Cable Modems Cable modems emerged in consumer markets as an alternative to ISDN and regular modems only a couple of years ago. Promises like speed up 30 Mbps to connect to the internet sounds very attractive given that the service charge is only $35-$55 a month. But the whole thing has been overhyped in my opinion, none of the cable companies mention that the subscribers share the bandwidth plus most cable modems use 10BaseT interface to connect to the PC or Mac which automatically limits the connection to 10 Mbps. Another limitation of cable modems is that ISP's are connected to the internet back-bone using T1 lines which puts an absolute limits on speed of cable connection to 1.5 Mbps The cable modem access network operates at Layer 1 (physical) and Layer 2 (media access control/logical link control) of the Open System Interconnect (OSI) Reference Model. Thus, Layer 3 (network) protocols, such as IP traffic, can be seamlessly delivered over the cable modem platform to end-users.
. Since cable modem technology is very recent the conflict exists between different standards. The only specification that has been approved by ITU (in 1998) is DOCSIS (Data Over Cable Service Interface Specifications) which was developed by MCNS, CabeLabs, Arthur D. Little and consortium of North American MCOs. Later that year CableLabs established certification program that would ensure interoperability among the equipment from different vendors. Certified cable modems are expected to appear in second quarter of 1999.
While waiting for the certificates vendor already started to develop the products that would meet new DOCSIS specification version 1.1 (not finalized yet). Also MCNS together with Broadcom and Terayon are working on implementing an IEEE 802.14 endorsed advanced PHY technology into the DOCSIS spec. The emerging standard will be known as DOCSIS 1.2. The technology will provide a more robust upstream and enable support for more business class applications. DOCSIS 1.0 is able to have download speed 27-36 Mbps per 6 Mhz channel (depends on modulation) and upload speed from 320 Kbps to 10 Mbps. DOCSIS will support Universal Serial Bus (USB) and IEEE 1394 (FireWire) technologies in the future in order to eliminate the need for Network Interface Card (NIC).
The IEEE 802.14 Working Group is a committee of engineers representing the vendor community that has developed a specification for data over cable networking. The group, which was formed in the early 90's, had intended to develop a specification that would be recognized as an international standard. However, MCNS' effort undermined the group's work and was able to define a spec much quicker than the IEEE. I mentioned above that MCNS is planning to implement advanced PHY specifications their DOCSIS 1.2 The reference architecture specifies a hybrid fiber/coax plant with an 80 kilometer radius (from the head end.).
At the physical layer, which defines modulation formats for digital signals, the IEEE and MCNS specifications are similar. The 802.14 specification supports the International Telecommunications Union's (ITU) J.83 Annex A, B and C standard for 64/256 QAM modulation, providing a maximum 36 Mbps of downstream throughput per 6 MHz television channel. The Annex A implementation of 64/256 QAM is the European DVB/DAVIC standard, Annex B is the North American standard supported by MCNS, while Annex C is the Japanese specification. The proposed 802.14 upstream modulation standard is based on QPSK (quadrature phase shift keying) and 16QAM, virtually the same as MCNS.
The main difference between these two standards lies in Media Access Layer. IEEE 802.14 uses Asynchronous Transfer Mode (ATM). IEEE 802.14 committee members say they chose ATM because it best provides the quality of service guarantees required for integrated delivery of video, voice, and data traffic to cable modem units. The group saw ATM as a long-term solution that would provide the flexibility to deliver more than just Internet access. MCNS members didn't buy the argument. Cable operators are clearly focused on delivering high-speed Internet services to consumers and believed ATM would add unnecessary complexity and cost to cable modem systems. By supporting a variable-length packet implementation, MCNS members plan to capitalize on the favorable pricing associated with Ethernet and IP networking technology.
The DVB-RCCL/DAVIC specification was developed by a combination of standards bodies including DAVIC, IEEE 802.14, and the ATM Forum. The specification has been recognized as the preferred technology by the ECCA, a consortium of European MSOs. As a result, it represents the only technology that rivals DOCSIS as a standard for international deployments.
About V.90 V.90 standard was finalized by ITU on February 6th, 1998 in Geneva, Switzerland. The battle between U.S. Robotics with it's X2 standard and Rockwell and Lucient who teamed up to bring 56Kflex is over. But the question is how long will they stay on the market? For a long time the only feasible way to connect to the Internet for the most consumers was to use an ordinary phone line and a modem. But with growing popularity of the Internet and vast use of multimedia on the web pages the consumers demand faster connection to the Internet. The analog telephone line has a limit on transmission speed at 35Kbps (upload/download), and since the connection from the telephone switch to the end user is analog for a long time it was thought to be the maximum speed that consumers would be able to achieve. Three years ago US Robotics announced the introduction of the first 56Kbps using its X2 standard. They were able to achieve such speed because of improvements that were undertaken by majority of telephone carriers to convert their lines to digital format. That is how it works: When the user dials to Internet Service Provider (ISP) his call is going through the analog to digital converter (ADC) at the telephone switching office which limits the transmission speed to 35Kbps, however when ISP transmits back and if its using a digital line the need for ADC is eliminated which makes a speed of 56Kbps possible.
Cable Modems V.S. V.90 A consumer has to be careful when deciding between cable and V.90 modems. First one has to determine if his/her ISP supports V.90 standard then they have to find out if the phone line that they are using is able to achieve speeds higher than 35Kbps and last the cost of the connection has to be analyzed. For users with only one phone line in the house been connected to the Internet means that they cannot receive any phone calls. Installation of the second phone line could be costly and one would have to pay around twenty dollars a month for the second line (considering they don't dial long distance to ISP which would run up the bill) plus the fee that ISP charges could be anywhere from fifteen to twenty five dollars month. All this comes to around forty dollars to have a phone line (voice) and connection to the Internet. The prices for cable modem range from thirty five to fifty five dollars a month and the only other costs involved are installation costs (equipment rental fee is usually included with monthly fee).
The transmission speed of cable modem is much higher than V.90 modem. It could be not the fastest on the market (compared to the ISDN, ADSL, T1) but when considering the costs of any other alternatives I came to the conclusion that cable modem even with all limitations (discussed above) is a much better way to go than V.90 modem.