Cable Vs V 90 Modems Essay
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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...
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