T1 Questions

Answered Question
Aug 22nd, 2007

Hello,

I've got a question about T1's that I haven't been able to clear up so I thought I'd ask it here.

From what I understand from my studies, a T1 is nothing more than a Layer 1 device that is capable of providing 1.544Mbps point-to-point. The Layer two protocols you run over it determine what "kind" of T1 you have. You can have a PRI T1, Frame Relay T1, ATM T1, etc...

Here is my confusion. Say, for instance, I want to get a T1 run to my local telco for internet access. Lets pretend the company I work for needs this high speed connection for internet access and they are running multiple webservers so it is a must. When I call up the Telco to order the service, what kind of T1 do I tell them I need? Lets pretend that this company is NOT using the T1 for any kind of Voice Communications only for data. Do I simply tell the telco that I want a Full T1? Does that make any sense? I mean, I wouldn't tell them I want a PRI T1 as I don't need to use the line for voice communication. I wouldn't tell them I need a Frame Relay (although I suppose one could still use a FR T1 soley for internet access as long as the telco was manging the FR T1 for that purpose).

So I guess, I'm confused about the kinds of T1's. Cisco default encapsulation is HDLC, correct? Can this encapsulation run over a T1? If one didn't want to use ISDN (it needs PPP), what kind of T1 could use HDLC?

As I see it, ISDN is headed out. So what are the other T1 options for internet access? What are the advantages of even using a PRI? Just to mutliplex voice and data on the same line (like if the business was running a PBX).

I hope I was clear in the above. It just frustrating as the book I have for my CCNA isn't really very clear on Leased-Lines...

I have this problem too.
0 votes
Correct Answer by pciaccio about 9 years 3 months ago

A telco will provide to you Internet access a layer 2 and 3 access. You will need to match the encap method (telco determines what they want to talk to you as ), You are correct in this. What I meant before is that if a telco provides to you a dedicated T-1 from your headquarters to your branch(for example), then you place the protocol onto that T-1 not the telco. But as for Internet access your telco will be sending to you a madia type (T-1 or other) and a layer 2 and 3 access....

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pciaccio Wed, 08/22/2007 - 12:27

You ask alot of questions regarding T-1's. To answer your first basic question is if you want to run data then a T-1 ordered as B8ZS lice coding and ESF (Extended Superframe) for the framing will support your data. If you wanted to run just voice you can order the line as D4 framing and AMI (Alternate Mark inversion) for linecode. These types of lines differ in that the B8ZS/ESF lines supports data rates of Nx64kbps and or Nx56kbps. A D4/AMI line onlt supports Nx56kbps. Traditional voice lines run up to 56kbps thus the D4/AMI is sufficiant. TO answer you other questions you would order a full T-1 if you require the full 1536kbps (1544k - 8k also equal to 24 Timeslots of 64kbps). IF you have budget restraints then a fractional T-1 can allow you to have any subrate amount of timeslots (64, 128, 256, 384,512,768,etc...)kbps. However T-1 costs today are cheap and a full T-1 is alot easier to support and provision. To answer your other questions the HDLC,ISDN, Frame Relay protocols can all be run onto a T-1 (no matter what type of T-1). Othe options for Internat access is Frame Relay, Straight HDLC or PPP or ATM to your ISP. Or DSL or PPPoE or PPPoA methods. I hope I shed some light on your questions. Good Luck...Please rate if helpful....

rodman.frowert Wed, 08/22/2007 - 12:46

Actually you post was very informing. So, if I just wanted to use the T1 for data and told the Telco that, I assume the telco would already know to use B8ZS with ESF and would set it up appropriately with that kind of setup. I also assume that they would tell me what encapsulation to use on the line. From what I have gathered, PPP is the most popular, but if they are running Cisco equipment than HDLC wouldn't be out of the question, correct?

Is Frame Relay popular at all for use for internet access on a T1? Just curious if a Frame Relay T1 is any more or less expensive than a PPP/HDLC T1.

pciaccio Wed, 08/22/2007 - 13:04

The telco does not care what protocol you place on a line. That is the network administrators job. The telco only porvides the media pipe that has a coding and framing to it as well as clocking (That is a whole other discussion for clocking). But you should always try to have the carrier supply your T-1 clocking... PPP encap is atractive because it provides for security. HDLC is Cisco's defualt for serial lines but any protocol you can use. Frame relay provides for traffic congestion and can support your needs better then just PPP but Frame Relay is a popular ISP solution. IT is supported within their huge ATM/Frame Relay networks and can be provisioned fairly easy for them. The price of a Frame Relay depends upon what CIR and burstable rate you require. There is no price to place PPP or HDLC ono a T-1. That is dependant upon you...

rodman.frowert Wed, 08/22/2007 - 13:22

"The telco does not care what protocol you place on a line. That is the network administrators job"

I guess I was under the impression that BOTH routers (on each end) had to have the same encapsulation method. So if the telco is providing internet service to me via the T1 that they just installed, my router would have to have the same encapsulation set as theirs... At least that is what I have gathered from my readin...

Ugh, I still have a lot to learn, but it is starting to sink in at least...

Correct Answer
pciaccio Wed, 08/22/2007 - 13:33

A telco will provide to you Internet access a layer 2 and 3 access. You will need to match the encap method (telco determines what they want to talk to you as ), You are correct in this. What I meant before is that if a telco provides to you a dedicated T-1 from your headquarters to your branch(for example), then you place the protocol onto that T-1 not the telco. But as for Internet access your telco will be sending to you a madia type (T-1 or other) and a layer 2 and 3 access....

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