tions provide an overview of some common networking devices

Repeaters

Repeaters are networking devices that exist at Layer 1, the physical layer, of the OSI

reference model To understand how a repeater works, it is important to understand

that as data leaves a source and goes out over the network, it is transformed into either

electrical or light pulses that pass along the networking medium These pulses are

called signals When signals leave a transmitting station, they are clean and easily

rec-ognizable However, the longer the cable length, the weaker and more deteriorated the

signals become as they pass along the networking medium The purpose of a repeater

is to regenerate and retime network signals at the bit level, allowing them to travel a

longer distance on the medium

The term repeater originally meant a single port “in” device and a single port “out”

device Today multiple-port repeaters also exist Repeaters are classified as Layer 1

devices in the OSI model because they act only on the bit level and look at no other

information

Hubs

The purpose of a hub is to regenerate and retime network signals The characteristics

of a hub are similar to those of a repeater A hub is a common connection point for

devices in a network, as shown in Figure 2-22 Hubs commonly connect segments of a LAN A hub contains multiple ports When a packet arrives at one port, it is copied to the other ports so that all the LAN’s segments can see all the packets

Figure 2-22 Hub

Because hubs and repeaters have similar characteristics, a hub is also called a multiport

repeater The difference between a repeater and a hub is the number of cables that

con-nect to the device Whereas a repeater typically has only two ports, a hub generally has from four to 20 or more ports, as shown in Figure 2-23 Whereas a repeater receives

on one port and repeats on the other, a hub receives on one port and transmits on all the other ports

Figure 2-23 Hubs Have Several Ports

The following are the most important properties of hubs:

■ Hubs amplify signals

■ Hubs propagate signals through the network

■ Hubs do not require filtering

■ Hubs do not require path determination or switching.

■ Hubs are used as network-concentration points

Hubs are commonly used in Ethernet 10BASE-T or 100BASE-T networks (You will

learn more about Ethernet networks in Chapter 6, “Ethernet Technologies and

Ether-net Switching.”) Hubs create a central connection point for the wiring medium They

also increase the network’s reliability by allowing any single cable to fail without

dis-rupting the entire network This feature differs from the bus topology, in which the

failure of one cable disrupts the entire network (Network topology is discussed later

in this chapter.) Hubs are considered Layer 1 devices because they only regenerate the

signal and repeat it out all their ports (network connections)

In Ethernet networks, all the hosts are connected to the same physical medium Signals

that are sent out across the common medium are received by all devices A collisionis

a situation that can occur when 2 bits propagate at the same time on the same network

The area within the network from where the data packets originate and collide is called

acollision domain All shared-media environments are collision domains, or

band-width domains You learn more about collision domains in Chapter 5, “Ethernet

Fun-damentals.”

As previously mentioned in this section, the function of Layer 1 devices is simply to

facilitate the transmission of signals The devices recognize no information patterns in

the signals, no addresses, and no data When two wires are connected using hubs or

repeaters, all the interconnections are part of a collision domain

Network Interface Cards

Network interface cards (NICs) are considered Layer 2 devices because each NIC

throughout the world carries a unique code, called a Media Access Control (MAC)

address This address controls data communication for the host on the LAN The NIC

controls the access of the host to the medium Figure 2-24 shows a NIC

Figure 2-24 Network Interface Card

Abridge is a Layer 2 device designed to create two or more LAN segments, each of which is a separate collision domain In other words, bridges were designed to create more usable bandwidth The purpose of a bridge is to filter traffic on a LAN to keep local traffic local yet allow connectivity to other parts (segments) of the LAN for traffic that is directed there Every networking device has a unique MAC address on the NIC The bridge keeps track of which MAC addresses are on each side of the bridge and makes forwarding decisions based on this MAC address list

Bridges filter network traffic by looking only at the MAC address Therefore, they can rapidly forward traffic representing any network layer protocol Because bridges look only at MAC addresses, they are not concerned with network layer protocols Conse-quently, bridges are concerned only with passing or not passing frames, based on their destination MAC addresses The following are the important properties of bridges:

■ Bridges are more “intelligent” than hubs That is, they can analyze incoming frames and forward (or drop) them based on addressing information

■ Bridges collect and pass packets between two or more LAN segments

■ Bridges create more collision domains, allowing more than one device to trans-mit simultaneously without causing a collision

■ Bridges maintain MAC address tables

Figure 2-25 shows how a bridge is used The appearances of bridges vary greatly, depending on the type

Figure 2-25 Bridge

Internet

What defines a bridge is its Layer 2 filtering of frames and how this is accomplished

To filter or selectively deliver network traffic, bridges build tables of all MAC

addresses located on a network segment and other networks and then map them to

associated ports The process is as follows:

Step 1 If data comes along the network medium, a bridge compares the

destina-tion MAC address carried by the data to MAC addresses contained in its tables

Step 2 If the bridge determines that the data’s destination MAC address is from

the same network segment as the source, it does not forward the data to

other segments of the network This process is known as filtering By

per-forming this process, bridges can significantly reduce the amount of

traf-fic between network segments by eliminating unnecessary traftraf-fic

Step 3 If the bridge determines that the data’s destination MAC address is not

from the same network segment as the source, it forwards the data to the appropriate segment

Step 4 If the destination MAC address is unknown to the bridge, the bridge

broadcasts the data to all devices on a network except the one on which

it was received This process is known as flooding

Abroadcastis a data packet that is sent to all nodes on a network A broadcast domain

consists of all the devices connected to a network that receive the data packet

broad-cast by a node to all other nodes on the same network Because every device on the

network must pay attention to broadcasts, bridges always forward them Therefore,

all segments in a bridged environment are considered to be in the same broadcast

domain

As was the case in the repeater/hub combination, another device, called a switch, is

used for multiple bridge connections The next section discusses switches in greater

detail

Layer 2 Switches

Layer 2 switches, also called LAN switches or workgroup switches, often replace

shared hubs and work with existing cable infrastructures to ensure that the switches

are installed with minimal disruption of existing networks Figure 2-26 shows a switch

Figure 2-26 Switch

Like bridges, switches connect LAN segments, use a table of MAC addresses to deter-mine the segment on which a frame needs to be transmitted, and reduce traffic Switches operate at much higher speeds than bridges

Switches are data link layer devices that, like bridges, let multiple physical LAN seg-ments be interconnected into single larger networks Similar to bridges, switches for-ward and flood traffic based on MAC addresses Because switching is performed in hardware, it is significantly faster than the switching function performed by a bridge using software Think of each switch port as a microbridge Each switch port acts as a separate bridge and gives each host the medium’s full bandwidth This process is called microsegmentation

Microsegmentationallows the creation of private or dedicated segments—one host per segment Each host receives instant access to the full bandwidth and does not have to compete for available bandwidth with other hosts In full-duplex switches, because only one device is connected to each switch port, collisions do not occur

However, as with a bridge, a switch forwards a broadcast message to all the segments

on the switch All segments in a switched environment are therefore considered to be

in the same broadcast domain

Some switches—mainly high-end and enterprise-level switches—perform multilayer functioning For example, the Cisco 6500 and 8500 series perform some Layer 3 func-tions The Cisco Catalyst 8500, shown in Figure 2-27, is a Layer 3-enhanced ATM switch that seamlessly integrates wire-speed Layer 3 switching and ATM switching The Catalyst 8500 family delivers campus and MAN solutions with scalable performance, lower cost of ownership, and intranet-based application features to deliver increased business productivity Unlike old first- or second-generation ATM switches, which force customers to have a costly, inefficient, multisystem solution, the Catalyst 8500 switch provides an integrated ATM and Gigabit Ethernet solution in a single chassis

Routers

Arouter, as shown in Figure 2-28, is a type of internetworking device that passes data packets between networks based on Layer 3 addresses A router can make decisions regarding the best path for delivery of data on the network

Working at Layer 3 allows the router to make decisions based on network addresses instead of individual Layer 2 MAC addresses Routers also can connect different Layer 2 technologies, such as Ethernet, Token Ring, and Fiber Distributed Data Interface (FDDI) Routers also commonly connect Asynchronous Transfer Mode (ATM) and serial con-nections However, because of their capability to route packets based on Layer 3 infor-mation, routers have become the backbone of the Internet and run the IP protocol

Figure 2-27 Cisco Catalyst 8500 Switch

Figure 2-28 Router

The purpose of a router is to examine incoming packets (Layer 3 data), choose the best

path for them through the network, and then switch them to the proper outgoing port

Routers are the most important traffic-regulating devices on large networks Routers

let virtually any type of computer communicate with any other computer anywhere in

the world

Voice, DSL, Cable Modem, and Optical Devices

Recent networking demands of voice and data network integration and fast data transmission for end users and network backbones have resulted in the development

of the following new networking devices:

■ Voice gateways for handling converged packetized voice and data traffic

■ DSLAMs used at the service provider’s central office for concentrating DSL modem connections from hundreds of homes

■ Cable Modem Termination System (CMTS) used at a cable operator’s headend or central location to concentrate connections from many cable modem subscribers

■ Optical platforms for sending and receiving data over fiber-optic cable, providing high-speed connection

Voice Gateway

A gateway is a special-purpose device that converts information from one protocol

stack to another The Cisco AS5400 Series Universal Access Server provides cost-effective platforms that combine routing, remote access, voice gateway, firewall, and digital modem functionality Figure 2-29 shows a Cisco AS5400 Series Universal Gate-way, which offers universal port data, voice, wireless, and fax services on any port at any time

Figure 2-29 Cisco AS5400 Series Universal Gateway

DSLAM

A digital subscriber line access multiplexer (DSLAM) is a device used in a variety of DSL technologies A DSLAM serves as the interface point between a number of sub-scriber premises and the carrier network Figure 2-30 shows a Cisco 6100 Series Advanced DSL Access Multiplexer

Figure 2-30 Cisco DSLAM 6100

CMTS

Cable operators use a Cable Modem Termination System (CMTS) at various

concen-tration points or hubs in the cable network to provide high-speed Internet access, voice,

and other networking services to home and business subscribers The Cisco uBR7100

(Universal Broadband Router) CMTS series is designed for MTUs (multitenant units)

such as apartment buildings and hotels High-capacity models such as the uBR10012

series, shown in Figure 2-31, can handle thousands of subscribers

Figure 2-31 Cisco CMTS uBR10012

Optical Platforms Several optical platforms are available for optical networking, which is primarily a backbone, wide-area technology Figure 2-32 shows a Cisco ONS 15454 dense wave-length division multiplexing (DWDM) optical network system (ONS) The Cisco ONS

15454 provides the functions of multiple network elements in a single platform