Hardware Reference
In-Depth Information
ranging
. It is important for the modem to know its distance to accommodate the
way the upstream channels operate and to get the timing right. The channels are
divided in time in
minislots
. Each upstream packet must fit in one or more con-
secutive minislots. The headend announces the start of a new round of minislots
periodically, but the starting gun is not heard at all modems simultaneously due to
the propagation time down the cable. By knowing how far it is from the headend,
each modem can compute how long ago the first minislot really started. Minislot
length is network dependent. A typical payload is 8 bytes.
During initialization, the headend also assigns each modem to a minislot to use
for requesting upstream bandwidth. As a rule, multiple modems will be assigned
the same minislot, which leads to contention. When a computer wants to send a
packet, it transfers the packet to the modem, which then requests the necessary
number of minislots for it. If the request is accepted, the headend puts an acknowl-
edgement on the downstream channel telling the modem which minislots have
been reserved for its packet. The packet is then sent, starting in the minislot allo-
cated to it. Additional packets can be requested using a field in the header.
On the other hand, if there is contention for the request minislot, there will be
no acknowledgement and the modem just waits a random time and tries again.
After each successive failure, the randomization time is doubled to spread out the
load when there is heavy traffic.
The downstream channels are managed differently from the upstream chan-
nels. For one thing, there is only one sender (the headend) so there is no con-
tention and no need for minislots, which is actually just time-division statistical
multiplexing. For another, the traffic downstream is usually much larger than
upstream, so a fixed packet size of 204 bytes is used. Part of that is a Reed-
Solomon error-correcting code and some other overhead, leaving a user payload of
184 bytes. These numbers were chosen for compatibility with digital television
using MPEG-2, so the TV and downstream data channels are formatted the same
way. Logically, the connections are as depicted in Fig. 2-42.
Getting back to modem initialization, once the modem has completed ranging
and gotten its upstream channel, downstream channel, and minislot assignments, it
is free to start sending packets. These packets go to the headend, which relays
them over a dedicated channel to the cable company's main office and then to the
ISP (which may be the cable company itself). The first packet is one to the ISP re-
questing a network address (technically, an IP address), which is dynamically as-
signed. It also requests and gets an accurate time of day.
The next step involves security. Since cable is a shared medium, anybody who
wants to go to the trouble to do so can read all the traffic zipping past him. To pre-
vent everyone from snooping on their neighbors (literally), all traffic is encrypted
in both directions. Part of the initialization procedure involves establishing en-
cryption keys. At first one might think that having two strangers, the headend and
the modem, establish a secret key in broad daylight with thousands of people
watching would be impossible to accomplish. Turns out it is not, but the technique
