another CM would receive.
Content
2.3 Deploying Cable System Technology
2.3.5 Provisioning a Cable Modem Data and TV
signals travel down the cable on different channels. Downstream
data travels to a user on a 6-MHz channel. On the cable, the
data looks just like a single TV channel and takes up the same
amount of space. When the user sends data back up the cable,
only 2 MHz are used. The reason the downstream and upstream
channel sizes are different is because it is assumed that most
users download more than they upload. Putting both upstream and
downstream data on the cable system requires two types of
equipment: - A CM on the customer end
- A CMTS at
the cable operator’s end
Between these two
components, all the computer networking, security, and
management of Internet access over cable television is
implemented. There are several steps used to provision a CM.
The CMTS at the headend must have operational provisioning
servers, such as DHCP and TFTP servers. DOCSIS defines the
initialization and registration steps. CMs are designed and
coded to follow specific steps : Step 1 Downstream
setup: The CM powers up and then scans and locks the
downstream path for the appropriate RF data channel (frequency)
that will be used for the physical and data link layers to be
established. Step 2 Upstream setup: The CM
listens to the management messages received through the
downstream path. The messages include information on how,
where, and when to communicate in the upstream path and are
used to establish the upstream physical and data link layers.
Step 3 Layers 1 and 2 establishment: The CM
communicates with CMTS to establish physical and data link
layers. Step 4 Obtaining an IP address:
After establishing Layer 1 and Layer 2 connectivity with the
CMTS, the CM requests IP configuration parameter information
(IP address, default gateway, and TFTP server) from the DHCP
server. Step 5 Getting the DOCSIS
configuration: The CM requests a DOCSIS configuration file
from the TFTP server. A DOCSIS configuration file is an ASCII
file created by special DOCSIS editors and includes settings,
such as downstream channel identification, class of service
(CoS) settings, baseline privacy settings, general operational
settings, network management information, and vendor-specific
settings. Step 6 Register QoS with
CMTS: The CM registers, negotiates, and ensures QoS
settings with the CMTS. Step 7 IP network
initialization: When the CM initialization and registration
is complete, the PC-based network initialization takes place.
That is, the PC requests its own IP configuration parameters
from the DHCP server. If multiple PC connections behind the CM
are required, a router can be used. A common scenario is for
the router to obtain a public IP address from the DHCP server
of the cable provider. The home router also performs Network
Address Translation (NAT), Port Address Translation (PAT) and
serves as a DHCP server for the PCs connected behind the
router.
Content 2.4 Describing DSL
Technology 2.4.1 What is DSL Several years
ago, research by Bell Labs identified that a typical voice
conversation over a local loop only required the use of
bandwidth of 300 Hz to 3 kHz. For many years, the telephone
networks did not use the bandwidth beyond 3 kHz. Advances in
technology allowed DSL to use the additional bandwidth above 3
kHz up to 1 MHz to deliver high-speed data services over
ordinary copper lines. As an example, asymmetric DSL (ADSL)
uses a frequency range from approximately 20 kHz to 1 MHz.
Fortunately, only relatively small changes to existing
telephone company infrastructure are required to deliver
high-bandwidth data rates to subscribers. Figure shows a
representation of bandwidth space allocation on a copper wire
for ADSL. The green area represents the space used by POTS,
while the other colored spaces represent the space used by the
upstream and downstream DSL signals.DSL is not a complete
end-to-end solution but rather a physical layer transmission
technology similar to dial, cable, or wireless technologies.
Service providers deploy DSL connections in the last step of a
local telephone network, the local loop. The connection is set
up between a pair of modems on either end of a copper wire that
extends between the customer premises equipment (CPE) and the
DSL access multiplexer (DSLAM). A DSLAM is the device located
at the central office (CO) of the provider and concentrates
connections from multiple DSL subscribers.Figure shows the key
equipment that you need to provide a DSL connection to a SOHO.
The two key components are the DSL transceiver and the
DSLAM.The DSL transceiver connects the teleworker’s computer to
the DSL line. Usually the transceiver is a DSL modem connected
to the teleworkers computer using a USB or Ethernet cable.
Newer DSL transceivers can be built into small routers with
10/100 switch ports suitable for home office use.The DSLAM is
at the central office and combines individual DSL connections
from users into one high capacity link to the Internet. The
advantage that DSL has over cable technology is that DSL is not
a shared medium. Each user has a separate direct connection to
the DSLAM. Adding users does not impede performance unless the
DSLAM Internet connection on the other side becomes saturated.
Content 2.4 Describing DSL Technology
2.4.2 How Does DSL Work? DSL transmission is either
downstream or upstream based on the direction of the
transmission. Downstream transmission is from a CO toward a
subscriber, and upstream transmission is from a subscriber
toward a CO. DSL types fall into two major categories, taking
into account downstream and upstream speeds:
- Symmetrical DSL: Upstream and downstream speeds are
the same.
- Asymmetrical DSL: Upstream and
downstream speeds are different. Downstream speed is typically
higher than upstream speed.
The term xDSL covers a
number of DSL variations, such as Asymmetric DSL (ADSL),
high-data-rate DSL (HDSL), Rate Adaptive DSL (RADSL), symmetric
DSL (SDSL), ISDN DSL (IDSL), and very-high-data-rate DSL
(VDSL).DSL types that do not use the voice frequency band allow
DSL lines to carry both data and voice signals simultaneously
(for example, ADSL and VDSL types), while other DSL types
occupying the complete frequency range can carry data only (for
example, SDSL and IDSL types). Data service provided by a DSL
connection is always on. The data rate that DSL service can
provide depends on the distance between the subscriber and the
CO. The shorter the distance: the higher the bandwidth
available. If a subscriber is close enough to a CO offering DSL
service, the subscriber might be able to receive data at rates
of up to 6.1 Mbps out of a theoretical maximum of 8.448 Mbps.
Content 2.4 Describing DSL
Technology 2.4.3 DSL Variants The following
properties differentiate DSL variants: - Nature:
The nature of DSL is the relationship between downstream and
upstream speeds. Symmetrical DSL has the same speed in both
directions, while asymmetric DSL has different downstream and
upstream speeds.
- Maximum data rate: This
defines the maximum speed that you can deploy with a certain
type of DSL.
- Line coding technology: This
describes the technique used to represent digital signals
transported over a copper twisted pair so that the receiver can
interpret the signals accurately.
- Data and voice
support: Depending on the usage of the available frequency
spectrum, certain DSL types support data and voice
simultaneously while other types do not.
- Maximum
distance: This describes the maximum distance that a
certain type of DSL connection can span.
Figure
shows some examples of DSL variants. DSL types include ADSL,
RADSL, VDSL, IDSL, SDSL, HDSL, and Symmetrical High-Data-Rate