Cisco Certification Exam Review:
Broadband And Remote Access Solutions
By Chris Bryant, CCIE #12933
Asymmetric DSL (ADSL) assumes users will do much more downloading that uploading, so it assigns much of the available bandwidth to downloading.
ADSL allows users to use the same phone line for simultaneous phone calls and Internet usage.
On powerup, a cable modem must scan for the proper Radio Frequency (RF) and achieve QAM Lock on that signal.
After receiving three maintenance messages from the Cable Modem Termination System (CMTS), the cable modem will have L1 and L2 connectivity.
For L3 connectivity, the cable modem will send a DHCP Request to the CMTS. The CMTS will answer with a DHCP Reply as well as the name of a necessary DOCSIS configuration file and the location of the TFTP server containing that config file.
And what is DOCSIS? The Data Over Cable Service Interface Specification. DOCSIS defines the bandwidths for data signals emanating from the cable provider.
The maintenance messages sent by the CMTS are:
SYNC, a time synchronization message
UCD (Upstream Channel Descriptor), defining parameters of communication with upstream devices
MAP (Bandwidth Allocation Map), telling the modem when it can transmit and for how long
After the cable modem acquires the DOCSIS configuration file from the TFTP server, it will send a Registration Request to the CMTS. If the CMTS sends a Registration Response in turn, the cable modem can now forward data.
G.lite is "splitterless ADSL", since no splitter is needed at the customer location. The drawback is speed - G.lite maxes out at 1.5 MBPS download and 512 KBPS upload. It's not as fast as "pure" ADSL, but it's still a lot better than dialup.
ADSL has two standards. CAP, the older standard, divides the line into three channels - one for voice, one for upstream traffic, and one for downstream traffic. The more-advanced standard, DMT, uses 250 channels to carry data.
ADSL has a distance limitation of 18,000 feet.
Symmetric DSL (SDSL) gives the end user the same amount of upload and download capability, but the phone cannot be used while someone is using that capability.
Very High Bit-Rate DSL (VDSL) can deliver up to 52 MBPS over copper, but there's a severe distance limitation - 4000 feet. Cisco's Long Range Ethernet technology is built on VDSL.
PPPoE is a more secure choice than RFC 1483 Bridging, but PPPoE still uses bridging. PPPoA is a pure routing solution.
The 800 series also makes an excellent DHCP server. Here's a sample configuration:
ip dhcp pool CLIENT_ADDRESS_POOL
network 20.2.2.0 0.255.255.255
domain-name thebryantadvantage.com
default-router 20.1.1.1
netbios-name-server 20.1.1.2
lease 1 0 0 (days, hours, minutes)
To your success,
Chris Bryant
CCIE #12933
chris@thebryantadvantage.com
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