Switches
Switches are smarter
than hubs and offer more dedicated bandwidth to users or groups of
users. A switch forwards data packets only to the appropriate port for
the intended recipient, based on information in each packet header. To
insulate the transmission from the other ports, the switch establishes
a temporary connection between the source and destination, and then
terminates the connection when the conversation is done.
A switch is analogous
to a phone system with private lines in place of the hub "party line."
Jane Tipton at the Berkley Hotel calls Bill Johnson in another room,
and the operator or phone switch connects the two of them on a
dedicated line. This setup allows more conversations at any one time,
so more guests can communicate.www.tartoos.com
Routers
Compared to hubs and
switches, routers are smarter still. Routers use a more complete
packet "address" to determine which router or workstation should
receive each packet next. Based on a network road map called a
"routing table," routers can help ensure that packets are traveling
the most efficient paths to their destinations. If a link between two
routers fails, the sending router can determine an alternate route to
keep traffic moving.
Routers also provide
links between networks that speak different languages—or, in computer
speak, networks that use different "protocols." Examples of protocols
include Internet Protocol (IP), Internet Packet Exchange (IPX), and
AppleTalk. Routers not only connect networks in a single location or
set of buildings, they also provide interfaces—or "sockets"—for
connecting to wide-area network (WAN) services. These WAN services,
which are offered by telecommunications companies to connect
geographically dispersed networks, are explained in more detail in the
next chapter.www.tartoos.com
Case Example
To understand routing, imagine the Berkley Hotel and all the other
fellow hotels in its chain have trained their operators to be more
efficient. When guest Jane Tipton at the Berkley Hotel calls guest
Rita Brown at the Ashton Hotel, the operator at the Berkley knows the
best way to patch that call through. He sends the call to the Pembrook
operator, who passes it to the Ashton. If there's ever a problem with
the switchboard at the Pembrook, the operator at the Berkley can use
an alternate route to get the call through—for example, by routing it
to another hotel's switchboard, which, in turn, sends the call to the
Ashton.
Networking
Technologies Overviewwww.tartoos.com
Local-Area Networks:
Ethernet, Fast Ethernet, and Gigabit Ethernet
Ethernet has been around since the late 1970s and remains the leading
network technology for LANs or networks contained in buildings or on a
single site. Ethernet is based on a standard referred to as carrier
sense multiple access with collision detection (CSMA/CD). (See Token
Ring in the "Glossary" section to learn about another basic style of
network communication.)www.tartoos.com
Simply put, an Ethernet workstation can send data packets only when no
other packets are traveling on the network—when the network is
"quiet." Otherwise, it waits to transmit, as a person might wait for
another to speak during a conversation.
www.tartoos.com
If multiple stations sense an opening and start sending at the same
time, a "collision" occurs. Each station then waits a random amount of
time and tries to send its packet again. After 16 consecutive failed
attempts, the original application that sent it must start again. As
more people try to use the network, the number of collisions, errors,
and subsequent retransmits grows quickly, causing a snowball effect.
Collisions are normal occurrences, but too many can slow the network.
When more than 50 percent of the total network bandwidth is used,
collision rates begin to cause congestion. Files take longer to print,
applications take longer to open, and users are forced to wait. At 60
percent or higher, the network can slow dramatically or even shut
down.
As noted in the previous section, Ethernet bandwidth or data-carrying
capacity (also called throughput) is 10 Mbps. Fast Ethernet and
Gigabit Ethernet works the same way—through collision detection—but
provide 10 times the bandwidth, at 100 Mbps, and 100 times the
bandwidth, at 1000 Mbps, respectively.
Shared Ethernet is like a single-lane highway with a 10 Mbps speed
limit. Shared Fast Ethernet is like a much wider highway with a 100
Mbps speed limit: there is more room for cars and they can travel at
higher speeds. Switched Ethernet look likes a multilane highway with a
speed limit of 10 Mbps in each lane. Switched Fast Ethernet also would
be a multilane highway, but with a speed limit of 100 Mbps in each
lane. Switched Gigabit Ethernet would have even more lanes, with a
speed limit of 1000 Mbps in each lane.
-
Networking
Technologies Overview
-
Wireless Connectionswww.tartoos.com
-
Wireless networking had not previously been
widely adopted due to the low data rates supported and the lack of
an international standard. The Institute of Electrical and
Electronics Engineers (IEEE) is the international group that creates
such standards—both Ethernet and Fast Ethernet are IEEE ratified
standards. The IEEE has recently created a high-speed standard for
wireless networking, IEEE 802.11b, which specifies the use of Direct
Sequence Spread Spectrum (DSSS) technology. This technology has a
shared throughput of up to 11 Mbps. With the advent of the 11 Mbps
IEEE 802.11b standard, wireless networking is now a viable solution
for today's business LAN.
Remote
Access and Wide-Area Networkswww.tartoos.com
LANs accommodate
local users—people within a building or on a campus. WANs connect
users and LANs spread between various sites, whether in the same city,
across the country, or even around the world. "Remote access" refers
to a simple connection, usually dialed up over telephone lines as
needed, between an individual user or very small branch office and a
central network. Your business gains access to the Internet through
some type of remote connection. A single user can dial up ISP via
modem. Multiple users within a campus might choose to rely on a router
to connect to the ISP, who then connects the campus to the Internet.www.tartoos.com
In general, LAN
speeds are much greater than WAN and remote-access speeds. For
example, a single shared Ethernet connection runs at 10 Mbps. Today's
fastest analog modem runs at 56 kbps-less than one percent of the
speed of an Ethernet link. Even the more expensive, dedicated WAN
services such as T1 lines don't compare (with bandwidth of 1.5 Mbps, a
T1 has only 15 percent of the capacity of a single Ethernet link). For
this reason, proper network design aims to keep most traffic
local—that is, contained within one site—rather than allowing that
traffic to move across the WAN.
Analog versus Digital
Analog Lines
Using analog lines to dial out to other networks or to the Internet—or
to allow remote users to dial into your network—is a straightforward
solution. Most ordinary phone lines are analog lines. Connect a modem
to your computer and to a wall jack, and you're in business. You pay
for a connection as you would pay for a phone call—by the minute, or
at a set rate per local call (long-distance charges are the same as
for a long-distance telephone call). At present, the fastest analog
modems operate at 56 kbps for transferring data. With today's larger
file sizes and graphically sophisticated Web sites on the Internet,
you should choose modems that operate at a minimum of 33.6 kbps (also
called V.34) and that have V.42 (error correction) and V.42bis (data
compression) capabilities. Although modems offer a simple solution for
dialout connections to other LANs and the Internet, they do not scale
well as your network grows. Each modem can support only one remote
"conversation" at a time, and each device that wants to connect with
the outside world needs a modem. (Find more information in the "Small
Business Networking Examples" section about overcoming this limitation
by installing a router for wide-area communications and your Internet
link.)www.tartoos.com
"Dial-on-demand"
routing (DDR) is sometimes used as a compromise between the dialup
method of connecting and full-fledged routing. With DDR, the router
establishes (and is charged for) a connection only when the connection
is in use. This solution uses a basic router paired with either a
modem or an Integrated Services Digital Network (ISDN) line, makes the
calls as requested by the router.
Modems versus Routers
ISDNwww.tartoos.com
ISDN operates at 128
kbps and is available from your phone company. Charges for ISDN
connections usually resemble those for analog lines—you pay per call
or per minute, usually depending on distance. ISDN charges also can be
a flat rate, if they are linked to a local Centrex system.
Technically, ISDN
comprises two 64 kbps channels that work separately. Load balancing or
"bonding" of the two channels into a 128 kbps single channel is
possible when you have compatible hardware on each end of a connection
(for example, between two of your sites). What's more, as a digital
service, ISDN is not subject to the "line noise" that slows most
analog connections, and thus it offers actual throughput much closer
to its promised maximum rate.
You can make ISDN
connections with either an ISDN-ready router or an ISDN terminal
adapter (also called an ISDN modem) connected to the serial port of
your router. Again, modems are best for single users, because each
device needs its own modem, and only one "conversation" with the
outside world can happen at any one time. Your ISDN router, modem, or
terminal adapter may come with analog ports, allowing you to connect a
regular telephone, fax, modem, or other analog phone device. For
example, an ISDN router with an analog phone jack would allow you to
make phone calls and send faxes while staying connected via the other
ISDN digital channel.www.tartoos.com
Leased Lines
Phone companies offer
a variety of leased-line services, which are digital, permanent,
point-to-point communication paths that are "open" 24 hours a day,
seven days a week. Rather than paying a fee for each connection, you
pay a set amount per month for unlimited use. Most appropriate leased
lines for small- and medium-sized businesses range in speed from 56
kbps to 45 Mbps (a T3 service). Because they all work the same way,
choosing the right line for you depends on the number of users and the
amount of remote traffic the network will carry (and how much
bandwidth you can afford). A common service for businesses with
substantial WAN usage is a T1 line with 1.5 Mbps of bandwidth.
By "point-to-point,"
we mean that leased lines use a direct, physical connection from your
business or branch office to the phone company's switch, and then to
your other offices. Often, the phone or data services company may need
to install new cabling.www.tartoos.com
Cable Modem/Router
Cable modems offer
extremely fast and relatively inexpensive access to the Internet. A
cable modem connects directly to the same line that provides cable TV
service to a home or business, and then to an Ethernet NIC in a PC.
Whereas a traditional
dialup modem provides access at speeds up to 56 kilobits per second
(kbps), a cable modem can deliver transmission rates of up to 10
megabits per second (Mbps)—nearly 200 times as fast. And unlike dialup
modems, cable modems have a connection to the Internet that is "always
on." In other words, you won't face busy signals or delays while your
computer connects to the Internet.
Likewise, because you
are sharing the connection with multiple users on your cable system,
your performance will depend on how many users are on line at once.
Another caution: in today's cable modem systems, users on a single
segment of cable are essentially connected to a single local-area
network (LAN). To prevent neighbors from accessing files on a
computer, the file sharing options of that computer should be turned
off.
Ready to sign up? You
might need to be patient. About 65 million locations in the United
States alone have cable service. But only a small percentage of those
links are cable modem-ready—the rest will require upgrades to handle
the two-way traffic of Internet surfing.
Upsides
-
Transmission rates
for downloading Web pages and files are extremely fast (uploading to
the Internet—mostly mouse clicks and e-mail—occurs at a slower but
still quick speed of up to about 2 Mbps).
-
Inexpensive
service—Service costs about $50 per month today, or close to the
cost of basic cable TV.
-
Minimal
requirements—All you need to make the connection is a relatively new
PC, a cable modem (often leased from the cable company), an Ethernet
card in the PC, and a connection point from your cable service
provider.
www.tartoos.com
Downsides
-
Limited
availability—At this writing, only about 10 percent of the United
States has access to the upgraded cable systems needed to support
cable modems. The experts say it will be about five years before the
technology is widespread.
-
Extra installation
steps—Gaining Internet access today can be as simple as installing a
modem and browser software and calling your Internet service
provider's (ISP's) access number. With a cable modem, the cable
company most likely will need to send out a technician to check your
wiring quality and install the device. You also will need to have an
Ethernet card installed in the PC, if it doesn't already have one.
www.tartoos.com
|
ãÏÑÓÉ
ÇáßãÈíæÊÑ 