NAVIGATION
My Investigation on Cable Modems
Investigation of CM's operation & architecture
Alternative Broadband Technologies
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Investigation and analysis of CM Standards 3.1 Analysis Of The Three Major CM Standards. 3.1.1 DOCSIS/MCNS 3.1.2 DVB/DAVIC 3.1.3 IEEE 802.14 3.2 Comparison Of Cable Modem's Standards. 3.2.1 Services available 3.2.2 PHY comparisons 3.2.3 MAC comparison 3.2.4 Performance comparison 3.3 Evaluation of CM Standards
Investigation and analysis of cable modems Standards Various standardization organizations are working to provide specification standards for cable modems. These standards will allow the development of compatible and interoperable hardware and also reduce the cost of implementation. The DVB/DAVIC, the MCSN/DOCSIS and the IEEE 802.14 protocols are focused on the current cable modems. All three of these have the same goal to provide interface specifications for the support of the same services.
Below is a list of all standard activities.
This section provides a listing of all CATV standard activities as well as an analysis of the three major standards. Furthermore after the analysis a comparison will be made of these three in terms of efficiency and characteristics.
3.1 Analysis Of The Three Major CM Standards. There are three established standards that certify cable modem devices. This section attempts to explain the three standards which are the DOCSIS/MCNS, the DVB/DAVIC and the IEEE 802.14. After the discussion of each one there is a comparison of all three.
3.1.1 DOCSIS/MCNS The DOCSIS specification standard was developed by MCNS, a consortium of North American MSO’s and CableLabs. It was developed in March 1998 for transmitting data over a cable network. In that date the specification was approved by the ITU as an international standard. This standard had a vast majority of operators supporting it. The most supportive operator, in the first period of the standards existence, was Broadcom. This big interest, shown by many large operators, made the DOCSIS leader in the Cable Modems Standard’s market. A few months after DOCSIS standard was established, CableLabs established some tests in order to provide an official certification to cable modems from various manufacturers. Furthermore after this certification procedure was established the next step by MCNS was to plan their future. The new development, DOCSIS 1.1 certification, supports fragmentation in the upstream and downstream, allowing providers to offer better services and support better quality software applications. Furthermore looking in the future of DOCSIS, the company mentioned above, Broadcom and another company called Terayon are working with the MCNS consortium to further increase the standard’s complexity by implementing an IEEE 802.14 endorsed PHY technologies into the already established DOCSIS specification. The new developing standard will be known as DOCSIS 2.0. The technology will provide a better upstream speed and offer better applications. Finally after conquering the US domestic market DOCSIS developers wanted to the same in Europe. To do so they developed a similar standard suitable for Europe with the name EuroDOCSIS. The EuroDOCSIS is exactly the same as DOCSIS apart from the Physical Layer. This Standard became a very good competitor to the already established European standard DVB.
3.1.2 DVB/DAVIC A competitor for the EuroDOCSIS specification in the European cable modem standard’s market is the DVB/DAVIC. This standard is also known as DVB-RCC and as ETS 300 800. The DVB is a European organization that develops standards for digital TV. This organization worked together with DAVIC (Digital Audio Visual Counsil) in order to provide a standard for cable modem manufacturers. Although the establishing of their standard was successful, very few manufacturers developed product for this standard making lose important support that would helped it win the race for the standard’s market. The number of vendors supporting this standard is not large but enough so that it does play a role. These specifications’ operation is based on an Asynchronous Transfer Method (ATM). This is a big difference between them since EuroDOCSIS does not support ATM. The use of ATM provides better Quality of Service (QoS). Moreover this standard is very well suited to both data and voice as pure ATM. Initially this standard had a big disadvantage over DOCSIS that lacked security but the problem was solved after a few months by releasing a new version of specifications called DVB/DAVIC 1.4 . Furthermore the DVB specifications are open to any implementations. Also and additional out-of-band receive data channel is provided
3.1.3 IEEE 802.14 The IEEE 802.14 group was established in 1994 in order to create standards for transferring data over CATV networks. This group tried to compete with the other two Standards discussed above but failed there were slow standardization procedures taken place within the IEEE. They did not manage to find the right timing for their specifications and lost all industry support. The latest specification will remain as a proposed specification within the IEEE for the next three years. If there is no interest by any group the specifications will be withdrawn. This specification supports new approaches for transferring data-over-cable. They implemented the ATM method for the transfers as well as new approaches for implementing the following:
The IEEE 802.14 working group developed three physical layer variations for their standard in order to reflect the European, the American and the Japanese requirements. They are known as types A, B and C respectively. They all are the same except a minor difference in the upstream channel. The difference is that type A’s upstream bandwidth is 8 MHz while the other two have 6 MHz.
3.2 Comparison Of Cable Modem's Standards.
In this section a comparison of the discussed standards is available. This comparison is split into four sections in order to see these standards comply in various areas like services, performance, MAC layer and Physical layer.
3.2.1 Services available All standards discussed above have many similarities but also have some very important differences that separate their technologies. The first step for defining these differences is to find what services each standard provides. All three of them support a variety of services which include the following:
On the other hand the new improved version of DOCSIS, version 1.1 protocol has been designed to offer some new features needed by the industry. These new features are the QoS and Voice Over IP (VoIP). To meet the needs of applications for better Quality of Service, the DOCSIS developers provided a new range of QoS. The full range is:
Table 2: The DOCSIS’s range of QoS
After CableLab's DOCSIS standard supported an improved QoS the other competitors start thinking about it and the IEEE 802.14 adopted this feature. A more technical comparison of cable modem’s standards, covering Upstream and downstream rates as well as performance, and services is available in the table below.
Table 3: Characteristics comparison of the three Standards As we saw above all standards support interactive set-top boxes plus the IEEE 802.14, as well as the DOCSIS 1.1 protocol, supports VoIP and QoS. Furthermore DVB/DAVIC supports internet telephony, VoIP, videoconferencing and multimedia home services. 3.2.2 PHY comparisons
DVB/DAVIC uses the QPSK modulating technique for the upstream direction. Also there are four data rates for the upstream transmission 6176, 3088, 1544 or 256kbps. On the other hand DOCSIS uses QPSK or 16 QAM for the same direction.
For downstream channel of DVB/DAVIC two modulations signals are provided. One is QPSK and the other is QAM. The data rates of the QPSK channel are 1554 or 3088Mbps are used. For the other channel it’s expected that multiples off 8Kbps will be used. On the other hand DOCSIS’s downstream channels have a less noisy modulation that can be either 64 or 256 QAM. In the downstream channel the DOCSIS MAC frames are carried using MPEG-TS. This is an important feature of DOCSIS protocol because it allows the coexistence of both data and MPEG video in the same channel. Finally as we discussed above the IEEE 802.14a has three specified physical layer specifications that reflect the different market requirements. These types are known as types A, B and C.
3.2.3 MAC comparison A data transmission scheme of request/grant is followed by the DOCSIS MAC layer. The Modem makes the request and waits to be granted by the CMTS. The CMTS provides the needed information to Cable Modems. Below are the types of information that the MAC layer of each modem on the network needs: · Timing reference · Registration · Transmission resolution · Contention resolution In the DOCSIS upstream transmission minislots are used. Each minislot can be used two ways. One way is to be used for contention and the other is to be used for contention and data simultaneously. In such a system the request is issues by the use of contention minis lots. In the upstream each minislot can be used either for contention, contention/data, and initial maintenance or station maintenance purposes. In this system when a cable modem needs to send data it has to issue a request, using the contention minislots. DVB/DAVIC on the other hand provides four access modes at its MAC layer. These are the four modes: Contention Access - Used to let the user send information anytime with the risk of data collisions. Fixed Rate - In this mode the INA provides an amount of slots to NIU Reservation Access - This mode provides a bit rate requested by a NIU Raging Slot Access - In this mode the slots are used for the calculation and adjustment of the timing offset.
Pic. 6: Sea of minislots
Finally the IEEE 802.14 MAC layer is like DOCSIS protocol that uses a scheme for data transmissions that is based on grand/request. The CMTS at head-end allocates transmission resources in the upstream channel to users for contention and reservation based data transfer. The IEEE and the DVB/DAVIC protocols use ATM transfer, while DOCSIS protocol uses another method. This method delivers variable length internet protocol packets. This method was chosen by CableLabs in a try to keep cost and complexity of cable modems down. However DOCSIS includes the required devices for the definition and transfer of ATM MAC Protocol Data Units (PDUs).
3.2.4 Performance comparison As we saw in the table above the performance of the DOCSIS protocol is much higher than DVB/DAVIC since it has higher upstream rates (10Mbps at 16 QAM) has lower upstream rates. Also in the downstream transmission DVB/DAVIC is not very efficient due to ATM that it uses and messaging. Furthermore the fragmentation of DOCSIS is more efficient than DVB/DAVIC’s that uses ATM for QoS. Finally DOCSIS is more flexible than DVB/DAVIC.
3.3 Evaluation of CM Standards The IEEE 802.14 might be good in theory, but it does not play an important role in the world of cable modem standards since the vendor support is minimal. On the other hand DVB/DAVIC has support but DOCSIS is the one that has conquered the market. It is an international standards based system that is well suited for high QoS requirement services such as Voice, Data and Video in HFC systems. |
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