Although, not applicable for all situations, if your organizations are with-in the same city, you might be able to set up a “campus” type system using sin-gle-line Digital Subscriber Lin
Trang 1Organizations with seve ral offices
located in the same city usually
want to link them into a single secure
n e t work Although, not applicable for all
situations, if your organizations are
with-in the same city, you might be able to
set up a “campus” type system using
sin-gle-line Digital Subscriber Line (SDSL )
modems connected by a single pair of
phone wires
This pair of wires, re f e r red to by
various names by different phone
com-panies, is most often called a “dry pair.”
This is an unconditioned telephone wire
c i rcuit from one location to another
without telephone equipment
connect-ed to it These circuits are often much
less expensive than other alternative s
offered by the phone company, and can
a c h i e ve synchronous speeds of 2
Megabits per second
(Mbps) in each direction within dis-tances not exceeding two miles
Connection speed begins to drop after distances exceeding two miles, but respectable speeds can be achieved at distances of four to five miles
SDSL modems are bridging devices used to connect to remote segments of your Local Area Network (LAN) These segments can be in different buildings
or separated by many floors in a build-ing The connecting pair of wires can
be user-owned as in a campus LAN or
p rovided by the phone company using its wiring circuits Traffic is synchro n o u s , meaning the same speed in both dire c-tions, which is an important considera-tion if you are sharing databases or files
or want to backup data over the link
C o n f i g u ration from point to point looks like the following:
Office A Hub <——> SDSL modem <——> dry pair
wiring <——> SDSL modem <——> Office B Hub
To give an example of their use and how easy or hard it is to connect offices with this setup, we will take you thro u g h
an actual installation we performed The objective was to link five offices, each with its own LAN into a single network where various facilities
a re located any w h e re from 4,000 to 13,000 feet line distance (Line distance
is the length of the wire between the
t wo end points and will have to go
t h rough a telephone wire center or switch office unless you are running your own direct wire.)
The Wide Area Network (WA N ) was constructed to share netwo r k
re s o u rces and to minimize the number
of Internet Service Provider (ISP ) accounts and the need to secure each access point (including emergency ser-vice providers), which is best done using
h a rd - w i red land line communications using a single broadband access point that is shared by all offices and secured with a firewall and antivirus system We looked at seve ral options, including a virtual private network (VPN) and wire-less networking, before deciding to use SDSL modem bridging
Although 80 2 11b wireless net-working was feasible and could provide
11 Mbps basic Ethernet performance,
80 2 11b is expensive, with pre l i m i n a r y
Create a WAN Using
SDSL Modems
These bridging devices can connect to remote
segments of your LAN.
By Joseph L Kashi and Thomas Boedeker
Bridging the Gap: The 300S modem was plugged in at each end point and connected phone
cords to wall jacks on each end for this installation.
Trang 2ments than using routers Some ro u t e r s offer encryption and can be used ove r
a dry pair link It’s also possible to use encryption techniques in a bridging setup such as the one that we described
h e re, but an extra level of hard wa re encryption complicates the installation and was not a significant concern When the remote segments we re added, our network branches all came together as a single network The SDSL modems are tra n s p a rent to various net-works and protocols The ability to con-nect to servers and other re s o u rces is simply based on the network opera t i n g system softwa re clients and pro t o c o l s that need to be installed on your re s p e c-tive servers and workstations regardless
of how the workstations physically con-nect to the network file servers All of the links under 13,000 feet line length operate at near 2 Mbps
N e t work packet collisions cause some reduction when multiple computers are trying to use the link simultaneously, but with small remote networks of four
We installed four dry pairs in the main office, one from each of the four remote locations, and then had the phone
com-p a ny install four regular com-phone jacks in our central office, one for each end of
e very dry pair link Then, we simply plugged in a 300S modem at each end point and connected the phone cord s
to the wall jacks on each end Wi t h i n about 30 seconds, the modem indica-tor lights showed good circuits We then connected the SDSL modem to our net-work using Ethernet cables
Initially, we had some internal net-work problems until we determined that
we needed to use an uplink port on our hub or switch or use a cro s s over cable if connecting the modems to a standard Ethernet port Once the modems we re
p roperly connected to our main net-work, individual segments we re able to see each other No other configura t i o n was re q u i red Although you give up some flexibility, security and contro l , simple WAN bridges are an easier method of connecting network
seg-estimates at about $25,000, and it’s
also potentially less secure VPN wa s
also possible, but in our local market,
a vailable VPN options we re not
syn-c h ronous with an $8,000 estimated
setup charge VPN also had a re c u r r i n g
monthly fee of more than $700 per
month for a mere 328 Kbps (kilobits
per second) bandwidth VPN also left no
option for connecting to another office
location located at a line distance of
24,000 feet because our local provider
planned to use an asymmetric Digital
Subscriber Line (ADSL), which could
not reach that far
Although our local telephone
com-p a ny doesn’t actively market the dry
pair SDSL service, we we re able to
obtain dry pair connections for a
monthly rate of only $53 for each
end-point to endend-point link, for a total re c u
r-ring cost of about $210 to $265 for all
f i ve offices This lower monthly cost
caused us to look closely at the
SDSL/dry pair option
Initially, our re s e a rch indicated we
could achieve ave rage line speeds in
excess of 1 Mbps over most of our
net-work, with published data indicating
that speed dropped dramatically as line
distances increased Based on our
actu-al field results, though, we can only
assume the published performance
esti-mates are very cautious and intended to
c over a wide variety of line quality and
o p e rating conditions because we have
consistently obtained better thro u g h p u t
for a given line distance than the ve
n-dors’ published estimates
We installed an ADC Megabit
Modem 300S (www.adc.com) with
switch selectable speed options of up to
2 Mbps, partly because we knew
some-one who already had good experience
with this product The 300S has an RJ
-45 port to connect to the network and
an RJ-11 port to connect to the dry pair
using a standard phone to wall cord It
uses an external power supply and is
priced at around $400
S e ve ral other SDSL modem models
a re available, including the NetTo N e t
SNE 2000-S, the MuLogic DSL - 2048
and the Netopia R7200 Howe ve r ,
some models are limited to 1.54 Mbps
Some other models also offer differe n t
connector options, so we suggest yo u
check carefully to be sure you have the
correct parts and connections on hand
Our plan to connect five separa t e
locations required us to choose our
cen-t ral office, where cen-the main servers are
located, as the WAN’s common point
Benefits and Downsides of SDSL
The benefits we achieved using dry pair SDSL WAN were:
1 Simple setup and maintenance
2 Synchronous connections at 2 Mbps speeds, which is faster than most common VPN options
3 Simplified data backup from remote segments to a central system
4 Easy sharing of a single Internet access point
5 Robust firewall at a single public access point with reduced network management costs
6 Intranet entirely within the local network, with no need to access the public Internet
7 Improved administration of remote computers
8 All LAN segments can be accessed from anywhere in the system as if they were in the same segment, although it’s slower than Ethernet
Dry pair SDSL has some disadvantages compared to other approaches, although these disadvantages were not critical in our installation:
1 Reduced security between the various network segments because all segments are accessible from anywhere in the system
2 Conflicts can arise from multiple DHCP servers if several are active on various LAN segments
3 If the DHCP server is down (due to a power outage), it’s possible that remote computers can lose their current address assignment, which causes difficulties using single-point Internet access or some database programs that require a current IP address for the workstation
4 Bridged SDSL is limited to 255 total computers or devices using an IP address unless we want to choose different firewall and antivirus
securi-ty devices This is really a limit of those devices because the SDSL bridge can recognize 8,192 network devices Although if you need that many network addresses, you probably need a more robust solution anyway
5 Traffic going out over the wiring is not encrypted by the SDSL modems
Trang 3to six computers we found no significant
reduction in speed
As we extended to three and a half
miles we saw throughput speed drop to
about 1.5 Mbps, still T-1 speed at a
small fraction of the cost These re s u l t s
e n c o u rage us to consider a longer link
of five miles Although that longer link
has not been installed and tested, we
h a ve read about similar SDSL bridges
achieving T-1 speeds over distances of
four to five miles, but these longer
dis-tances work better if the copper wire is
the heavier 24 AWG rather than the
26-gauge wire that is common in older
areas
S e ve ral other points should be
noted If you have been using a
Dynamic Host Configuration Pro t o c o l
( DHCP) server on each LAN segment
to assign Internet Protocol (IP) addre s
s-es, you need to shut off all but one in
o rder to avoid potential conflicts in
a d d ress assignment And, with many security systems, you are limited to one range of IP addresses This will limit yo u
to 255 devices connected to the net-work on all segments combined
If you have a need to connect
sev-e ral officsev-es locatsev-ed within two to ssev-evsev-e n miles of each other, then SDSL modems using dry pair wiring to bridge between offices is a viable solution
Our re s e a rch leads us to conclude that given the price of ISP and tele-phone company services, the cost of dry pair wiring offers some distinct cost sav-ing possibilities worth considersav-ing The simplicity is such that the connection easily can be made without extensive technical consultant costs In addition, in some areas, it might prove to be a faster, less expensive alternative than other types of broadband access
A B O U T T H E A U T H O R S
THOMAS BOEDEKER is the city man-ager of Soldotna, Alaska He is the for-mer borough attorney for the Kenai Peninsula For a break from work and computers, Boedeker plays golf and reads mysteries and science fiction Boedeker has been an active attorney and litigator for more than two decades JOSEPH L KASHI practices law in Soldotna, Alaska He has held various posi-tions in the ABA’s Law Practice Management Section He received his bach-elor’s and master’s degrees from the Massachusetts Institute of Technology in
1973 and his law degree from Georgetown University in 1976 He is admitted to prac-tice before the Alaska Supreme Court, the U.S Supreme Court and the U.S Court of Appeals for the 9th Circuit and speaks fre-quently on legal automation topics.