eth tutor

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n the previous

two parts of this

article series, I cov-

ered various aspects ofEthernet. I discussed LANs as well as

100- and 1000-Mbps Ethernet. Ill con-

clude this series by examining the

many different hardware components

used to construct a LAN. The focus of

these articles has been the underlying

hardware in an Ethernet system. A

complete treatment must also include

the software required for proper net-

work communication, such as the

NetBEUI and TCP/IP protocols, as

well as network applications. Visit

my web site (www.sunybroome.edu/

~antonakos_j) and look for

Telecommunications III for more

information.

THE NICThe Network Interface Card (NIC)

is the interface between the PC (or

other networked device) and the phys-

ical network connection. In Ethernet

systems, the NIC connects to a seg-

www.circuitcellar.com/online CIRCUIT CELLAR ONLINE April 2002

1

TechnicallySpeaking

????

ment of coaxial or UTP cable (fiber

NICs are available but not common

yet). The NIC is responsible for opera-

tions that take place in the physicallayer of the OSI network model. It is

only concerned with sending and

receiving zeros and ones, using the

IEEE 802.3 Ethernet standard (or IEEE

802.5 token ring). Windows identifies

the installed NIC in network proper-

ties.

To use a protocol with a NIC you

must bind the protocol to the adapter

card. This is typically done automati-

cally when the protocol is added. The

driver type of the NIC may beMicrosofts Network Driver Interface

Specification (NDIS), which allows

multiple protocols to use a single NIC.

An Open Data-Link Interface (ODI)

driver, developed by Novell, performs

the same function for multiple proto-

col stacks used with the NetWare net-

work operating system. Figure 1

shows the NDIS/ODI interface. Both

are designed to decouple the protocols

from the NIC.

The protocols do not require any

specific information about the NIC.They use the NDIS/ODI drivers to

perform network operations with the

drivers responsible for their specific

hardware. It is important to mention

that all NICs are manufactured with a

unique 48-bit MAC address (for exam-

ple, 00-60-97-2B-E6-0F). You can view

your NICs MAC address using the

Windows WINIPCFG utility.

James Antonakos

Ethernet Technology

Part 3Network Building Blocks

i

Application

Protocol stack

NDIS/ODI

Driver

NIC

TCP/IPNetBEUI

NDIS

TCP/IP

IPX

ODIInterface

Figure 1NDIS and ODI network driver interfaces.

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2/72April 2002 CIRCUIT CELLAR

ONLINE www.circuitcellar.com/online

REPEATERS AND TRANSCEIVERS

A repeater connects two network

segments and broadcasts packets

between them. Because signal loss is a

factor in the maximum length of a

segment, a repeater is used to amplify

the signal and extend the usable

length. A common Ethernet rule is

that no more than four repeaters may

be used to join segments together. This

is a physical limitation designed to

keep collision detection working prop-

erly. Repeaters operate at layer one

(i.e., physical layer) of the OSI model.

A transceiver converts from one

media type to another. For example, a

UTP-to-fiber transceiver acts like a

repeater, except it also interfaces UTP

cable with a fiber optic cable. It is

common to use more than one media

type in an installation; many different

kinds of transceivers are available.

HUBSHubs, also called concentrators,

expand one Ethernet connection into

many. For example, a four-port hub

connects up to four machines (or other

network devices) via UTP cables. The

hub provides a star connection for the

four ports. Many hubs contain a single

BNC connector as well to connect the

hub to existing 10Base-2 network

wiring. The hub also can be connected

via one of its ports. One port isdesigned to operate in either Straight-

Through or Crossover mode, selected

by a switch on the hub.

A hub is similar to a repeater, except

it broadcasts data received by any port

to all other ports on the hub. Most

hubs contain a small amount of intelli-

gence as well, examining received

packets and checking them for integri-

ty. If a bad packet arrives or the hub

determines that a port is unreliable, it

will shut down the line until the error

condition disappears. Because of its

slight delay when processing a packet,

the number of hubs that may be con-

nected in series is also limited. Figure

2 shows how several hubs are used to

connect five Ethernet segments within

the accepted limits. Because each UTP

cable may be as long as 100 m, the

maximum distance between nodes is500 m (the network diameter).

BRIDGES AND SWITCHESWhen a network grows in size, it is

often necessary to partition it into

smaller groups of nodes to help isolate

traffic and improve performance. One

way to do this is to use a bridge,

whose operation is indicated in Figure

3. The bridge keeps segment A traffic

on the A side, and segment B traffic on

the B side. Packets from segment A

that are meant for a node in segment Bwill cross the bridge (the bridge will

permit the packet to cross). The same

is true for packets going from B to A.

The bridge learns which packets

should cross as it is used.

A switch is similar to a bridge, but

with some important enhancements.

First, a switch may have multiple

ports, thus directing packets to several

different segments, and further parti-

tioning and isolating network traffic

in a way similar to a router. Figure 4

shows an eight-port N-way switch,which can route packets from any

input to any output.

Some or all of an incoming packet is

examined to make the routing deci-

sion, depending on the switching

method that is used. One common

method is called store and forward,

which stores the received packet

before examining it to check for errors

before retransmitting. Bad packets are

not forwarded. In addition, a switch

typically has auto sensing 10/100-

Mbps ports and will adjust the speed

of each port accordingly. Furthermore,

a managed switch supports SNMP for

further control over network traffic.

Switches operate at layer two (i.e.,

data link) of the OSI model.

ROUTERSA router is the basic building block

of the Internet. Each router connects

two or more networks together by pro-

viding an interface for each network

to which it is connected. The router

examines each packet of information

to determine whether the packet must

be translated from one network to

another, performing a function similar

to a bridge. Unlike a bridge, a router

can connect networks that use differ-

ent technologies, addressing methods,media types, frame formats, and

speeds.

A router is a special-purpose device

designed to interconnect networks.

For example, three different networks

can be connected using two routers, as

illustrated in Figure 5.

If a computer in network A needs to

send a packet of information to net-

work C, both routers pass the packets

from the source network to the desti-

nation network. Routers maintain

routing tables in their memories tostore information about the physical

connections on the network. The

router examines each packet of data,

checks the routing table, and then for-

wards the packet if necessary. Every

other router in the path (between a

source and a destination network) per-

Segment 3 Segment 4

Segment 2

Segment 1

Segment 5

Figure 2In accordance with the five/four/three rule(five segments, four repeaters, three segments withnodes), five Ethernet segments are connected by fourhubs.

Networkdevices

Packetenters

here

Packetexit

here

1

2

3 4

5

6

78

Figure 4An 8-port switch showing how network trafficis forwarded between ports.

A B

Local packet

does notcross bridge.

This packet crosses

bridge to locatedestination node.Bridge

This is alsoa local packet.

Segment A

Segment B

Figure 3The bridge selectively allows packets tocross from one network segment to another.

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forms a similar procedure. Note that a

router does not maintain any state

information about the packets; it sim-

ply moves them along the network.

Routers operate at layer three (i.e.,

network) of the OSI model.

HUBS VERSUS SWITCHESThe essential difference between

hubs and switches is that hubs broad-

cast frames received on one port to all

other ports, while switches forward a

received frame to a specific port. This

is illustrated in Figure 6, where a

small network of six stations (AF) is

connected two different ways.

In Figure 6a, station A transmits a

frame with a destination of station F.

This does not matter to the four-port

hubs, which simply broadcast copies

of the frame from station A to the

other five stations. This amounts to agood deal of wasted bandwidth.

Furthermore, all six stations operate

in the same collision domain, making

them compete for bandwidth. Figure

6b shows the same network with the

four-port hubs replaced by four-port

switches. A frame transmitted from

station A with a destination of station

F is forwarded between the switches

and sent directly to station F on port

4. Stations B, C, D, and E do not

receive copies of the frame, as they do

in Figure 6a. Thus, network traffic hasbeen reduced.

The switches also partition the net-

work into six separate collision

domains. Each station now has unre-

stricted access to its own dedicated

bandwidth (the speed of the switch

port). The switch is capable of specific

forwarding because it learns what

MAC addresses are associated with

each port. Recall that every Ethernet

frame contains a source MAC address

and a destination MAC address. When

a frame is received by a port on aswitch, the switch will save a

copy of the source MAC address

and its associated port number in

a special internal look-up table.

Although we are storing the

source MAC address, it is also a

destination MAC address to every

other station in the network.

When a frame requires forward-

ing, the switch examines the des-


3

C without affecting the F-to-D trans-

mission.

INSIDE A SWITCHIf you wanted to start your own net-

working company and begin designing

and manufacturing switches, where

would you begin? Lets examine the

block diagram of a simple switch,

shown in Figure 8.

What would be required of each

component in the switch? Well, the

input port logic contains the Ethernet

receiving logic and buffers for received

frames. Buffering received frames low-ers the rate of collisions and allows

the switching fabric to be busy for

short periods of time without losing

data. If the frame buffer fills up, any

new frame received by the port will

trigger a collision. The random delay

of CSMA/CD will then give the

switch time to empty a portion of the

input buffer before the station

attempts retransmission.

The second component, the output

port logic, contains an Ethernet trans-

mitter and output frame buffer. Again,the buffer allows the switch fabric to

service multiple output ports on a

demand basis. For example, several

frames may arrive simultaneously,

with each frame directed to the same

output port. The buffer is required to

prevent the switching fabric from

stalling. In addition, the input and

output frame buffers allow different

speeds between ports (e.g., port 1 oper-

ating at 10 Mbps and port 3 operating

at 100 Mbps). The buffers may be

filled at one speed and emptied atanother speed.

The switching fabric is respon-

sible for directing the received

frames from each input port to

the appropriate output port. In

addition, the switching fabric

must be able to handle a broad-

cast to all output ports. In gener-

al, there are two ways to build

the switching fabric: crossbar

tination MAC address stored in the

frame and looks for it in the look-

up table. If the destination MAC

address is found in the table, the

frame is forwarded to the associated

port. If the destination MAC

address is not found, the frame is

broadcast to all ports. Eventually,

the destination station will most

likely respond with its own frame,

and its port will be identified.

Further broadcasts for that station

will not be required.

Figure 7 shows the results

obtained when a hub and switch

are used together. Stations A, B, and

C are in one collision domain (com-

peting for bandwidth).

Stations D, E, and F are in their

own collision domains. Each has

full access to the available band-

width. In Figure 7a, station A trans-mits a frame destined for station F.

The frame is broadcast by the hub

and forwarded by the switch.

Stations B and C must contend

with the broadcast frame, waiting

their turn for access. Neither sta-

tion is allowed to transmit while

the hub is broadcasting or a colli-

sion will result. Even though sta-

tion A is sending a frame to station

F, stations B and C are affected.

Figure 7b shows station A sending a

frame to station C. The hub stillbroadcasts the frame, which affects

station B, but not stations D, E, or F.

In Figure 7c, station F sends a

frame to station A. The frame is

forwarded by the switch and broad-

cast by the hub. Stations B and C

are affected by the frame of station

F; however, stations D and E are

not affected. Figure 7d shows sta-

tion F sending a frame to station D.

Station E is unaffected and may

transmit a frame to stations A, B, or

Network A Network B Network C

Routers

Figure 5Two routers used to connect three networks.

Figure 6aThe hubs broadcast received packets to all ports. bThe switches forward received packets to specific ports.

1 2 3 4

1 2 3 4A ACB

D AFE

4-Port hub

4-Port hub

Collision domain

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switch or high-speed multiplexed bus.

Both methods are shown in Figure 9.The crossbar switch in Figure 9a is a

two-dimensional set of data buses. Any

combination of input-to-output con-

nections is possible, even broadcasting.

Each intersection of input wires and

output wires in the crossbar switch

contains an electronic switch that is

either open or closed. A small amount

of control information is required to

configure the crossbar switch. Changing

the control information changes the

input-to-output connections.

The multiplexed bus in Figure 9beffectively makes one input-output

connection at a time, with each input

port getting its turn at using the bus.

When many signals are multiplexed in

this fashion, the data rate on the mul-

tiplexed bus must be much faster than

the individual speeds of each port. For

example, on a four-port switch with

each port running at 100 Mbps, the

multiplexed bus would need to oper-

ate at 400 Mbps. An eight-port switch

would require an 800 Mbps bus. The

speed requirement of this techniquemakes it unsuitable for switching at

high speeds. However, this problem is

overcome by the parallel nature of the

crossbar switch.

The control logic is the fourth com-

ponent. It must perform several

chores, including: update and search

the MAC address table; configure the

switching fabric; and maintain proper

flow control through the switch fabric.

Recall that the switch learns which

ports are associated with specific sta-tions by storing copies of the source

MAC address from each received

frame. The MAC address and port

number are stored in a special high-

speed memory called Content

Addressable Memory (CAM). The

hardware architecture of the CAM

allows its internal memory to be

quickly searched for a desired data

value, such as a 48-bit MAC address.

Figure 10 shows a simple example of a

CAM being searched for the MAC

address 00-C0-F0-3C-88-17.It is important to note that all of the

MAC addresses stored in the CAM are

compared to the input value simulta-

neously. For example, the MUSIC

LANCAM MU9C1480 from Music

Semiconductors stores 1024 64-bit

entries and performs comparisons in

70 ns. The control logic uses the look-

up table results from the CAM to con-

figure the switching fabric. In the

event that an output port becomes

unavailable because of congestion or

some other problem, a flow controlmechanism will prevent access to the

port until it becomes available again.

SWITCHING METHODSInitially, switches handled frames

using a technique called store-and-for-

ward. Using this technique, the entire

frame is stored as it is received. If the

FCS is valid, the destination MAC

address is used to select an output

port, and the frame is forwarded to the

appropriate output port via the switch-

ing fabric. Because the entire frame is

stored before any decisions are made,

there is a delay (or latency) between

the time the frame is received and the

time it begins transmission on the

appropriate output port. The latency

varies depending on the length of the

frame. The minimum latency is

obtained with a minimum size frame.

For 10 Mbps Ethernet, the minimum

latency is 57.6 s (576-bit times at 100

ns per bit, including the preamble).

Maximum length frames have a laten-

cy of just greater than 1.2 ms. Some

applications, such as streaming audio

and video, are sensitive to latency.

A second technique of switching is

cut-through switching. This method

reduces the latency of a switch

tremendously. As soon as the destina-tion MAC address of an incoming

frame is received, the forwarding

process can begin (assuming there is a

free output port and the switching fab-

ric is available). This reduces the

latency to just 11.2 s plus any addi-

tional time for internal switch opera-

tions. In addition, the latency of the

cut-through method is fixed, because

forwarding can always begin as soon

as the destination MAC address is

received. Unfortunately, errors can be

propagated using the cut-throughmethod, because there is no way to

know if a frame being forwarded is

good until it has been completely

received. Cut-through switches will

revert to the store-and-forward

method when multiple errors occur

while using the cut-through method.

Control logic MAC storage

1

2

3

4

1

2

3

4

Switchingfabric

Input

ports

Outputports

Figure 8A four-port switch contains the followingcomponents: input port logic, output port logic, switch-ing fabric, and control logic.

Figure 7Hubs and switches can be mixed in a network. You can follow the action: first, station A sends a frameto station F (a); second, station A sends a frame to station C (b); third, station F sends a frame to station A (c);and fourth, station F sends a frame to station D (d).

1 2 3 4

1 2 3 4

A ACB

D AFE

4-Port switch

4-Port hub

Collision domains

1 2 3 4

1 2 3 4

A ACB

D AFE

4-Port switch

4-Port hub

1 2 3 4

1 2 3 4

A ACB

D AFE

4-Port switch

1 2 3 4

1 2 3 4

A ACB

D AFE

4-Port switch

4-Port hub 4-Port hub

a) b)

d)c)

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at the network layer in the TCP/IP

and OSI protocol stacks. Using

routers, various types of networks are

connected together to form one logical

network. The Internet is an example

of a logical network. On the Internet,

the routing protocols are based on the

Internet Protocol and use IP addresses.

Each router must follow some ground

rules to allow it to process network

layer data.

In general, communicating on a

LAN just like any other station. For

example, on an Ethernet network, a

router communicates using

CSMA/CD and monitors the media

for the MAC address and any broad-

cast messages. The first ground rule

for the router is that it must maintain

tables with routing information for all

reachable networks. The second rule

is to forward or block traffic based onthe destination network address. The

fourth rule is to drop all frames to

unknown destinations. The fifth rule

requires the router to block all broad-

cast messages between networks. And,

the sixth ground rule is to perform

CRC checks on each transferred packet.

Using a router, messages are passed

from one device (host computer or

router) to another until the message

eventually reaches the destination.

Figure 11 shows a typical network

connected to the Internet through arouter. Any traffic exchanged between

any of the nodes on the LAN can be

delivered directly without a router. All

traffic that is destined for the Internet

must be passed on to the router. A

Windows application program called

NETSTAT is used to show the routes

that are currently active on a PC run-

ning the Windows operating system

(see Listing 1).

To deliver a message to a

remote network, the message

must be transmitted from thesource host to a local router

(sometimes called the default

gateway). In the NETSTAT dis-

play, the default gateway has

the address 24.24.78.1. Do any

of the other addresses look

familiar, such as the loop-back

address or the network masks?

After the data is sent to the

default gateway router, it is

packets between networks in the mostefficient manner.

Choosing which type of routing pro-

tocols to use is complicated. An organi-

zation bases its choices on the answers

to (at least) the following questions.

What is the size and complexity of the

network? What types of physical net-

works must be connected? Which serv-

ice provider will handle the network

data? What are the network traffic lev-

els? What are the security needs? What

level of reliability is required? What are

the organizational policies within theorganization? How does the organiza-

tion implement changes? What type of

hardware and software support from

the manufacturer is required? How

long will it take to repair or replace the

equipment if it fails?

Routing protocols perform a differ-

ent type of switching than discussed

previously. Routing protocols operate


5

SWITCHES VERSUS ROUTERSSwitches are considered layer-twodevices, using MAC addresses to for-

ward frames to their proper destina-

tion. Routers, layer-three devices, are

much more complex, using micro-

processor-based circuitry to route

packets between networks based on

their IP address. Routers provide the

following services: route discovery;

selection of the best route to a desti-

nation; adaptation to changes in the

network; translation from one tech-

nology to another, such as Ethernet totoken ring; packet filtering based on IP

address, protocol, or UDP/TCP port

number; and connection to a WAN.

Because of the additional processing

required for each packet, a router has a

higher latency than a switch. In addi-

tion, a router requires an initial set-up

sequence, in which the ports are pro-

grammed and certain protocols and

characteristics are enabled or

disabled. A switch may be

simply plugged into the net-

work, automatically learninghow to forward frames as the

network is used. Note that

some protocols (e.g.,

NetBEUI) cant be routed;

instead, they will pass

through a switch. Finally,

switches are used within net-

works to forward local traffic

intelligently. Routers are used

between networks to route

Figure 9Switching fabrics. (a) crossbar switch, (b) multiplexed bus.

1

2

3

4

1 2 3 4

In

Out

2 Connected to 3

4 Connected to 1

1

2

3

4

1 2 3 4

In

Out

1 Connected to 3

2 Connected to 13 Connected to 4

4 Connected to 2

1

2

3

4

1 2 3 4

In

Out

2 Broadcasting to

1, 3, and 4

1

2

3

4

1

2

3

4

MUX DEMUXIn Out Data bus Port 1 Port 2 Port 3 Port 4 Port 5

Internal frametransfer time

Less than the time ofa minimun frame

a)

b)

Figure 10Operation of content addressable memory.

00-C0-F0-27-64-E2

00-20-78-C6-78-14

00-C0-F0-3C-88-17

00-C0-F0-26-FC-83

3

1

4

1

Destination MAC addressfrom received frame

00-C0-F0-3C-88-17

All address are

compared with theinput address at

the same time.4 Output

1 Address match

1-Match0-Not found

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passed on to another router or to the

host computer on the destination

LAN. Each router implements the

routing process by forwarding mes-

sages (one hop at a time) toward their

final destination using information

stored in a routing table. The routing

table contains an entry that indicates

the best path (or interface) for sending

data to its destination.

The routing table can be created and

maintained using two different meth-ods: static or dynamic. In a static

router, a number of predefined routes

are created and the router lacks the

ability to discover new routes. In a

router with statically configured rout-

ing tables, the network administrator

needs a detailed knowledge of the net-

work topology and must take the time

to manually build and update the

routing table as conditions change.

This involves programming all of the

routes into the router memory. Static

routers can work well for small net-works, but do not work well in large

or dynamically changing networks

because of the manual effort required.

In addition, static routers are not

fault-tolerant. The lifetime of a manu-

ally configured static route is infinite.

Therefore, statically configured

routers do not recover from a bad link

or a malfunctioning router.

In contrast, using dynamic routing,

new routes can be discovered or old

routes updated as required. Dynamic

routing consists of maintaining rout-ing tables automatically using either

periodic or on-demand messages

through an ongoing communication

between routers using the routing pro-

tocols. Except for their initial configu-

ration, dynamic routers require little

ongoing maintenance and are fault-tol-

erant. Dynamic routes learned from

other routers have a finite lifetime. If

a router or link goes down, the routers

sense the change in the network

topology through the expiration of the

lifetime of the learned route in the

routing table. This change then can be

propagated to other routers so that all

of the routers on the network realize

the new network topology. The router

chooses the best path to send the data

by implementing a distance-vector or

link-state algorithm. In the distance-

vector algorithm, each router in the

network contains a partial view of the

complete network topology. In the

link-state algorithm, each router is

aware of the entire network.

LAYER-THREE SWITCHINGA layer-three switch is essentially a

switch and a router combined into one

package. Layer-three switching has

become popular as a result of the ever-

increasing demand for bandwidth andservices. Traditional routers have

become bottlenecks in the campus

and corporate LAN environments

because of their microprocessor-based

operation and high latency. Layer

three switches use ASIC technology to

implement the routing functions in

hardware. This enables the layer-three

switch to perform router duties while

forwarding frames significantly faster

than an ordinary router. In fact, layer-

three switches are capable of forward-

ing millions of frames per second,compared to only several hundred

thousand for a router.

Replacing the campus or corporate

routers with layer-three switches, or

adding layer-three switching to a

router-less network, has many bene-

fits. First of all, layer-three switches

are less expensive than routers. There

are fewer network components to man-

age (via SNMP). You get faster forward-

ing (close to wire speed, the speed of

the frames on the wire). Using switches

on layer three helps provide quality of

service (QoS) to the LAN environment.

Theyre compatible with existing rout-

ing protocols (RIP, OSPF). And, theyre

easier to configure than a router.

There is a great deal of information

about layer-three switching availableon the Internet. You should read one

of 3Coms write papers on the subject

(www.3com.com). Figure 12 summa-

rizes the layer-based networking com-

ponents we have examined, including

the layer-three switch. Lets see how

these hardware components and proto-

cols work together in an ISP.

INSIDE AN ISPFigure 13 shows an overhead view

of the networking and telecommuni-

cations room at a small InternetService Provider. Along the east wall

are the incoming phone lines (200

pairs), modem bank (groups of 64 56-

Kbps modems in a rack-mountable

case), and the 44.7-Mbps T3 connec-

tion (to a higher-level ISP). The

Listing 1Running the NETSTAT program with ther option produces the output. As you can see, NETSTATshows the routing table and active connections for the computer.

C:\WINDOWS>netstat -rRoute TableActive Routes:Network Address Netmask Gateway Address Interface Metric0.0.0.0 0.0.0.0 24.24.78.1 24.24.78.84 124.24.78.0 255.255.255.0 24.24.78.84 24.24.78.84 124.24.78.84 255.255.255.255 127.0.0.1 127.0.0.1 124.255.255.255 255.255.255.255 24.24.78.84 24.24.78.84 1127.0.0.0 255.0.0.0 127.0.0.1 127.0.0.1 1224.0.0.0 224.0.0.0 24.24.78.84 24.24.78.84 1255.255.255.255 255.255.255.255 24.24.78.84 24.24.78.84 1Active ConnectionsProto Local Address Foreign Address StateTCP server:1025 sbccab.cc.sunybroome.edu:139 ESTABLISHEDTCP server:4424 ftp-eng.cisco.com:ftp CLOSE_WAITTCP server:4970 mail3-1.nyroc.rr.com:pop-3 TIME_WAITTCP server:4981 sunc.scit.wlv.ac.uk:80 CLOSE_WAIT

Figure 11A router connecting a LAN to the Internet

Internet

Router

Host

computer #1Host

computer #2

Hostcomputer #3

24.24.78.14 24.24.78.84 24.24.78.39

24.24.78.1

Figure 12Hierarchy of hubs, switches, and routers.

Network

Data-link

Physical

Layer 3 Router

Bridge, switch

Repeater, hub

Layer 3switch

Layer 2

Layer 1

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7

routers and switches that make up the

ISP topology and logical networks are

along the west wall. The center of the

room contains the server farm, where

all of the servers required for opera-

tion of the ISP reside. These include

servers for DNS, DHCP, e-mail, web

pages, and authentication. One

machine is dedicated to monitoring

the network via SNMP and another

for performing backups. Along the

south wall is server space for individ-

ual and corporate servers, which,

along with the dial-up users, help gen-

erate income for the ISP. Numerous

Uninterruptable Power Source (UPS)

units provide 30 min. of power in the

event of a main power loss.

Figure 14 illustrates the actual lay-

out of the network. The T3 connec-

tion is the WAN connection to the

higher-level ISP providing the actualInternet connection. Traffic in the T3

connection is filtered by the firewall.

The I-router connects the individual

subnetworks together and acts as the

gateway to the Internet through the

firewall. Employee computers (some of

which have 100-Mbps switched serv-

ice) communicate with their own file

server or may tap into the server farm

via the I-router. Individual and corpo-

rate servers share their own switch, as

do the modems in the modem bank

and the servers in the server farm. TheF-router is used to lighten the load on

the I-router for traffic moving between

the server farm and the modem bank.

The network was designed in this

fashion to allow the subnetworks to

keep operating in the event that the

main I-router goes down. Dial-up

users can still check e-mail or workon their web pages. Employees can

continue to work as well, although

without access to the Internet or the

server farm.

BIGGER, FASTER, MOREThe Internet continues to grow.

New computers and other networked

devices are added every day, increasing

the demand for bandwidth and reliable

communication. These demands are

being met by new technologies, such

as 10-Gbps Ethernet and dense-wave-division multiplexing, with more to

come. What a long way weve come

since the use of acoustic-coupled 300-

bps modems! And where are we going?

The end is not in sight, but we are

moving there faster and faster each

day, with Ethernet leading the way. I

RESOURCE

3Com Corp., Title of White Paper

Needed, Need Number if

Available, Need Date ofPublication.

SOURCE

MUSIC LANCAM MU9C1480

Music Semiconductors

(732) 469-1886

www.music-ic.com

Figure 14Network diagram for the small ISP

Telephone lines

200 POTS

connections

64

64

64

64

Modem bank

F Router

100-Mbps

switch

100-Mbpsswitch

Authentication

DHCP

Backup

DNS E-mail Web

T3 Interface

I Router

100-Mbps

switch

Company

file server

10/100-Mbpsswitch

Server farm

T3 Cable

Employee stations

10/100-Mbpsswitch

Individual/coraporateservers

Firewall

Figure 13Communication room layout in a small ISP.

Air

conditionerAir

conditioner

Router

and

switch

rack

Tools

and test

equipment

Individual/corporate serversModern

bank

T3

Dial-up

phone lines

Status

monitorDNS

Authentication DHCP

BackupCompany

file server

E-mailE-mail

Web Web

Server farm

DoorWindow

T3

MUX

CSU DSU

James Antonakos is a professor in the

Department of Electrical Engineering

Technology at Broome Community

College, with over 25 years of experi-

ence designing digital and analog cir-

cuitry and developing software. He isalso the author of numerous text-

books on microprocessors, program-

ming, and microcomputer systems.

You may reach him at antonakos_j

@sunybroome.edu or visit his web

site at www.sunybroome.edu/~anton-

akos_j.

Circuit Cellar, the Magazine for Computer

Applications. Reprinted by permission.

For subscription information,

call (860) 875-2199, or www.circuitcellar.com.

Entire contents copyright 2001 Circuit Cellar

Inc. All rights reserved.

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