Tuesday, January 18, 2011

Technical Comparison of ADSL and CABLE MODEM

In the narrowband access technology from the existing optical access solutions to end the process of evolution, there will be many very viable transition technology, which, based on ADSL technology and the telephone line cable of Cable Modem based technology is the mostpotential.

ADSL line rate with upper and lower asymmetric features, so its application is mainly applied to provide users with Internet access and VOD on demand and other services, does not apply to LAN interconnection services.Provide Internet access services using "ADSL + ATM / Ethernet" approach.ADSL client remote device configuration, the Council configure DSLAM (ADSL central office equipment), between them, with ordinary telephone twisted pair connections.ADSL users PC, the remote device to provide Ethernet interface, DSLAM through the ATM or Fast Ethernet and connected to ISP.Through the network, users can achieve broadband Internet access.ISP DSLAM can be placed in the room, through the access network access to ISP; also be directly placed in the ISP engine room, and the ISP directly connected to the LAN access platform use.Using ADSL VOD video on demand service available, you can use "ADSL + ATM" approach.

Cable Modem is also a downlink and uplink bandwidth asymmetry of technology, to provide Internet access and VOD for the two businesses.Which can be used to provide Internet access service "HFC + Cable Modem + Ethernet / ATM" approach.Central Office needs with a HFC Headend Equipment, by ATM or Fast Ethernet and Internet connectivity, and complete signal modulation and mixing functions.Data signals through the HFC network (HFC) transmitted to the user at home, Cable Modem signal the completion of decoding, demodulation and other functions, and digital signal through the Ethernet port to send to the PC.Conversely, Cable Modem PC, came to receive the uplink signal, encoded, modulated pass through HFC head-end equipment.

ADSL and Cable Modem networking are two techniques can provide a variety of business and can basically meet the needs of current broadband services.ADSL bandwidth is lower than in the Cable Modem, but the use of HFC Cable Modem and to network in stability, reliability, power supply and operation and maintenance systems are still some problems.In addition, because of its network line bandwidth is shared, in fact, after the user reaches a certain size can not provide broadband data services, users share the bandwidth is very limited.The following is the author's comparison of these two technologies.

- Security.As the Cable Modem signal of all users were carried out with a coaxial cable transmission, so there is the risk of wiretapping.Problem solving wiretapping to protect the cable first, followed by the lines to understand the process of establishing a good set initialization parameters, the latter point is technically difficult to resolve.The ADSL not have this problem.

- Reliability.As the CATV network is a tree, so very easy to create a single point of failure, such as damage to the cable, amplifier failure, transmitter failure will cause the node's customer service interruption.The ADSL uses a star network, a fault will only affect ADSL equipment to a user.

- Stability.Cable Modem users will be able to enjoy the early stage of very high quality service, this is because the small number of users under bandwidth and band lines are very abundant.However, each addition of Cable Modem users will increase noise, occupied channel, reduce the impact of circuit reliability and quality of service on the existing users.This will be the Cable Modem urgent need to address a major problem.ADSL access network is not affected by the number of users and traffic impact.Of course, if the DSLAM bandwidth of less than all of the export total bandwidth users may need, there will be congestion at peak times, but this time as long as the bandwidth by increasing exports can solve this problem.

- Compatibility.Although the Cable Modem and ADSL have been introduced technical norms and standards, but there are different manufacturers of the products can not be compatible to the market expanding so bring some difficulties.

- Network cost.Cost in the network, ADSL equipment cost is clearly higher than the Cable Modem, but the latter need to be able to apply after the completion of HFC transformation.At present, China can only satisfy the majority of HFC 450MHz band of frequencies, while the use of HFC two-way business at least 750MHz bandwidth.This obviously does not meet the requirements need to replace all the coaxial cable.At the same time, to achieve a two-way HFC cable television line to replace the current use of one-way amplifier, this part of the reconstruction cost is very high.

Tuesday, January 4, 2011


FTTx: HFC Vs FTTh: "Since the early launches of Fiber to the Home (FTTH), advocates of this system architecture have touted its superior bandwidth as one of the..."


FTTx: GPON VS GEPON: "GPON vs. GEPON what's the difference. Advantages of GEPON over GPONPON is the acronym for Passive Optical Network is a point-to multipoint n..."

Monday, January 3, 2011


Since the early launches of Fiber to the Home (FTTH), advocates of this system architecture have touted its superior bandwidth as one of the main justifications for naming it the broadband network architecture of the future. However, while it is indisputable that fiber information capacity is vastly greater than that of a coaxial cable entering the home, today’s FTTH implementations yield no clear advantage over the near term possibilities for existing coaxial infrastructure. There are several points of confusion that arise when comparing cable’s Hybrid Fiber/Coax (HFC) architecture to a telco’s FTTH implementation.

Much of the confusion begins with comparing the number of subscribers in a given FTTH system to the number in a given HFC service group. Some writers on the subject begin with the assumption of 500 subscribers per cable TV service group, based on the late 90’s architecture statements from some of the largest cable system operators at the time. For HFC, a service group is usually defined by the number of subscribers served by a single optical node. However, the 500 subscriber figure is rapidly becoming a dated one. Much of the US HFC infrastructure was designed with segmentable optical nodes so that an optical node feeding 500 homes at the time could be reconfigured into 4 service areas of roughly 125 subscribers each. When those systems were designed, the optical wavelengths available to CATV system designers were limited to either 1310 or 1550 nm for purposes of economy, so eight separate fibers and 2 spares to each node were often part of the fiber network’s design to provide for unique content on each pre-planned, segmented branch.

Today, with the availability of more optical wavelengths made for cable television distribution (CWDM or even DWDM), the fibers that were once planned for use at a single node station could be extended to other newly installed, deeper node stations, thus further reducing the number of subscribers in a service area.
In fact, with the addition of just a single 2x2 node in each branch of the originally installed big 4x4 node’s service area, it is possible to cut the original 500 subscriber service area model down to 8 service areas of little more than 32 subscribers each- the magic number that happens to coincide with the telco GPON definition of 32 subscribers per service area.

Once the HFC service area is down to 32 subscribers, the coaxial system approaches the capacity of current GPON systems. This is simply because, whether in the coaxial or fiber part of cable’s HFC plant, the system carries digital information using radio-centric signaling, whereas GPON FTTH adaptation uses basic line-code (on-off) centric signaling.
Radio-centric signaling commonly separates different services into discrete silos, referred to as channels, a method known as frequency division multiplexing. In HFC digital radio signaling, each channel is configured to transport digital information by a specific amplitude modulation method known as Quadrature Amplitude Modulation (QAM).

A common QAM level in CATV is 256 QAM, which amounts to about a 38 MBPS data rate per each channel. HFC systems are migrating toward 1,002 MHz upper frequency limit, so at 6 MHz per channel, there’s about 158 channels of capacity on these upgraded systems. Simply put, that’s 6 GBPS full spectrum that fits on a single wavelength using the radio-centric cable signaling method. While this capacity is no big secret, it is almost never presented this way in the HFC vs. FTTH forum because cable employs so many different radio formats over the typical system that it’s hard to make a direct comparison given the state of a cable system today, with its mix of analog channels, digital channels, cable modem channels, etc.

One of the tricky things about having this kind of capacity, until very recently, has been the difficulty in accessing the available data rate. Since each channel is 38 MBPS, the maximum bandwidth available to one user would be 38 MBPS under DOCSIS 1.X and DOCSIS 2.0. But with the adaptation of channel bonding, a new feature in DOCSIS 3.0 a cable modem can access 8 channels simultaneously, with plans for 10 channels at a time. So, with current technology, it is possible for a radio-centric network to provide access of up to 1 GBPS per radio.
So far we’ve covered the downstream capacity of an HFC system and have found, that using radio-centric signaling, the capacity flowing from the headend to the subscriber is quite large given a subscriber group size comparable to a GPON system and a spectrum filled exclusively with digital carriers.
The next consideration is what to do with the upstream performance.
US Cable systems are divided downstream and upstream by frequency splitting passives, known as diplex filters, usually located in the line equipment- the amplifiers and nodes that deliver the signal to the subscribers via coax and fiber. The upstream path occupies spectrum from 5 to 42 MHz and the downstream occupies 54 to as high as 1,002 MHz currently. In short, the upstream path is just a tiny sliver compared to the downstream path. This legacy frequency split is due exclusively to the legacy frequency allocations of broadcast stations in the US. Since channel 2 is designated as the lowest carrier in television broadcast, and it occupies the spectrum from 54 to 60 MHz, then cable also currently begins its downstream spectrum at 54 MHz so that legacy analog television tuners can be used to receive cable signals without the need for a converter box. However, with the end of analog broadcast television upon us, this utility will soon be obsolete, even though cable companies can continue to carry analog stations beyond broadcast’s conversion to all digital transmission. Once consumers settle into an all digital world, the legacy frequency split will no longer serve any meaningful purpose. So, those networks that have certain modular equipment in the outside plant will be able to reclaim the lower end of the formerly analog spectrum for upstream signal path use by changing the frequency split (diplex) modules in their line equipment, thereby expanding the bandwidth of the return path up to a level nearly symmetrical with that used for downstream radio access. That means potential bidirectional access of 1 GBPS per subscriber radio (modem).
The scenario described above is somewhat arbitrary. That is, the 1 GBPS access could be any other rate that is or will be provided for in current or future multiple access radio standards. However, we will extend the 1 GBPS bidirectional scenario into the remainder of this part of the discussion.
We’ve taken up 2 GBPS of the roughly 6 GBPS of capacity providing for a pretty seriously high data rate, bidirectional data service. That leaves us 4 GBPS for the last but still very important component of signal carriage- the video services. Put another way, we’ve taken 20 channels of frequency spectrum to provide for data services (just ignoring the extra bandwidth in the 5 to 42 MHz range). That leaves us 138 of 158 channels for video service, assuming a 1,002 MHz system. Now, these remaining channels can be allocated for analog, standard definition video or high definition video, but continuing on the scenario of a premium, all-digital network we should look at providing for a good number of high definition stations. 5 HD services take up about 2 channels, so 160 HD services would take up 64 channels, leaving 74 channels for standard definition. At 7 services per carrier, there’s room enough left for 518 standard definition services- far more if switched digital video is part of the solution.
So, by matching service area size, migrating to all digital, and using multiple access radios for data access, an HFC system can rival the performance of today’s FTTH implementations, and even beat some of the older implementations. In the next installment of this discussion, some ideas for an even more robust implementation of a radio-centric network will be offered. Stay tuned

Friday, December 31, 2010


GPON vs. GEPON what's the difference. Advantages of GEPON over GPON
PON is the acronym for Passive Optical Network is a point-to multipoint network. A PON consists of optical line terminal at the service provider’s central office and many number of optical network units near end users. The goal of PON is to reduce the amount of fiber.

There are two standards of the Passive Optical Network available and they are listed here.
  • GPON

            GPON (Gigabit PON) is the evolution of broadband PON (BPON) standard. The protocols used by GPON are ATM, GEM, and Ethernet. It supports higher rates and has more security.

            GEPON or EPON (Ethernet PON) is an IEEE standard that uses Ethernet for sending data packets. In current there are 15 million EPON ports installed. GEPON uses 1 gigabit per second upstream and downstream rates.
            EPON/GEPON is a fast Ethernet over passive optical networks which are point to multipoint to the premises (FTTP) or fiber to the home (FTTH) architecture in which single optical fiber is used to serve multiple premises or users.

The Differences between GEPON and GPON:
The differences that make the GEPON the best are discussed here.
            One important distinction between the standards is operational speed. BPON is relatively low speed with 155 Mbps upstream/622 Mbps downstream operation. GE-PON/EPON operates at 2.5 Gbps symmetrical operation. GPON supports 1 Gbps asymmetrical operation. Another key distinction is the protocol support for transport of data packets between access network equipment.
             BPON is based on ATM, GPON uses native Ethernet and GEPON supports ATM, Ethernet and WDM using a superset multi-protocol layer.
            GEPON is still evolving; but, it requires the multiple protocols through translation to support the native Generic Encapsulation Method (GEM) transport layer. This emulation supports ATM, Ethernet and WDM protocols. It is widely deployed in Asia and uses Ethernet as its native protocol and simplifies timing and lowers the costs by using symmetrical 2.5 Gbps data streams. The complexity is lower and cost is less than GPON. GEPON has an installation cost advantage.
            GEPON Supports Class of Service (CoS) operation for time-sensitive transport of data payloads such as video. This video frames must be delivered in sequence and it should maintain time constraint to prevent malfunction. It functions with VoIP.
            When compared to GPON, GEPON is highly scalable and flexible; it provides service for more than 2,300 subscribers. It is used in telecommunication services. GEPON supports effective education and public outreach (EPO). This network will:
·        It provides infrastructure to facilitate collaboration between scientists, educators etc.
·        Provides support systems for professional development.

There are many advantages of the GEPON. They are listed and discussed here.
  • Service flexibility: The GEPON does lots of services and it is of very flexible type.
  • Easy, modular planning and rollout: The GEPON is the easiest mechanism and there is modular planning and roll out that is attached with the GEPON which adds lots of benefits to the GEPON differentiating from the GPON.
  • Highest density and availability.
  • Price. GEPON solutions at the time of writing are more cost effective
  • Much more easy configuration - easier to use, almost plug and play technology.