Understanding the Internet-a short course
Analog 28.8 kbps/33.6 kbps/56 kbps
ISDN 64 kbps
ISDN 128 kbps
|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:
*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:
|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.
World Wide Web
Transmission Control Protocol/Internet Protocol (TCP/IP)
Internet Protocol (IP) Address
Network Address Translation (NAT)
Dynamic Host Configuration Protocol (DHCP)
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