Layer 1 and Layer 2 between the Home and the Head

Cable TV systems originally were built to send TV video and audio signals to lots of places, with no need to receive a signal back. In other words, the idea of having someone's TV send some information back to the cable company was not even under consideration. Because the original CATV architecture allowed for sending signals from the head end outward, and the capability for two-way communication was added later, data over cable standards treats data going toward the home differently than data coming from the home. In fact, CATV terminology refers to the data going toward the home as downstream data, and data from the home as upstream data.

Downstream data uses standards that are consistent with some of the standards for sending digital video over cable. In fact, you can think of the downstream data as being sent over another TV channel. For downstream data, the data over cable standards takes advantage of the fact that the signals are broadcast to all subscribers in a section of the cable plant. Just like the TV channels' signals go to every home, the signals for the downstream data go to every home. In many ways, the concepts are similar to an Ethernet broadcast domain: When a broadcast Ethernet frame is sent, everyone in the broadcast domain receives the frame. With downstream cable transmissions, not just broadcast frames, but all data, is broadcast to all receivers. Yes, the data that you receive over the web actually could be captured with a network analysis tool by one of your neighbors.

Because every home in a part of the cable network receives the same data channel, some form of addresses must be used so that only the correct device tries to process incoming data. For instance, your PC does not need to process any data being sent to your neighbor's PC. So, CATV standards call for the use of a data-link protocol called Multimedia Cable Network Systems (MCNS) MAC. (You might remember that MAC stands for Media Access Control.) MCNS is similar to Ethernet's MAC, as defined in the IEEE 802.3 specification, including the use of Ethernet MAC addresses. So, although all downstream data is sent to all drops in the cable system, only those with a cable modem know that data has been received, and only the PCs with the correct MAC address process the data.

MCNS also defines the physical encoding details. MCNS calls for the use of a modulation method called quadrature amplitude modulation (QAM). Two options can be used for downstream data, one called QAM-64 and the other called QAM-256. QAM-64 represents 6 bits per baud, and QAM-256 represents 8 bits per baud.

Table 15-6 summarizes some of the key reference information about downstream data over cable.

Table 15-6 Downstream Data over Cable: Interesting Facts

Downstream Rate

OSI Layer 1

QAM-64 and QAM-256 encoding

OSI Layer 2

MCNS MAC and IEEE 802.2 LLC

Multiplexing used

Frequency-division multiplexing

Speed

30 to 40 Mbps

Upstream Data

The upstream data channel uses a totally separate frequency range than the downstream channel, so no collisions occur between downstream and upstream data. However, all upstream data from multiple cable subscribers does share the same frequency range—the same channel, essentially—so collisions can occur between data sent toward the Internet by the different home users.

Noticing that a collision has occurred in an upstream cable channel is much more difficult than with an Ethernet. Cables inside the CATV cable plant might be miles long, which means that a device would have to wait longer for the electrical signal from a collision to return. So, the CSMA/CD algorithm used by Ethernet does not work well on the upstream channel. Instead, MCNS defines the use of a multiplexing method called time-division multiple access (TDMA), in which each home user is granted regular time periods during which to send upstream data. These time slots happen multiple times per second. By using TDMA, most collisions can be avoided.

The upstream channel uses the same data-link protocols as the downstream channel, with MAC addressing, but it uses different modulation schemes. The upstream channel uses quaternary phase-shift keying (QPSK) or QAM-16. QPSK modulates the signal using phase shifts, while QAM uses amplitude modulation.

Both the downstream and upstream channels compete with other users for the use of the channel. So, as more subscribers are added, the actual throughput of the connection actually can slow down.

Table 15-7 summarizes some of the key points about the upstream data channel. Table 15-7 Upstream Data over Cable: Interesting Facts

Downstream Rate

OSI Layer 1

QPSK and QAM-16

OSI Layer 2

MCNS MAC and IEEE 802.2 LLC

Multiplexing used

Time-division multiple access (TDMA)

Speed

320 kbps to 10 Mbps

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