hotshot

I just bought an ADSL+ modem/router.
My question has to do with sth that i found in the configuration of the device named ATM protocol.
What is that,is there any connection with TCP/IP.I need to note that my PC with the modem/router form a LAN.

heartbomb

ATM is a layer 2 switching technology "asynchronous transfer mode" it allows IP to ride on top of it since IP is layer 3. It uses fixed 53 byte cells to do very fast hardware switching.

hotshot

Point to point over ATM is differing form of DSL service from Point to Point of Ethernet. In my experience PPoE is more popular but we use PPoA for our DSL customers for now. Most DSL routers you buy at Best Buy don't offer PPoA or at least have not in the near past. It could be your DSl router is one that does both PPoE and PPoA. The ones I know that do it are brands from Cisco, Netopia, and Alcatel.

edited typos

heartbomb

That being said, ATM equipment is probably not cost-effective for home use. Because of its speed, it is used for videoconferencing, VoIP telephony, and large data transfers.

Check this out:
http://www.hpc.gatech.edu/Internet2/atm/

hotshot

FastLearner it is an ADSL modem. DSL is very common interface. The point to point protocol over ATM is one way phone companies/ISP's introduce DSL service.

heartbomb

yes, but the A in ADSL does not stand for ATM. I have ADSL service at home and I do not use any ATM-supported hardware whatsoever. Would albatros not need special ATM-standardized switching equipment to take advantage of ATM's speed advantages from a home environment?

hotshot

hmm, how about a primer...

The A in ADSL doesn't have anything to do with the ATM. The "A" in ADSL stands for "Asymmetric" which means, basically, that the up and down speeds are not the same. ADSL has a higher download speed than upload.

The PPP in both PPPoE and PPPoA most people are familiar with, back in the days of dial up modems, most used PPP to connect. PPP is a protocol that simply specifies a way of authenticating connections. It sends a username and password and can use encryption to do so.

DSL is a technology that breaks the frequency range that the copper wire is capable of into slices. Someone figured out that normal voices don't really take advantage of the full frequency range allowable over normal copper telephone wires. So they thought “how we can take advantage of the unused frequency sitting there already installed in 95 percent of homes”... what they did was slice up the range on the line - thus.. 0-4 kHz is used for voice, 25-160kHz is used for upstream and 200kHz to 1.1Mhz is used for the downstream. As you can see with DSL the upstream is much more limited.

PPPoA and PPPoE are about the way the ISP handles the broadband connection. It is about frame types and encapsulation. When you send information over a network it is broken into packets. These packets are structured based on rules so that both sides understand how to parse/analyze the packet. PPPoA creates a virtual circuit between the client’s router (called customer premise equipment or CPE) it uses a globally defined rule called RFC 1483 (we use 1483 so I will use it here). (RFC stands for Request for Comment. An RFC is a means by which globally accepted standards can be set forth. You will see a term like RFC XXX compliant, what this means is that the device, protocol or whatever follows established guidelines. Think of it in terms of a treaty between nations. Everyone agrees on a set of rules and they try to follow those rules. Without standards communication between devices is problematic. RFC's are ever changing and the newer version for PPPoA is 2364) RFC 1483 operates in either Logical Link Control-Subnetwork Access Protocol (LLC-SNAP) or VC-Mux mode. These modes dictate the values placed in the Header and footer of the packet. When a packet from your PC goes to the router it is encapsulated in an Ethernet frame (packet type) when it arrives at the DSL modem/router a PPP header is attached. It then undergoes encapsulation based on the RFC1483 mode used. It basically chops the packets into smaller ATM cells (53 bytes) and fires them off to the DSLAM.

A DSLAM is a network device usually owned by a telephone company. It is the device that concentrates all the DSL connections coming from customers in to one point. It is very important because the distance a client is from the DSLAM dictates the speed they will get in their broadband connection. The maximum distance you can be is 18 thousand feet. The farther you are from a DSLAM the lower your broadband DSL connection will be.

PPPoE became popular at the outset of DSL. You needed to install the software on a CD given to you by the DSL company, input the username and password by following the simple instructions the software asked, as it walked you through the set up; it was easier to get the ball rolling so to speak. PPPoE adds a wrinkle in the form of 8 bytes of data to the packet that is not added by PPPoA. This also runs into a issue with MTU or Maximum Transmission Unit. Anyone who wants to tweak their DSL is probably familiar with this term. The problem is that a normal Ethernet packet is 1500 bytes. Remember that PPPoE uses 8 bytes so the packet size will be 1492 bytes when it goes to the ISP. There are fixes for this but I won’t go in to them.

For several years DSL routers have come with PPPoE software built in, in most cases, using the software provided by the DSL company is not needed if you configure the router directly.

Now, I hope this explains most of the questions you have. If you have more you can google any of the topics above and learn all the details you would ever want to know. Reading RFC's is almost as much fun as picking ticks off a dog, almost...


Variable

heartbomb

the "A" in adsl means assymetric which means that the download speed is different from the upload speed opposed to SDSL which is synchronous meaning the upload and download speeds are the same. DSL runs on ATM by the way of whats called a DSLAM that connects the end user CPE to the ATM backbone, ISDN also rides on ATM as well. ATM basicly is what allows high speed data transmission over existing copper lines. PPPoE and PPPoA are basicly authentication and authorization protocols used by xDSL. PPPoA is now becoming more popular since it is more secure and not susceptable to DOS unlike PPPoE.

edit I guess we posted at the same time LOL!

heartbomb

yep, I do. The adsl connection I am using is PPPoA (wanadoo in the UK)and it works on an ordinary ethernet port with a standard ethernet card. most of the ISPs in the uk seem to use PPPoA

hotshot

Thanks for the great explanations, guys. I for one definitely learned something new about ATM here. My only point, however, was that ATM is more expensive than Ethernet in general. I was thinking strictly from the client side, and not too much about how the ISP uses ATM. Please correct me if the following is wrong:

First, a home PC with a standard 10/100 Ethernet card cannot use ATM (or PPPoA). This means that if you have a network with 50 computers and you want to use PPPoA, then you need to invest in 50 ATM NICs, which are more expensive than an Ethernet card. There's one expense.

Then of course the switch/router at the SoHo location needs to be ATM (RFC 2364) compliant, as well.

Do you disagree with this?