Home Connectivity: Broadband + DSL

Upstream traffic is traffic going from a host device to the cable company and/or ISP, and downstream traffic is traffic going from the ISP to the host device

Broadband Delivery Options


Cable Modem

The end user’s connection is carried through a preexisting cable TV connection, enabling many cable companies to offer “do-it-yourself” broadband connectivity kits. when a cable modem is powered on or reloaded, it begins to look for a signal from the service provider as it boots up.

When that signal is found, the cable modem synchronizes its timing with the downstream provider device, and the connection procedure continues from there.

The end user can simultaneously access the Internet while watching cable television due to the Data Over Cable Service Interface Specification (DOCSIS) standards. (Wiki for more details)

By using the specific bandwidths outlined by DOCSIS, the same cable can be used to deliver cable television service, transmit data to the client, and receive data from the client simultaneously.

Modulation Standards

  • National Television Standards Committee (NTSC) is used in primarily in North American and Japan.
  • Phase Alternating Line (PAL) is used, well, almost everywhere else.
  • Sequential Color Memory (SECAM) is used primarily in France, Africa, and Eastern Europe.


One step up from the cable modem, we have Digital Subscriber Line, or DSL. DSL uses a preexisting phone line for broadband delivery. There are several different kinds of DSL as below:

Asymmetrical DSL (ADSL) works under the assumption that the user will download more information than they send, and for the average Internet user, that’s a safe assumption. The connection speed from the provider to the user is going to be 3 – 4 times faster than the speed from the user to the provider.

For example, an ADSL connection of 512 kbps will give the user 384 KBPS download capabilities, but only 128 KBPS uploading capability. ADSL actually offers download speeds of up to 8 MBPS and upload speeds of up to 1 MBPS or 1.5 MBPS (ADSL 1?)

ADSL is susceptible to that 18,000-feet distance limitation.


The Original High Data-Rate DSL (HDSL) has the capability to deliver T1 (1.544 MBPS) or E1 (2.048 MBPS) speed over copper, and this service is symmetrical – the download and upload capabilities are the same. Unlike ADSL, you cannot use the phone while you’re using the HDSL connection.

Second-generation HDSL (HDSL2) does allow for Voice Over IP (VOIP) and video to be carried along with data.

Rate-Adaptive DSL (RADSL) is just what it sounds like – the software calculates the maximum download and upload speeds on the customer’s pre-existing phone line and dynamically adjusts those rates.

G.SHDSL provides symmetric tranmission for upstream and downstream data rates of anywhere from 192 kbps to 2.3 MBPS. Estimates of G.SHDSL’s maximum distance range from 20,000 feet to 28,000 feet, depending on whose documentation you’re reading.

DSL Drawbacks

As mentioned earlier, there is an 18,000-foot limitation on DSL services. However, attenuation – the gradual weakening of a signal – occurs before the actual distance limitation is reached.

A bad splice or overall corrosion can lead to an impedance mismatch. As with attenuation, the signal isn’t totally lost, but it is degraded. Those impedance mismatches can be introduced by using wires with different wire gauges. The American wire gauge (sometimes called the Brown & Sharp wire gauge, according to Wikipedia) refers to a standardized system of measuring wire and cable thickness.

Signal interference can come from “inside” and “outside” our network as well. The “inside” interference can result from crosstalk, the result of a signal “crossing over” and interfering with another signal.

The “outside” source, I kid you not, is AM radio. Certain AM frequencies can interfere with the DSL signals.

Data Transport Over ATM

There’s an unusual type of Asynchronous Transfer Mode (ATM) switch as use in this type of network, a DSLAM. DSLAMs are just ATM switches with DSL cards in them.

When it comes to transporting data over this setup, we’ve got three choices:

  1. PPP over Ethernet (PPPoE)
  2. PPP over ATM (PPPoA)
  3. RFC 1483/2684 Bridging

RFC 1483 Bridging has some advantages and some serious drawbacks.


  • Easy to set up, install, and configure
  • Offers multiprotocol support
  • Excellent for single-user environments


  • Uses a lot of broadcasts, which can quickly use most or all available bandwidth.
  • Not a scalable solution.
  • Wide open to intruder attacks, including ARP spoofing, IP address hijacking, and broadcast attacks


More likely, you’ll use Point-to-Point Protocol over Ethernet (PPPoE). Defined in RFC 2516, this process involves bridging an Ethernet frame from the host PC to an aggregation router such as the Cisco 6400 series.

However, the Ethernet frame will actually contain a PPP frame, enabling a PPP session to be built between the host device and the aggregation router. While the PAP / CHAP PPP option is there, CHAP will typically be used due to its encryption options.

There are actually two encapsulations taking place. First, the host data is placed into a PPP frame, and then that PPP frame is placed into an Ethernet frame. Finally, the “frame inside a frame” is ready to transmit.

At the very beginning of the PPPoE process, the host device will perform Discovery – and what the host needs to discover is the MAC address of the PPPoE server. That server will be the aggregation router. This establishes the client-server relationship, identified by a PPPoE SESSION_ID value. Once Discovery has concluded, the PPP process can continue as it normally would over an ISDN link.

*Refer to the study guide for a real world example of PPPoE and DSLAM configuration.

*This is an excellent training video from the CCNP ROUTE course courtesy of Chris Bryant. I think this is a bit of a bonus as when I took CCNP ROUTE I don’t recall anything on any of the below. Very interesting all the same!