Cable modems receive data much faster than they can send it. Cable modem manufacturers have designed their modems to use less than a full 6 MHz carrier channel for upstream traffic. Typically 2 MHz wide bands are used for upstream data traffic. Cable TV networks transfer data using sophisticated digital modulation schemes which greatly increase the amount of data that can be sent. 64-state quadrature amplitude modulation (64 QAM) is digital modulation technique used for sending data downstream over a coaxial-only cable network. A single downstream 6 MHz television channel may support up to 27 Mbps of downstream data throughput from the cable head-end using 64 QAM transmission technology. HFC networks are able to implement 256 QAM, which supports 36 Mbps of downstream data throughput. However, 64QAM and 256 QAM are susceptible to interfering signals, making them unable to support noisy upstream transmissions. Quadrature Phase-Shift Keying (QPSK) is a digital frequency modulation technique used for sending data upstream over coaxial cable networks. QPSK is suitable for sending data upstream over a cable data network because it is fairly resistant to noise. Depending on the amount of cable RF spectrum allocated, upstream channels may deliver 500 Kbps to 10 Mbps, using 16 QAM or QPSK modulation techniques, with 16QAM being the fastest transfer method of the two (Salent, 1999).
Upstream cable modem traffic is always sent in bursts. Each modem transmits upstream bursts in time slots. These time slots can be designated as reserved, contention, or ranging slots. As the name implies, a reserved slot is a time slot that is reserved to a particular cable modem. No other cable modem is allowed to transmit in this reserved time slot. The CMTS allocates the reserved time slots to the various cable modems under its control through a bandwidth allocation algorithm. Reserved slots are normally used for longer data transmissions (Ostergard, 1998).
Contention time slots are open for all cable modems to transmit in. If two cable modems attempt to transfer simultaneously in the same contention slot, their packets collide and the data is lost. The CMTS detects the collision and signals that no data was received, which makes the each cable modems try to retransmit the data after waiting a random length of time.
Ranging is the process of automatically adjusting transmit levels and time offsets of individual cable modems. Ranging is performed to insure that bursts coming from different modems line up in the right time slots and are received at the same power level at the CMTS. A uniform power level for bursts reaching the CMTS facilitates collision detection. If two cable modems transmit at the same time, but one is much weaker than the other one, the CMTS will only detect the strong signal and assume that no collision took place. If the two colliding upstream signals are the same strength, they will both be detected by the CMTS as garbled. The CMTS will then know that a collision took place and will instruct the cable modems to retransmit their packets (Ostergard, 1998).
Ranging slots are also used to compensate for the differences in physical distance between the CMTS and each of the cable modems. The large geographic reach of a cable data network poses special problems as a result of the transmission delay between users close to head-end versus users at a distance from cable head-end. To compensate for cable losses and delay as a result of distance, the CMTS performs ranging, which allows each cable modem to assess its time delay in transmitting to the head-end. Large CATV networks can experience long delays in the millisecond range. The ranging protocol compensates for these delays by moving the “clock” of each cable modem forward or backward to make up for they delay. Ranging is performed periodically by the CMTS for each cable modem under its control. Three consecutive time slots are set aside for ranging. The CMTS commands the cable modem to transmit in the second time slot. The CMTS then measures the transmission time and gives the cable modem a small positive or negative correction value for its local clock. The two time slots on either side of the second time slot are required to insure that other traffic does not interfere with the ranging burst (Ostergard, 1998).
The cable modem itself is comprised of the following major components; the Tuner, the Demodulator, the Burst Modulator, the Media Access Control (MAC) Mechanism, the Interface, and the Central Processing Unit (CPU). External cable modems have an on-board CPU to handle instruction processing. Internal cable modems are being developed that will use the PC’s CPU much like the way internal dial-up modems do. The cable modem’s tuner connects directly into the CATV outlet. For two-way data transfer, a tuner must have a two-way diplexer to break out the upstream and downstream traffic (Ostergard, 1999).
The Cable Modem’s Demodulator receives the downstream IF signal from the tuner and, as the name implies, demodulates it. The Demodulator is composed 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 is a 188/204 byte block, with a single fixed sync byte in front of each block. The Reed-Solomon error correction algorithm reduces the block size from 204 bytes to 188 bytes, which leaves 187 bytes for MPEG header and payload (Ostergard, 1998).
The upstream data traffic is modulated by the cable modem’s Burst Modulator. The Burst Modulator feeds the cable modem’s Tuner, performs Reed Solomon encoding of each downstream burst, performs QPSK or QAM16 modulation on the designated upstream frequency, and D/A conversion. The Burst Modulator’s output signal is fed through a variable output amplifier, so the signal level can be adjusted to compensate for cable loss(Ostergard, 1998).
Both the upstream and downstream traffic travels through the cable modem’s Media Access Control mechanism. The MAC mechanism’s functions are fairly complex. The MAC mechanism’s main purpose is to implement MAC protocols under the direction of the CMTS. MAC protocols are used to time-share the cable media among the various cable modems in a cable data network. The MAC processes can be implemented in hardware, or a combination of software and hardware. Both the CMTS and the MAC mechanism implement MAC protocols to perform ranging procedures to compensate for cable media delays and line losses. The CMTS also interfaces with the MAC mechanism in each cable modem to assign upstream frequencies and upstream time slots. The CMTS controls data traffic on the cable network through the use of a special control channel. When the cable modem is turned on, it scans all its assigned channels to locate the control channel, which can be identified by its unique header signal. The CMTS control channel tells each subscriber’s cable modem when it can transmit, on which frequency band, and for how long. The data that passes through the MAC mechanism goes into the computer interface of the cable modem, which is 10Base-T Ethernet for the majority of current cable modems (Ostergard, 1998).
A cable data system is comprised of many different technologies and standards. The first generation of cable modems used various proprietary protocols that made it impossible for the CATV network operators to use multiple vendors cable modems on the same system. Cable operators have long believed success in the high-speed data business would require that cable modems be interoperable, low-cost and sold at retail like telephone modems and data network interface cards. This way, MSOs could avoid the capital burden associated with purchasing cable modems and leasing them back to subscribers, and consumers would be able to choose products from a variety of manufacturers.
The Institute of Electronic and Electrical Engineering’s (IEEE) 802.14 Cable TV Media Access Control (MAC) and Physical (PHY) Protocol Working Group was formed in May 1994 by a number of vendors to develop international standards for data communications over cable. The original goal was to submit a cable modem MAC and PHY standard to the IEEE in December 1995, but the delivery date slipped to late 1997 (Van Matre, 1999).
The cable operators were anxious to get into the high-speed data business as soon as possible, and became impatient waiting IEEE 802.14. So, the cable operators combined their purchasing power to jump-start the standards process. In January 1996, cable operators Comcast, Cox, TCI, and Time Warner, operating under a limited partnership dubbed Multimedia Cable Network System Partners Ltd. (MCNS), issued a request for proposals (RFP) to retain a project management company to research and publish a set of interface specifications for high-speed cable data services by the end of the 1996 (Van Matre, 1999).
MCNS released its Data Over Cable System Interface Specification (DOCSIS) for cable modem products to vendors in March 1997. Afterwards, IEEE released its standard, but by that time, the cable operators had already wed themselves to the MCNS DOCSIS standard. To date, more than 20 vendors have announced plans to build products based on the MCNS DOCSIS standard. The cable companies MediaOne (formerly Continental Cablevision), Rogers Cablesystems, and CableLabs, also signed on to 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. Even though DOCSIS is the dominant US cable data network standard, it has yet to be formally certified by any independent standards body. The DOCSIS requirements are now managed by CableLabs. A CableLabs certification program administers vendor equipment compliance to the DOCSIS requirements and interoperability tests. Standardized DOCSIS cable modems started shipping in limited quantities in the third and fourth quarters of 1998 with wider availability expected in the first quarter of 1999. No major vendors are currently building modems based on the initial IEEE standard (Van Matre, 1999).
The differing cable modem specifications advocated by IEEE 802.14 and MCNS reflect the priorities of each organization. IEEE 802.14 is a vendor-driven group, and has focused on a creating a future-proof standard based on industrial-strength technology. The MSO members of MCNS, on the other hand, are far more concerned with minimizing product costs and were in an extreme hurry to get into the high-speed data market. To achieve its objectives, MCNS sought to minimize technical complexity and develop a technology solution that was adequate for its members’ needs (Van Matre, 1999).
Under the MCNS DOCSIS specifications, to enable transparent transfer of Internet Protocol messages across a cable system, three of the protocol layers of the International Organization for Standardization’s (ISO) 7 Layer Open System Interconnect (OSI) Reference Model are used. These three layers are the Network Layer, Data Link Layer and the Physical Layer. The functions of each layer are described below (Salent,1999).
Network Layer The Network Layer uses the Internet Protocol (IP), which enables IP traffic to be seamlessly delivered over the cable modem platform to end-users.
Data Link Layer The Data Link Layer is comprised of three sublayers: a Logical Link Control (LLC) Sublayer, which conforms to Ethernet standards, a Link-Security Sublayer that supports the basic needs of privacy, authorization, and authentication, and a Media Access Control Sublayer, suitable for cable system operation, that supports variable-length protocol data units (PDU).
Physical Layer The Physical Layer which defines the upstream and downstream modulation format. There is minimal coupling between physical and higher layers which accommodates the incorporation of future physical layer technologies.
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 standards 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 and 16QAM, virtually the same as MCNS (Van Matre, 1999).
The MAC sublayer provides the general requirements for many cable modem subscribers to share a single upstream data channel for transmission to the network. These requirements include collision detection and retransmission, timing and synchronization, bandwidth allocation to cable modems at the control of CMTS, error detection, error handling, and error recovery, as well as procedures for registering new cable modems.
For the MAC sublayer, 802.14 specified Asynchronous Transfer Mode (ATM) as its default solution from the head-end to the cable modem. MCNS went a different route, using a scheme based on variable-length packets that favors the delivery of Internet Protocol (IP) traffic. Although the MCNS MAC is based on packets and the IEEE specifies fixed ATM cells, both cable modem solutions specify a 10Base-T Ethernet connection from the cable modem to the PC (Van Matre, 1999).
IEEE 802.14 committee members say they chose ATM because it best provides the quality of service (QoS) 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.
Initially, cable operators were solely 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. However, QoS guarantees were added under DOCSIS version 1.1 (Van Matre, 1999)..
The Link-Security Sublayer insures the privacy of cable modem user data by encrypting link-layer data between cable modems and CMTS. Security is a major concern with cable modems because the total bandwidth is shared by all cable modems in a local loop. This means all downstream data is received by all the cable modems in a loop. Each cable modem uses the Ethernet frame format to filter out the data it needs from the downstream of data. The CMTS encrypts the payload data of link-layer frames transmitted on the cable network. The Security Association (SA) assigns a set of security parameters including keying data to a cable modem. All of the upstream transmissions from a cable modem travel across a single upstream data channel and are received by the CMTS. In the downstream data channel, the CMTS must select appropriate the appropriate SA parameters based on the destination address of the target cable modem. Baseline privacy employs the data encryption standard (DES) block cipher for encryption of user data. The encryption can be integrated directly within the MAC hardware and software interface.
Cable modem technology offers tremendous advantages. A cable modem user can get the performance of a T-1 line at a fraction of the cost. Current cable modem service connection speeds are much greater than that of a dial-up ISP at roughly the same price. Dial-up ISPs offer 56 Kbps connections for around $20 per month. Emerald Coast Cable TV, the MSO for the Fort Walton Beach, Florida area currently charges $30 a month for unlimited Internet access with a cable modem. A cable modem offers speeds between 500 Kbps to 1.5 Mbps. Even the low end of this range is an order of magnitude faster than a 56 Kbps connection. Cable modems currently retail for approximately $300, but the prices are forecast to drop rapidly. Emerald Coast Cable TV charges $15 a month to rent a cable modem.
In addition to their blazing speed, another advantage of cable modems is constant connectivity. Cable modems are online as soon as the computer is turned on. The cable modem user does not have to dial-in to begin an online session. There are no busy signals and tied up telephone lines like there are with dial-up modems. Another advantage of cable modem technology is that it has tremendous upgrade capacity. Twisted pair telephone lines have already used up a sizeable portion of their inherent bandwidth capacity (Halfhill, 1996). On the other hand, MSOs have already created a tremendous amount of shared bandwidth with their upgrades to HFC networks. Furthermore, as the number of cable modem users grows, and too many users try to share the available bandwidth, the cable operators have the capability to add more. Many MSOs have six optical fibers in their cable bundles and are only currently using two of them. The MSOs could “light up” these unused fibers and greatly increase the amount of bandwidth to be shared. Another option is to allocate additional 6 MHz channels for high-speed data. Still, another option for adding bandwidth is to subdivide the physical cable network by running fiber-optic lines deeper into neighborhoods. This reduces the number of cable modems served by each node segment, and thus, increases the amount of bandwidth available to subscribers (Medin, 1999).