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Questions about T1

Ok, figured I would ask this since I never really grasped the concept.
What in God's name is a T1 lol?  I know it is a type of connection to a service provider, or a way to connect your company's different sites together etc etc. But is it purely physical?  What type of encapsulation does it use?   Do you have to do any special configuration in your routers (like you do if you run frame relay encapsulation) when you have a T1?

How do T1 and Frame relay relate? I noticed in my cisco book it says Frame is good for the occasion bursty traffic, but not good for constant data transfer. They said constant data transfer calls for a T1.  Someone help, I'm drowning here.   I was under the impression everyone uses T1 "lines."  But they choose their encapsulation method (frame, ppp etc...).  And since I'm here, I guess I should ask my next stupid question. I'm assuming FDDI cannot run over a T1?  

Thanks for entertaining these questions
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1 Solution
According to my copy of _Newton's_Telecom_Dictionary_ (16th Edition), "T1" (or "T-1") stands for "Trunk Level 1". Quoting:

A digital transmission link with a total signalling speed of 1.544 Mbps.... T-1 is a standard for digital transmission in North America.... T-1 is part of a progression of digital transmission pipes - a hierarchy known generically as the DS (Digital Signal Level) hierarchy.
I know the answer you are looking for, but I thought I see what answers every one else came up with out of curiosity. I though for sure there would have been many enlightened answers for this by now, but all we have so far is the textbook description of what the word T1 stands for, but not what it is.
A T1 is a digital line of 24 multiplexed DS0’s (digital signal 0, best analogy one phone line, or a 64K digital channel, sometimes called timeslots). The 24 X 64K are how you arrive at the 1.536 Megs of capacity for a T1.

To connect a router to a T1 you need a CSU/DSU (channel service unit/data service unit), you can kind of think of this as a kind of like a modem that can take data coming in on 1 or more DS0’s, and output it to a serial interface that can be connected to a router. The CSU/DSU can an external, or internal unit, but for what we a concerned about, functionally they are the same.

Once you have the physical part of a data link in place, you need some protocol to control how data is transmitted across the link. This is where PPP, HDLC, and frame relay encapsulation come into play. They provide the framework of how the data is sent across the T1, encapsulating it in a manor to abstract the underlying transport mechanism from the network protocol being retransmitted across the T1. So to the various networking protocols it looks like a native data link.

Now this is where PPP and HDLC diverge from frame relay. PPP and HDLC are point to protocols, that’s what PPP stands for. So to get a link from office A to office B, you need a dedicated T1 circuit that goes all the way from that office to the other one.

With frame relay you no longer need to have a dedicated link since it uses packet switching technology. The data to be sent is encapsulated in packets, with markers to identify it, allowing it to be sent over a frame relay network commingled with data from other networks, eliminating the need for dedicated point to point circuits, reducing costs.

In a typical T1 frame relay circuit, the data goes through a local T1 to the nearest access point of the frame relay network, travels though it to the nearest frame relay access point for the receiving network, and is sent through another local T1 to its final destination. So instead of having to have a dedicated link from one side of the country to the other, you have two short T1 links to the frame relay network, and it bridges the gap at a lower cost since the long haul links of the frame relay network are shared.

There are two types of frame relay service PVC (permanent virtual circuit), and SVC (switched virtual circuits). A PVC circuit acts a lot like a dedicated point-to-point circuit, it’s always on, and always connected. Unless the frame relay network is congested, it’s almost as good as having a point-to-point circuit, except at a lower cost.

SVC circuits on the other hand only connect when there is data to be sent, and disconnects after a preset idle time. The connection process is pretty fast, but there still is a performance impact since data has to wait until the connection is made before it can flow. Because it’s only connected when it’s being used, SVC are less expensive than PVC circuits, so SVC circuits are only good for those who have intermittent data transmit needs, and won’t be impacted by the connection delay. A good candidate for an SVC would be for a store that needs a way to send sales data to the main office in batches. A poor one would be for a store that is connected to a remote database at the home office that is under constant use.

I suspect you are going to also want the why for frame relays existence; after all we have the Internet. Well that’s today, but in the days when frame relay came about, you really had only one choice, point-to-point circuits, and they where very expensive in those days. Frame relay was created to make more effective use of the existing telecom info structure, allowing them to offer less expensive data services, and even today, with cheep internet, it can still be a cost effective solution for a company the wants to network several remote offices, but the days of frame relay may be limited. As companies are using VPN’s across the Internet to replace circuits they have traditionally used frame relay for, but I don’t see an end coming to frame relay anytime soon. As most frame relay networks are fully amortized, so expenses for providing it are very low.        




dissolvedAuthor Commented:
Wow thanks for the info.  So you're saying alot of companies are moving towards VPN. This way, they only need their primary  connection to their service provider (instead of point to point links or using Frame Relay and it's "cloud") to their other sites ?

Thanks again Dr IP
I am not saying there is a mass extrados from frame relay, but VPN solutions are making inroads into markets that have been traditionally frame relay markets. I don’t think you will see a lot of frame relay users abandoning it any time soon, but you will see a lot of new potential customers taking a hard look at VPN based alternatives. Those highly concerned about security will probably continue to choose frame relay at least for now, but those whom have lesser security concerns won't shy away from the alternatives, and many will weigh their ultimate choice on cost. So I wouldn’t say frame rely is dead yet, but I wouldn’t consider it a growth industry either.    
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