Understanding the Internet-a short course

Dial-up
DSL
T1
DNS

Dial-Up Access
Analog 28.8 kbps/33.6 kbps/56 kbps
ISDN 64 kbps
ISDN 128 kbps

Dedicated Access
DSL 1.5 Mbps/256kbps
T1 1.544 Mbps

 

56k Fun Facts:
Only a few short years ago, the highest speed for dial-up access was 28.8 kbps (before that, do you remember the old 300 and 1200 baud modems (0.3 and 1.2 kbps)).   Then, an enhancement provided some modems a 33.6 kbps speed.  Then, the 56k modems came along.  Basically, there were 2 technologies.  The first technology was the US Robotics x2.  The second technology was the Rockwell 56KFlex.  The 2 standards fought for dominance until a standards committee was formed to merge the 2 incompatible standards in 1998.  The merged standard is called v.90 (or sometimes called v.PCM, referring to the technology used to break the 28.8 barrier and produce the 56k speeds-pulse code modulation).  The 56k download speed is a maximum theoretical speed, the actual speed in practice tops out at 53 kbps.  The upload speed with 56k modems is generally a maximum of 31.2 kbps.  US Robotics was purchased by 3Com in 1998.  For more information regarding 56k modems and modem technologies, please visit www.3com.com or the ITU at www.itu.int.   More recently, newer standards have has been ratified by the International Telecommunication Union (ITU), called v.91 and v.92.  The v.91 standard is supposed to allow modems to communicate over special digital lines, such as those used in PBX's.   The v.92 standard allows for 3 benefits:  modem-on-hold (which allows a user to take an incoming call while suspending the internet session which can then be resumed after the phone conversation is completed), quick-connect (the modem remembers the dial-in parameters, shortening the time to connect on successive dial-in sessions), v.PCM upstream (allows for up to 48 kbps upstream).  Access America currently supports the v.90 standard in our modem banks.

With the 56k technologies, the data transfer rates are asymmetric:

  • Speed from you towards the internet:  28.8 kbps
  • Speed from the internet towards you:  56 kbps*
  • *Due to current FCC regulations and local loop restrictions (a telephone line will only transmit data at a maximum of 64 kbps), the maximum obtainable download speed is 53 kbps.  Whether or not you can obtain this speed will vary depending upon the quality of your phone line connections.  Typically, the speed is approximately 45-49k.

     

ISDN Fun Facts:
ISDN (Integrated Services Digital Network) is a digital subscriber line technology developed in the 1970s.  ISDN is carried over a single copper pair.  The modems on either end (one at the telephone company central office and the other at your location) divide the capacity into 2  64k B Channels and a 9.6k D Channel.  The D Channel is generally only used for call setup purposes.  The 2  64k B Channels can be software bonded together to achieve the 128k capacity.  You should make sure the modem you purchase adheres to the National ISDN Standard.  In order to get your ISDN modem to work, you will need to obtain your SPID numbers (subscriber profile identification/telephone numbers) from your local exchange carrier.  Most commonly you will receive 2 SPID Numbers (one for each B Channel).  The SPID Numbers are in the format of NPA-NXX-XXXX-01 or NPA-NXX-XXXX-0001 or NPA-NXX-XXXX-0101.  Please refer to your modem software installation instructions to determine whether you add the -01 or -0001 or -0101 to the end of your SPID.  You will also need a 128k ISDN account with Access America Internet.  ISDN is an excellent way for a small office to share Internet access with several users.  You will need a LAN in your office, with each computer connected using TCP/IP routing (Win95/98/NT machines or newer Macs).  One of the computers will handle the Internet connection, generally through an external router/firewall.  These external modem/router units can be purchased from any of several vendors (i.e. 3Com:   the OfficeConnect series, Ascend/Lucent:  the Pipeline series).

There are at least 10 ISDN routers/modems currently on the market.  The only type that we support and that we feel will work reliably long-term for business use is the Ascend (purchased a couple of years ago by Lucent) Pipeline series of ISDN routers:

  • Lucent (Ascend) Pipeline 50 or 75

 

DSL Fun Facts:
DSL (Digital Subscriber Line) is also carried over a single copper pair.  The DSL modem on the customer end and the DSLAM (DSL access multiplexer) at the other end in the telephone company central office communicate to provide a much higher data communications rate.  DSL technologies are sometimes referred to as xDSL (there is ADSL, IDSL, SDSL, HDSL and VDSL).  The most common form of DSL is ADSL (asymmetric DSL) which is generally 1.5 Mbps download speed and 256 kbps upload speed.  Higher (and lower) speeds are also available with this technology.  The other forms of DSL are ISDN DSL (IDSL), Symmetric DSL (SDSL), High bit rate DSL (HDSL), and Very high bit rate DSL (VDSL).  The most common question is why isn't this technology available at my location.  Various factors are involved, but the most common one is that the length of the copper loop generally cannot exceed 18,000 feet.  Also, their cannot be load coils and/or long bridge taps on the line.  DSL is a signal carried on the high frequencies on the copper loop (voice is carried on the lower frequencies, between 0-4000 Hertz).  DSL is carried upstream in the 25-160 kHz band and is carried downstream in the 240 kHz to 1.5 Mhz band.  Thus, you can be on the internet and talk on the phone simultaneously!
T1/DS1 Fun Facts:
A T1 (also known as a DS1, Digital Service or Digital Signal) is generally carried over a 4-wire copper pair.  A T1 or DS1 can also be carried over fiber, but this is fairly rare done, due to the expense of fiber.  A T1 is also referred to as a high-cap (high capacity) service.  The T1 speed in the United States is 1.536 Mbps (1,536 kbps).  A T1 is composed of twenty-four (24) 64 kbps channels or timeslots (24 x 64 = 1,536).  A single channel is referred to as a DS0.  As an aside, you can purchase connectivity at a DS0 level (either 56 kbps or 64 kbps, depending upon your carrier, if they support 64 kbps or if they have to use the 8 kbps for signalling and give you 56 kbps for your data.  In Europe, the T1 is known as an E1 and has 32 channels/timeslots with a capacity of  2.048 Mbps.  A T1 can be "channelized" using a mulitplexer (MUX) and only a few channels used (usually called fractional T1 service) or the entire T1 can be used either channelized or unchannelized.  Thus, a T1 is flexible so that a few channels can be used or the entire capacity can be used.  A T1 can be used for point-to-point communications (usually linking phone systems or data servers together), also called a private line.   A T1 can go from an end-user to the ISP (an internet T1 or dedicated internet T1 (DIA T1)).  A T1 can also go from an end-user to a long distance carrier for terminating 1+ long distance calls and/or for 800 toll-free calls.  Thus, there is an important distinction that needs to be made when talking about T1s as to whether the T1 is a point-to-point T1 or a T1 that goes to an ISP or long distance carrier.  In this case, since we are generally talking about internet, the rest of the discussion will be about dedicated internet T1s (DIA T1).  The T1 technology was developed in the 1970s (and if you read the ISDN section above, you may notice a parallel in time).  The ISDN technology is a subset of T-carrier technology and was developed to provide extra features for voice use.  Why 64 kbps?  The reason that 64 kbps is important is that it was determined at Bell Labs that when voice was digitized, sampling at a rate of 64,000 times per second provided a reasonable representation of the speaking voice.   A T1 for internet access is usually purchased as clear channel, giving the full 64 kbps for each channel.

T3/DS3
A T3 (DS3) is 28 T1s, 672 channels or 45 Mbps.  A T3 is carried over fiber and provides high-capacity data communications.  A T3 to the internet is common for web servers or other servers.  It is uncommon in small business, but large businesses may have multiple T3s to carry server traffic and voice communications.

OCx
OC3, OC12, OC48, OC192 are all optical carrier (OC) technologies and are carried over fiber.  OC3 is 155 Mbps, OC12 is 622 Mbps, OC48 is 2.488 Gbps, OC192 is 9.952 Gbps.  The Optical Carrier technology is based on SONET (synchronous optical network) technology.

Ethernet
Ethernet is commonly used in local area network applications and is widely used for data communications.  Wide-area ethernet is becoming more commonplace and provides the opportunity to easily hookup multiple business locations by using a switch, instead of a router and CSU/DSU.  This ease of use is very attractive as it can decrease the workload of the IT department, if the network is planned well.  Ethernet can be used for point-to-point communications as well as dedicated internet access.

Internet Technologies

Internet
The Internet is a network of networks, linked by routers.  You can have (and probably do have at your workplace and at your home) a small network.  If it uses TCP/IP it sometimes is called an internet (small "i").  The Internet started as an Advanced Research Projects Agency project (ARPANet) which went through various iterations (DARPANet) and merged with NSFNet and became a non-government project after which is became the Internet.

World Wide Web
The World Wide Web (WWW) is the term given to the Internet once hypertext was added and linking was added to the Internet enabling a user to easily navigate from one piece of information to another.

NAP
The Network Access Points (NAPs) are the major peering points in the United States where internet traffic is exchanged between backbone providers.  Some of the major NAPs are in Washington, DC, Chicago, California.  This is where the big routers do their stuff.  Originally there were 4 NAPs.  These are still active, but there are many more peering points now.  The term NAP is often used to describe any of these peering points of a certain size.

Transmission Control Protocol/Internet Protocol (TCP/IP)
This is the language of the internet and the control protocol.  This is only one of many languages that are used for data communications, but was used for simplicity and fail-over reasons as the language of the network that became the Internet.

Internet Protocol (IP) Address
The numeric "location" of a device connected to the internet.  For example, accessam.com is on a server with a particular IP address.  When you are connected to the internet, you have a unique IP address.  There are public IP addresses and private IP addresses.  The public IP addresses used to be referred to as Class A, B and C.  Now, there is class-less internet domain routing (CIDR) so that Class A, B and C are more historical terms.  It used to be that you reserved a Class C block of addresses.  However, you might not use all of them, so to keep the world from running out of addresses, subnets were formed.  Thus, you now see an IP address and its subnet mask as a pair of numbers.  If you have a LAN, then you likely have private addresses.  One of these sets of private addresses is the 192.168.XXX.XXX block.  Most small business networks make do with a single set of 256 addresses.   For example, you may have used the addresses at work of 192.168.1.1 through 192.168.1.254.  Note that 192.168.1.0 and 192.168.1.255 are reserved and can't be used for machines, they are used for communications on the network.  Thus, this network is a 192.168.1.0 network.  The subnet mask is referred to as either /32 or 255.255.255.0.  Lots of times a network will be set up in an organized fashion so that routers and switches have the addresses 192.168.1.1 through 192.168.1.10, printers and servers 192.168.1.11 through 192.168.1.20 and then the individual machines will be 192.138.1.21 through 192.168.1.254, but it is completely up to you how you want to set up IP addressing for your LAN.  Note that these private addresses are non-routable and so you will need a router that performs network address translation (NAT).

Network Address Translation (NAT)
Network Address Translation (NAT) performs the function of translating a public IP address into the private addresses used on a LAN.  NAT also functions as a pseudo firewall in that the devices behind the router cannot be seen directly.  However, it is possible to setup ways for servers to be accessible to the internet through NAT (using port address translation (PAT)).  These are all things that can be done by your IT support or by Access America.

Dynamic Host Configuration Protocol (DHCP)
Dynamic Host Configuration Protocol (DHCP) is most often used to automatically assign IP addresses to various machines on a LAN, although it can be used by an ISP to dynamically assign IP addresses to dial-up or cable modem customers.  The router performing DHCP hands out IP addresses to anyone requesting them and provides them on a time-dependent basis as a "lease."  A lease can be short or long, depending on the network administrator's whims.  Usually routers come with a default DHCP lease time of 1 hour, but this can be changed to virtually any amount of time (shorter or longer).

Routers
These are the computers that send the data packets from one place to another.   More specifically, a router sends a data packet from one network to another.   Switches can also be used to move data, but only within a network.  Routers are the workhorses of the internet.

DNS
This is the technology that allows the internet to function using letters instead of humans having to use internet protocol numbers.  This way, we can type in www.accessam.net instead of the IP address 199.236.99.13.  Note that many times the www is not required if the DNS is set up correctly.  DNS also allows us to create "subdomains," such as mail.accessam.net which points to our mailserver.  An excellent short description of DNS can be found at:  www.rscott.org/dns, although there are few examples of syntax on these pages, the website gives a nice overview of what the DNS records are and how they are used.

E-mail
Electronic mail is one of the main applications used on the internet.  E-mail is handled by an e-mail server.  Your inbound e-mail is generally handled by a POP3 (post office protocol 3) server.  Your outbound e-mail is handled by an SMTP (simple mail transport protocol) server.  These protocols can be handled by a single server (i.e., mail.accessam.net) or can be in two different servers.

Web browsing
Browsing the world wide web (WWW) is the other of the main applications used on the internet.  There are several programs that can be used (called browsers).  These include Microsoft Internet Explorer, Mozilla Firefox, and Opera.


Access America 1998-2007
673 Emory Valley Road
Oak Ridge, TN  37830
865-482-2140
800-860-2140