8. Appendix: Miscellaneous

8.1. Links

8.2. Glossary

A dictionary of some of the jargon used in this Document, and in the telco and DSL industries.


Asymmetric Digital Subscriber Loop. "Asymmetric" in that the downstream potential is greater than the upstream. ADSL is capable of sharing on a single telco wire pair. Maximum speed is 8 Mbps, though typically is limited by the provider to lesser speeds. The most popular DSL at this time.


ADSL Network Termination (a.k.a. the ADSL modem).


Address Resolution Protocol. Converts MAC addresses to IP addresses.


Alcatel's terminology for a DSLAM.


Asynchronous Transfer Mode - provides high-speed packet switching from 155 Mbps to (currently) 2Gbps. Used to provide backbone switching for the Internet, and by many telcos since it can carry both voice and data. This is a common transport protocol for many telco DSL networks.


ATM Forum Interface - 25Mbps speed, provided by a PCI NIC card. One of the interfaces used between the modem and PC.


A combination DSL modem that can be configured to act as either a bridge or a router.


Carrierless Amplitude Phase. A proprietary ADSL line encoding technique, that is (or was) in competition with "DMT". DMT has won the standards battle. CAP and DMT modems are not compatible with each other.

Central Office, or CO

Usually refers to one of two meanings: 1) The local Telco building that houses telephone equipment, and where local loops terminate. 2) The Telco voice switch that provides dial tone. Often referred to as just "CO". Typically, the CO houses one or more DSLAMs that make DSL possible. But, increasingly, DSLAMs are being deployed remotely.


Competitive Local Exchange Carrier. "Competitors" to the ILECs. They do not own any lines, and must lease their lines from ILEC in order to provide any service.


Customer Premises Equipment - The Telco term for customer owned equipment (i.e. the stuff you are responsible for fixing). Examples are analog modems, fax machines, and your telephone.


Dynamic Host Configuration Protocol - A protocol used to distribute dynamically assigned IP addresses and other important networking parameters. The DHCP server "leases" an IP from its pool to clients on request. The lease is renewed at regular intervals. This is a common protocol on bridged DSL networks, and cable modem networks.


Discrete Multitone Technology. This is a line encoding common among ADSL deployments, and now is the standard. Sometimes referred to as "Alcatel compatible". Most telcos in the U.S. are now standardizing on DMT. The other, less common, ADSL encoding is "CAP". CAP and DMT modems are incompatible with each other.


The basic digital circuit for Telcos - offered at 56 Kbps or 64 Kbps. Can support one analog voice channel.


Digital Subscriber Loop Access Multiplexer - The Telco equipment installed at the CO that concentrates and multiplexes the DSL lines. One end of the copper loop connects to the DSLAM, the other to your modem. The DSLAM is essentially what makes DSL work. Increasingly, smaller devices that perform similar functions, are being deployed in remote locations in order to extend the reach of DSL.


Digital Subscriber Loop - A term describing a family of DSL services, including ADSL, SDSL, IDSL, RADSL, HDSL, VDSL, SHDSL, etc. that enable high speed Internet connections over standard copper telephone lines. The main limitation is distance.


Synonymous with "full rate" ADSL. Used to distinguish between full rate ADSL, CAP based ADSL and G.Lite. See DSL Family for more.


A lesser version of ADSL that has lower maximum speeds, and requires no splitter or filters. Not DMT compatible. See DSL Family in this HOWTO for more.


High bit rate DSL. See DSL Family in this HOWTO for more.


Incumbent Local Exchange Carrier. The Regional phone company that physically owns the lines. Examples: Bell Atlantic and Pacific Bell. FCC regulations are forcing the ILECs to open up their networks to independent providers. This is allowing an independents like Covad to offer competitive services. This is a good thing for consumers IMHO.


Interleaving is a tunable aspect of DMT/ADSL line encoding. It essential controls the 'interleaving' of bits in the transmission, and is used as a form of error correction. As interleaving increases, so does stability of marginal lines. It also increases latency.


Internet Protocol. Also, often used to simply refer to an IP address.


Internet Service Provider. Even full-time connections require an ISP to provide basic Internet services and connectivity.


Local Area Network. A network of computers that are segregated from the WAN (Wide Area Network, i.e. the Internet). Often using private, non-routable IP addressing, e.g. or


Link Control Protocol, one of the sub-protocols used by PPP, and derivative protocols like PPPoE. As the name sounds, it used by both the client and server to determine if the connection is viable. Either end may terminate the session if LCP indicates the connection is not responsive.


The two wire twisted pair from the telco Central Office that terminates at a customer location. For DSL, a "clean" copper loop within the distance limitations is required.

MAC Address

Media Access Control Address. Sometimes also called "hardware" address, it is a unique identifier of network devices and is an important aspect of some network environments.


Remote Access Multiplexer, a mini DSLAM. Typically with very few connections -- eight is common. Used for remote areas too far from a CO.


Maximum Transmission Unit, the largest packet size, measured in bytes, that a network can transmit. Any packets larger than the MTU are divided into smaller packets, or "fragmented", before being transmitted.


Network Address Translation is a means of allowing computers on a LAN to access the WAN while "masquerading" with the IP address of a host with a suitable address and configuration. With Linux this is called "ip-masquerading". Often used to share one public, routable IP address among hosts located on a LAN behind a masquerading proxy where the local addresses are private and non-routable.


Network Interface Device - The telco housing on the side of your house. Typically where the telco's responsibility ends, and the owner's begins. Also, sometimes called the "SNI", "TNI" or "ONI" or other descriptive acronyms.


Network Interface Card - An internal PC card that supports the required network interface. Often an ethernet 10/100baseT or an ATMF-25Mbps card in this context.


Network Service Provider. An ISP's upstream provider or backbone provider.


A fiber optic line capable of 155 Mbps.


Plain Old Telephone Service - The service that provides a single analog voice line (i.e. a traditional phone line).


Point-to-Point Protocol over ATM (RFC 2364) is one of the PPP protocols being used by some DSL providers. This is really a device specific driver, and in many respects quite different from PPPoE. A hardware device, i.e. a combination modem/router, is one alternative if this is the only option available to you.


Point-to-Point Protocol over Ethernet (RFC 2516). Another PPP protocol in use by providers. This one is more common, and there are several Linux clients available. See the Links section for more. Not to be confused with PPPoA (PPPoATM) since there are fundamental differences.


Used to refer to PPPoE and PPPoA collectively.


Rate Adaptive DSL. See DSL Family in this HOWTO for more.


Regional Bell Operating Company. The "Baby Bells". The U.S. phone companies that have had a state sponsored monopoly since the break up of AT&T.


Radio Frequency Interference. DSL is susceptible to RFI if in the right frequency range, and if close enough to the DSL signal. This can disrupt and consequently degrade the DSL signal. Unfortunately, DSL seems to operate in the frequency range of quite a few potential disrupting influences.


Shorthand for 'Receive Window', aka the TCP Receive Window, a tunable aspect of TCP network stacks.


Single Line DSL. Or, sometimes also "Symmetric DSL". See DSL Family for more.


Subscriber Network Interface - The Telco term for the phone wiring housing on the side of your house. It designates the point between the Telco side and the Inside Wire. This is also called the Demarcation Point. Sometimes called a "NID" also.


The passive device (low-pass filter) at or near the NID that splits the DSL signal into separate voice and data channels. Filtering is required for most DSLs that share a regular voice phone line (whether POTS or ISDN).


A DSL installation that does not require a splitter. For higher speeds, a RJ11 filter (sometimes called microfilters) is placed on every extension phone jack where an analog phone or other non-DSL device is used, thus filtering the DSL signal at the jack, rather than at the NID. For lower speeds, no filter is necessary. Without a filter or splitter, the DSL signal tends to cause audible interference on voice phones. G.Lite needs no splitter, nor filter, but this is the exception to the rule.


Small Office HOme

Sync Rate

The speed as negotiated by the DSL modem and the telco's DSLAM. This represents the theoretical maximum speed of the connection before any networking protocol overhead is taken into account. Real world throughput is always something less than the modem's sync rate.


German Telekom's ADSL implementation. See DSL Family for more.


a.k.a DS1 - A digital dedicated line at 1.544 Mbps comprised of 24 channels, used for both voice (24 DS0s) and data.


a.k.a DS3 - T1's big brother, a digital dedicated line at 44.736 Mbps, used for both voice (672 DS0s or 28 DS1s) and data.


VPI is "Virtual PATH Identifier" and is part of an ATM cell header. VCI is "Virtual Circuit Identifier", also part of an ATM cell header which contains circuit information. Technically speaking, these are really remote VPI and VCI (RVPI, RVCI). They are both important configuration aspects for modems and routers attached to ATM networks. They must match what the provider is using. Frequently used VPI/VCI pairs include 0/32, 0/35 and 8/35.


Very high bit rate DSL. See DSL Family for more.


Video on Demand.


Voice over DSL.


Wide Area Network, a large publicly accessible network. For example, the Internet.


Used to refer to the entire DSL family of related technologies: ADSL, SDSL, IDSL, etc.

8.3. Other Consumer Class High Speed Services

8.3.1. Cable Modem vs DSL

The Telcos see DSL as a competitor to the Cable Company's Cable Modem, and as such, are providing competitive pricing and configuration offerings. Although Cable Modems are advertised as having 10-30Mbps potential bandwidth, they use a shared transmission medium with many other users on the same line, and therefore performance may vary, sometimes greatly, with the amount of traffic, time of day, and number of other users on the same node. But YMMV.

It is often heard that DSL has an advantage in that it is a private pipe to the Internet, with dedicated bandwidth. This is mostly a myth. You do have a private pipe to the DSLAM, but at that point, you enter the telco's ATM (or frame relay) network, and start sharing bandwidth. You are at the mercy of how well your DSL provider and ISP manage their networks. The consensus seems to be that DSL providers and ISPs are mostly doing a better job of managing bandwidth than the Cable companies. It is easier for them to add and adjust bandwidth as needed to meet demand. You are less likely to have speed fluctuations due to other users being on line at the same time. But, again, this gets down to how well the specific network and bandwidth are managed.

DSL probably has a small security advantage too. With most Cable modem networks, it is like being on a big LAN. You are sharing your connection (and bandwidth) right at the point of connection. But if you are not doing something to filter incoming connections already, you are asking for trouble either way. And, the cable companies are addressing this now, with more secure approaches. This should not now be a major deciding factor.

There also seems to be a better chance of having ISP alternatives with DSL than Cable. At least, in the U.S. this is true. Choice is a good thing, and so is competition. It seems most Cable outfits give you just one choice for an ISP. If you don't like it, you are out of luck. The number of options with DSL probably varies greatly by geographic areas. Populous areas, like Northeast U.S., seem to have many options.

So which is better? The differences aren't as much with the technology, as they are with the implementations. If you look around, you can find plenty of horror stories on either. And plenty of happy customers too. The way to know what may be the best for you, is to do comparative shopping based on experiences of other users in your area. Don't base your choice on one person's opinion. This is statistically invalid. Likewise, don't base your choice on someone's opinion who has had a particular service for only a short time. Again, statistically not worth much. Get as many opinions from those that are using the exact same services that you are looking at.

8.4. Compatible Modems

This list is limited to those modems and delivery systems that are readily available, and should work with any current Linux distribution without having to go to extraordinary lengths. Alpha and Beta projects are not included. All modems listed are POTS (analog) modems at this time.

Ethernet Interface

PCI (Internal)


8.5. Setting up Linux as a Router

Depending on your local setup, you should consider some other issues. These include a firewall setup, and any associated configurations. For my setup, shown in Figure 5 below, I use an old i486 machine configured as a firewall/router between the DSL connection and the rest of my home network. I use private IP addresses on my private LAN subnet, and have configured my router to provide IP Masquerading and Firewalling between the LAN and WAN connections.

See the IP Masquerade HOWTO , and Firewall HOWTO for more information. For 2.4 kernels see the Linux 2.4 Advanced Routing HOWTO. My experience is that Linux is more flexible and provides superior routing/firewalling performance. It is much less expensive than a commercial router -- if you find an old 486 machine that you may be using as a doorstop somewhere. There any number of brands of "DSL/Cable" routers on the market as well. These might be the way to go for pure ease of use, but lack the sophistication of what Linux can do.

Figure 5: A typical SOHO Network Setup

 <--Private Subnet/LAN-> Linux <-----ISP's Public Subnet----><--inet-->

 X--+   -------- 
    |   |      |        --------      (eth0:0)---------
    +--=| Hub/ |       | Linux  |     +------=|  DSL  |=-DSL-> ISP's
 X-----=|Switch|=-----=| System |=----+       | Modem |       Gateway
    +--=|      |  eth1 |(Router)| eth0        ---------
    |   --------    |   --------    |
 X--+               |   IP_Masq     |
                    |  IP_Firewall  |
   |                |    Gateway    |
   |                |               |
   |                V               V
   V          Dynamic or
 192.168.1.x   LAN Gateway         Static IP
LAN Addresses  IP Address          from ISP pool


What I did is setup a Linux router (Redhat Linux 5.0 on a i486) with two ethernet interfaces. One interface routes to the ISP subnet/gateway (eth0 in above example), and the other interface (eth1 above) goes to a hub (or switch) and then connects the LAN with private network addresses (e.g. 192.168.1.x). Using the private network addresses behind your router/firewall allows some additional security because it is not directly addressable from outside. You have to explicitly masquerade your private addresses in order to connect to the Internet from the LAN. The LAN hosts will access the Internet via the second NIC (eth1) in the Linux router. Just set their gateway to the IP address of the second NIC, and assign them addresses on the same network.

Caution Make sure your kernel is complied with IP forwarding and the IP forwarding is turned on. You can check this with 'cat /proc/sys/net/ipv4/ip_forward'. The value is "1" for on, and "0" for off. You can change this value by echoing the desired value into this file:

 # echo 1 > /proc/sys/net/ipv4/ip_forward

You will also need to set up "IP Masquerading" on the Linux router. Depending on your kernel version, this is done with ipfwadm (2.0), ipchains (2.2), or iptables (2.4). See the documentation for specifics on each. AND -- do not forget to have that firewall set up too!

There are also several projects that are devoted specifically to using Linux as a router, just for this type of situation. These are all-in-one solutions, that include security and various other features. Installation and configuration, is reportedly very easy. And these will run on very minimal hardware -- like a floppy drive only. The best known is http://www.linuxrouter.org. You might also want to look at http://www.freesco.org and http://www.coyotelinux.com. There is also http://www.clarkconnect.org/index.html, which is a similar concept but more full-featured and is designed to be monitored and configured with a set of Windows based utilities.