Cable Modems: Cable Tv Meets The Internet Essay Example

the Internet" provides an informative overview of cable modems and cable data systems. Extensive research was conducted to explore how cable modems function and how they are integrated into cable data systems. The cable industry was only given permission to enter the ISP business less than three years ago. Because cable modems are relatively new devices and cable data network technology has rapidly improved, we used the most current and reliable sources of information to ensure accuracy.

Recent magazine articles and Internet sites provided the latest and most up-to-date information, while hardcover books contained obsolete and outdated information. The paper presented the results of the research conducted on the subject, citing the references used as sources of information.

Results

Cable modems have been proven to be a reliable technology, offering high speeds and the potential for upgrades in cable data networks.

Discussion

The content provided demonstrates the strength of cable modem technology and its significant potential for further advancements. Cable modems serve as an important milestone towards the complete integration of networking into our daily lives. Although this progress is still far from being achieved, it is inevitable and will occur relatively soon. Despite the sluggish speeds offered by conventional dial-up telephone modems, residential Internet usage has experienced rapid growth.

These voiceband connections have a speed limit of 56 Kbps or lower, resulting in a sluggish internet experience with dial-up modems. However, there is a high demand for faster internet connections. In 1996, the Telecommunications Act allowed cable TV companies to improve their services by offering two-way voice and data communication along with TV programming (Clark, 1999).

Multiple Service Operators (MSOs), also known as cable companies,

offer extended services to meet the demands of web surfers who desire high-speed internet access. These aspiring MSOs recognize the sizable market for fast internet and aim to cater to it. Cable modems enable fast internet access through a cable television network, functioning similarly to traditional dial-up telephone modems but with greater power. Unlike telephone lines, cable modems utilize the cable TV programming cable as their connection medium to the internet.

Cable modems utilize the broadband capacity of cable TV infrastructure to achieve connection speeds significantly faster than dial-up connections. The increased bandwidth translates to higher speed. Depending on the architecture and traffic load of the cable network, cable modem users can experience access speeds ranging from 500 Kbps to 1.5 Mbps or more (Halfhill, 1996). With their impressive speed, cable modems can quickly download large audio and video files, offering genuine multimedia capability. Alongside speed, cable modems also provide constant connectivity, another important advantage.

Cable modems enable instant online access upon computer power-up and utilize a connectionless technology, comparable to an office LAN (Ostergard, 1998). Users are spared the inconvenience of dialing in or encountering busy signals, thus freeing up their telephone line. Moreover, cable modem Internet service is remarkably cost-effective with monthly expenses ranging from $40 to $60 inclusive of rental and unlimited access fees when contrasted with other high-speed data systems.

Cable companies are facing the challenge of providing high-speed internet service to their subscribers. It is not as simple as just connecting cable modems to customers' PCs. They need to invest in constructing a complex and expensive IP networking infrastructure capable of handling numerous users, including internet backbone connectivity, routers, servers, network management tools,

security systems, and billing systems (Salent, 1999). Additionally, cable data systems incorporate various technologies, so there was a need to establish standards for cable modems to ensure compatibility among products from different vendors. However, the primary obstacle for cable companies is that their television systems were initially designed for one-way communication and now require upgrades to support two-way networks for advanced communication services (Medin, 1999).

CATV systems were originally created to transmit broadcast television signals to households, making it a costly and technically intricate endeavor. In the cable industry, this process is referred to as downstream traffic, and the central distribution point for a CATV system is known as the Head-end.

The Head-end receives video signals from satellites or other sources, which are then frequency modulated to the appropriate channels. These signals are transmitted downstream through the cable medium into the subscriber's homes. The subscriber's television tuner or set-top cable converter box demodulates the signal to display a video image. To ensure that consumers can use their existing TV sets to receive cable service, cable operators recreate a portion of the over-the-air RF spectrum within a sealed cable line. Older coax-only cable systems usually have a capacity of 330 MHz or 450 MHz. On the other hand, newer and more expensive hybrid fiber-optic/coax systems (HFC) can operate at 750 MHz or even higher.

HFC networks incorporate fiber-optic and coaxial cable lines, giving them the advantage of both technologies. Approximately half of cable users in North America are connected to HFC cable systems. Despite being more cost-effective than pure fiber-optic networks, HFC networks still offer many of the benefits in terms of reliability and bandwidth that fiber-optic networks provide.

The fiber-optic section of the HFC network follows a star configuration, with optical fiber feeder lines extending from the cable head-end to clusters of 500 to 2,000 subscribers (Van Matre, 1999).

These groups of subscribers are referred to as cable nodes or cable loops. A trunk-and-branch configuration of coaxial cable is used to connect the optical-fiber feeders to each subscriber. Originally, CATV systems were primarily designed for one-way signal transmission, resulting in only a small portion of the available bandwidth being allocated for upstream transmissions. In a CATV system solely used for television signal transmission, there is very little need for upstream communication. The allocated upstream bandwidth occupies a narrow 5 to 42 MHz band located at the lower end of the cable TV RF spectrum (Barnes, 1997). Downstream cable TV program signals begin at 50 MHz, equivalent to channel 2 for over-the-air television signals.

The standard RF spectrum allocated for each television channel is 6 MHz. Therefore, a traditional coaxial cable system with a downstream bandwidth of 400 MHz can accommodate up to 60 analog TV channels. On the other hand, a modern HFC system with a downstream bandwidth of 700 MHz can handle up to 110 channels (Salent, 1999). In order to facilitate two-way data transmission over a cable network, one unused 6 MHz television channel within the range of 50 - 750 MHz is usually designated for downstream data traffic. Additionally, another unused 6 MHz channel within the range of 5 - 42 MHz is utilized for transmitting upstream data. Whenever an individual engages in activities such as clicking on a hyperlink, sending e-mail, or uploading files, they are transmitting data upstream.

Regrettably, the upstream

band is susceptible to various interference sources that can distort data. This limitation greatly hinders the practicality of utilizing a solely coaxial cable system for bidirectional high-speed data transmission. Coaxial cables are prone to picking up noise emanating from motors, CB radios, microwave ovens, and other household appliances. Notably, activities such as operating ham radios or VCRs can generate significant disruptions to upstream data. Only HFC plant upgraded CATV systems possess the capability for efficient two-way high-speed data transfer.

The use of optical fiber enhances upstream data transmission and reduces noise. Additionally, it allows signals to be transmitted over longer distances without the need for amplification. To transmit data over the HFC network, laser transmitters convert signals from the head-end into optical signals. At each cable node, laser receivers convert the signals back so they can be transmitted over coaxial cable plant, which is configured in a tree-and-branch layout to reach each household. The availability of high-quality two-way HFC plant is crucial for implementing two-way cable data services, although the upgrading cost is significant.

It costs a cable company $200 - $250 per home to upgrade to HFC plant (Clark, 1999). Some cable companies that have not upgraded to HFC are offering cable modems that use the RF coaxial cable spectrum for fast downstream transmission and a traditional dial-up modem to handle upstream communications over the public telephone network. However, telephone-return modems do not provide some key benefits available with two-way cable modems, such as ultra-fast upstream speeds, constant connectivity, and not tying up a subscriber's telephone line. The Cable Modem Termination System (CMTS) is the central device for connecting the cable TV network to the Internet.

The CMTS resides at the cable head-end. All the traffic to and from the cable modems in a cable data network travel through the CMTS.

The CMTS connects to an IP router that sends and receives the data from the rest of the Internet. It interprets the data it receives from individual customers and tracks their offered services. It also modulates the Internet data for specific subscribers. Some Cable Modem Termination Systems allow the MSO to create different service packages based on bandwidth needs (Clark, 1999).

For instance, the CMTS can program a business service to both receive and transmit with high bandwidth, while limiting a residential user to high bandwidth downstream traffic and low bandwidth upstream traffic. The architecture of cable data networks is comparable to Ethernet Local Area Networks (LANs) (Halfhill, 1996). Present cable modem systems employ the Ethernet frame format for upstream and downstream transmissions. In essence, cable operators are constructing some of the largest intranets worldwide.

Cable operators prioritize high-speed intranet access over direct Internet access due to the principle that a network connection's speed is limited by its slowest component. Most multi-system operators (MSOs) connect their head-end to the Internet using a T1 line, which offers a data rate of 1.5 Mbps – considerably slower than a cable modem capable of delivering up to 30 Mbps (Brownstein, 1997). However, the speed of the Internet is ultimately determined by the slowest server it accesses. Thus, even with a 1.5 Mbps T1 connection, its effectiveness diminishes if attempting to access content hosted on a Web server connected via a 56-Kbps line. Consequently, the gateway to the Internet and the Internet itself are typically the main

bottlenecks for Internet traffic in a cable network system.

The cable companies propose solving this issue by bringing the Internet content closer to the subscriber. The cable operator's server stores many well-liked websites. Therefore, when a cable modem subscriber attempts to access a popular webpage, they will be redirected at high speeds to the server in the head-end. However, if a site is not stored in the cache, the head-end server must search for it on the busy Internet, similar to how a traditional ISP's server operates.

Cable modem users can expect high speeds (multiple MBit/sec) within their local cable network. However, when accessing the internet, data transfer rates may decrease significantly. Like LANs, cable modem systems rely on a shared access platform (Ostergard, 1999). All subscribers connected to a cable loop share the available bandwidth with the head-end. The entire local cable loop utilizes the same cable with a total bandwidth capacity of about 30 Mbps. Consequently, as more individuals connect cable modems, the same amount of bandwidth will be divided among more users.

There are concerns about the performance of cable modem users as more subscribers join the network. Cable operators can increase bandwidth capacity in cases of congestion by using two methods. The first method is to allocate an additional 6 MHz video channel for high-speed data, effectively doubling downstream bandwidth for users. The second method involves extending fiber-optic lines deeper into local communities to subdivide the physical cable network.

This reduces the number of cable modems served by each node segment, and thus, increases the amount of bandwidth available to subscribers. Based on bandwidth alone, it would seem that 200 cable modem subscribers sharing a

27-Mbps connection would each get approximately 135 Kbps of throughput, which is not much better than a 128-Kbps ISDN connection (Salent, 1999). However, unlike circuit-switched telephone networks where a caller is allocated a dedicated connection, cable modem users do not occupy a fixed amount of bandwidth during their online session. Instead, they share the network with other active users and use the network's resources only when they actually send or receive data in quick bursts. So instead of 200 cable online users each being allocated 150 Kbps, each user is able to use all the bandwidth available during the short period of time they need to download their data packets.

The current interconnection used between the cable modem connect and the subscriber’s PC is another bottleneck in cable data networking. In the subscriber’s home, a splitter is utilized to split the coax cable into two lines: one for the TV set and one for the cable modem. Cable modems, which are external devices, connect to the coax cable through a standard “F” port connector (Barnes, 1997).
To connect the cable modem to the PC, Ethernet10Base-T twisted-pair wiring and RJ-45 connectors are employed. The twisted pair wiring from the cable modem is connected to the RJ-45 jack of a 10Base-T Ethernet card that has been installed in the subscriber’s PC.

Although cable modems can receive data at speeds up to 30 Mbps, the PC's Ethernet interface limits its speed. Ethernet has a theoretical speed of 10 Mbps but typically operates at a slower maximum of 4 Mbps (Barnes, 1997). Cable operators often need to install an Ethernet card when connecting a new customer for cable modem service, as most

home computers do not have one installed. This seemingly straightforward procedure surprisingly becomes a significant bottleneck in the cable modem installation process.

First, the user's computer must have an available ISA or PCI card slot for the Ethernet adapter. Additionally, configuring the card installation within the operating system settings is often necessary to prevent conflicts with other hardware devices. This complexity often requires a specialized computer technician to handle the installation, which can take more than 20 minutes per subscriber. Furthermore, the need to open each customer's PC to install the hardware poses a potential liability for the cable operator. In order to avoid the headaches associated with Ethernet card installations, cable operators have sought an alternate approach. They have found a solution in a device called a dongle, which is essentially a Universal Serial Bus (USB) adapter.

USB, an abbreviation for Universal Serial Bus, is a technology that enables the connection of peripheral devices like modems, keyboards, printers, and scanners to computers (Van Matre, 1999). USB ports serve as external interfaces, eliminating the need to open the computer for device installation. The development of external USB modems and internal PCI modem cards is currently underway. It is worth noting that cable modems can receive data at a much faster rate than they can send it.

Cable modem manufacturers have created modems that utilize less than an entire 6 MHz carrier channel for upstream traffic. Usually, upstream data traffic relies on 2 MHz wide bands. Cable TV networks utilize advanced digital modulation techniques to enhance the amount of data that can be transmitted. The technique commonly used for sending data downstream over a coaxial-only cable network is called

64-state quadrature amplitude modulation (64 QAM). By implementing 64 QAM transmission technology, a single downstream 6 MHz television channel has the capability to support up to 27 Mbps of downstream data throughput from the cable head-end.

HFC networks have the capability to implement 256 QAM, which can support downstream data throughput of 36 Mbps. However, both 64QAM and 256 QAM are susceptible to interference, making them unsuitable for supporting noisy upstream transmissions. For sending data upstream over coaxial cable networks, Quadrature Phase-Shift Keying (QPSK) is used as a digital frequency modulation technique. QPSK is a good choice for sending data upstream in cable data networks because it is resistant to noise. The speed of upstream channels depends on the allocated amount of cable RF spectrum and can range from 500 Kbps to 10 Mbps. Modulation techniques such as 16 QAM or QPSK are used, with 16QAM being the faster of the two transfer methods (Salent, 1999). Traffic from upstream cable modems is always sent in bursts.

Each modem sends upstream bursts in time slots categorized as reserved, contention, or ranging slots. A reserved slot is exclusively assigned to a specific modem, preventing other modems from transmitting during that time. The CMTS uses a bandwidth allocation algorithm to allocate reserved time slots to the cable modems it controls.

Typically, reserved slots are utilized for lengthier data transmissions (Ostergard, 1998). On the other hand, contention time slots are available for all cable modems to transmit their data. When two cable modems attempt to transfer data at the same time in a contention slot, their packets collide and the transmitted data is lost. The collision is detected by the CMTS, which

then signals that no data was received. Consequently, each cable modem attempts to retransmit the data after a random duration of waiting.

The process of ranging automatically adjusts transmit levels and time offsets of individual cable modems. This ensures that bursts from different modems align properly in time slots and are received at the same power level at the CMTS. Having a consistent power level for bursts at the CMTS helps with collision detection. If two cable modems transmit simultaneously but one has a much weaker signal, the CMTS will only detect the stronger signal and assume there was no collision.

If the upstream signals that collide have the same strength, they will both be detected by the CMTS as garbled. This indicates a collision occurred and the CMTS will instruct the cable modems to retransmit their packets (Ostergard, 1998). Ranging slots are used to account for the variations in physical distance between the CMTS and each cable modem. The extensive coverage of a cable data network brings challenges due to transmission delay experienced by users near the head-end compared to those farther away. To overcome cable losses and delays caused by distance, the CMTS performs ranging. This allows each cable modem to assess its time delay in transmitting to the head-end.

Large CATV networks may encounter significant delays in the millisecond range, which are compensated by the ranging protocol. This protocol adjusts the "clock" of each cable modem to account for the delay, either by moving it forward or backward. The CMTS periodically performs ranging for each cable modem it controls, setting aside three consecutive time slots specifically for this purpose.

The cable modem is instructed by the

CMTS to transmit during the second time slot. The CMTS then measures the transmission time and provides a slight adjustment value to the cable modem's local clock. To ensure that other traffic does not disrupt the ranging burst, the two neighboring time slots are necessary (Ostergard, 1998). The major components of the cable modem include the Tuner, Demodulator, Burst Modulator, Media Access Control (MAC) Mechanism, Interface, and Central Processing Unit (CPU). External cable modems have an onboard CPU to handle instruction processing. There are also internal cable modems in development that will use the PC's CPU similar to internal dial-up modems.

The cable modem's tuner is directly connected to the CATV outlet. To facilitate two-way data transfer, the tuner requires a two-way diplexer to separate upstream and downstream traffic (Ostergard, 1999). The demodulator of the cable modem receives the downstream IF signal from the tuner and, as its name suggests, demodulates it. The demodulator consists of an A/D converter, a QAM64/256 demodulator, MPEG frame synchronization, and Reed Solomon error correction. Downstream data is framed according to the MPEG-TS (transport stream) specification.

The frame format for this specification consists of a 188/204 byte block that contains a single fixed sync byte at the beginning of each block. The size of the block is reduced from 204 bytes to 188 bytes using the Reed-Solomon error correction algorithm. This leaves 187 bytes for the MPEG header and payload (Ostergard, 1998).

To modulate the upstream data traffic, the cable modem's Burst Modulator is utilized. The Burst Modulator performs various tasks such as feeding the Tuner of the cable modem, encoding each downstream burst using Reed Solomon, modulating the designated upstream frequency using

QPSK or QAM16 modulation, and performing D/A conversion.

Furthermore, the output signal of the Burst Modulator is passed through a variable output amplifier to allow adjustment of the signal level to compensate for cable loss (Ostergard, 1998).

Both the upstream and downstream traffic pass through the Media Access Control (MAC) mechanism of the cable modem. The MAC mechanism is responsible for implementing MAC protocols under the direction of the Cable Modem Termination System (CMTS). These protocols are used to allocate the cable media to different cable modems in a cable data network. The MAC processes can be implemented either in hardware or a combination of software and hardware.

Both the CMTS and the MAC mechanism utilize MAC protocols for conducting ranging procedures, which help compensate for delays and losses in the cable media. The CMTS is responsible for interacting with the MAC mechanism in individual cable modems, assigning upstream frequencies and time slots for transmission. To manage data traffic on the cable network, the CMTS employs a specialized control channel. Upon activation, the cable modem scans its designated channels to locate the control channel, identified by its distinct header signal. Through the CMTS control channel, each subscriber's cable modem receives instructions on when and how it can transmit data, including the assigned frequency band and duration.

The data that flows through the MAC mechanism enters the computer interface of the cable modem, which typically uses 10Base-T Ethernet (Ostergard, 1998). Cable data systems consist of various technologies and standards. In the past, the first generation of cable modems used different proprietary protocols, making it impossible for CATV network operators to use cable modems from different vendors on the same

system. However, cable operators have always believed that for success in the high-speed data business, cable modems should be interoperable, affordable, and sold at retail just like telephone modems and data network interface cards. This would allow MSOs to avoid the capital burden of purchasing cable modems and leasing them to subscribers, while giving consumers the freedom to choose from a variety of manufacturers’ products. The IEEE 802.14 Cable TV Media Access Control (MAC) and Physical (PHY) Protocol Working Group was established in May 1994 by a group of vendors with the aim of developing international standards for data communications over cable.

Originally, the goal was to present a cable modem MAC and PHY standard to the IEEE in December 1995, but the deadline was extended to late 1997 (Van Matre, 1999). However, the cable operators were eager to enter the high-speed data industry as soon as possible and became impatient waiting for IEEE 802.14. Consequently, the cable operators joined forces to accelerate the standards process. In January 1996, cable operators Comcast, Cox, TCI, and Time Warner, operating under a limited partnership known as Multimedia Cable Network System Partners Ltd. (MCNS), released a request for proposals (RFP) to find a project management company that could research and publish a set of interface specifications for high-speed cable data services by the end of 1996 (Van Matre, 1999).

MCNS released its Data Over Cable System Interface Specification (DOCSIS) for cable modem products to vendors in March 1997. Despite the subsequent release of IEEE's standard, the cable operators had already embraced MCNS DOCSIS. Currently, more than 20 vendors have announced their intention to develop products based on the MCNS DOCSIS

standard. Additionally, the cable companies MediaOne (formerly Continental Cablevision), Rogers Cablesystems, and CableLabs also joined the DOCSIS RFP. Together, this coalition represents the majority of the North American cable industry, serving 85% of U.S.

cable subscribers and 70% of Canadian subscribers. While DOCSIS is the primary US cable data network standard, it has not yet received formal certification from any independent standards organization. CableLabs now manages the DOCSIS requirements, and they have implemented a certification program to ensure vendor equipment compliance and interoperability with the DOCSIS standard. In late 1998, limited shipments of standardized DOCSIS cable modems began, with broader availability expected in early 1999.

No major vendors are currently developing modems that follow the original IEEE standard (Van Matre, 1999). The cable modem specifications suggested by IEEE 802.14 and MCNS differ because each organization has its own priorities. IEEE 802.14 is driven by vendors and aims to establish a robust standard using advanced technology. On the contrary, the MSO members of MCNS are more focused on reducing product costs and were in a rush to enter the high-speed data market.

MCNS aimed to minimize technical complexity and develop a suitable technology solution for its members' requirements (Van Matre, 1999). Under the MCNS DOCSIS specifications, three layers from the International Organization for Standardization’s (ISO) 7 Layer Open System Interconnect (OSI) Reference Model are utilized to enable transparent transfer of Internet Protocol messages across a cable system. These layers include the Network Layer, Data Link Layer, and the Physical Layer. The functions of each layer are explained below (Salent, 1999).

Network Layer: The Network Layer utilizes the Internet Protocol (IP) to seamlessly deliver IP traffic over the cable modem

platform to end-users.

Data Link Layer: The Data Link Layer is composed of three sublayers. The Logical Link Control (LLC) Sublayer adheres to Ethernet standards. The Link-Security Sublayer provides privacy, authorization, and authentication capabilities. The Media Access Control Sublayer facilitates cable system operation by supporting variable-length protocol data units (PDU).

The Physical Layer is responsible for defining the modulation format used for upstream and downstream communication. There is limited connection between the physical layer and higher layers, allowing for the integration of future physical layer technologies. Both the IEEE and MCNS specifications have similar modulation formats at the physical layer, specifically for digital signals. The 802.14 specification supports the ITU's J.83 Annex A, B, and C standards for 64/256 QAM modulation. This enables a maximum downstream throughput of 36 Mbps per 6 MHz television channel. Annex A corresponds to the European DVB/DAVIC standard, Annex B to the North American standard supported by MCNS, and Annex C to the Japanese specification. The proposed 802.14 upstream modulation standard aligns with MCNS, using QPSK and 16QAM (Van Matre, 1999).

The MAC sublayer is responsible for various requirements in cable modem networks, such as collision detection and retransmission, timing and synchronization, bandwidth allocation, error handling, and registering new cable modems. In this context, 802.14 standardizes the use of Asynchronous Transfer Mode (ATM) from the head-end to the cable modem. On the other hand, MCNS adopts a different approach using variable-length packets that prioritize Internet Protocol (IP) traffic delivery. Despite the difference in packet structure, both solutions require a 10Base-T Ethernet connection between the cable modem and the PC. The IEEE committee decided on ATM for its ability to provide quality of

service (QoS) guarantees for integrated video, voice, and data traffic.