Refer to the T-carrier system for the basics, it is better to see the whole picture , I think by reading a bit on this it will give a better perspective of chnannalized and fractional T1s and its many applications.
for example, a channalized T1 is simply a leased T1 but you asked the provider to provisioned individual circuits off that T1 for different uses, as you already know the T1 has 24 64k channels, you may want to use 4 individual 64k channnels/circuits for data and the rest as voice circuits (telephone lines ).
Fractional T1 is when you only lease portions of the 24 channels.
A channelized T-1 is just a T-1 that has its timeslots (DS0's) separated into various circuits either all of the timeslots are 64k or 56k or any combination of the whole. A Fractional T-1 is pretty much the same in that the useable bandwidth from the T-1 is a subrate speed of the whole. Hope this helps...Please rate....
To add to pciaccio good info, in cisco parlance, a fractional T1 is an interface that has the capability to combine on single set of timeslots to form the needed bandwidth, but no more than one set, or channel-group.
A channelized interface, again in cisco terms, on the other hand can be divided in as many channel-groups as necessary
In agreement to all others, I shall try to put it as simply as:
1. Channelized T1 is as good as any T1 i.e. 24*64kbps = 1.544Mbps, however over here, its the customer who decides how to use those T1 channels i.e. you may use it completely for Data or Completely for Voice or a mix of both, now if you make use of the mix, then in true sense it becomes a Channelized T1 e.g. 512kbps for Data and rest for Voice.
2.Fractional T1 is like, subscribing only a subset BW of the entire T1 (1.544 Mbps), e.g. the WAN BW requirement between two sites is on;y 256kbps, since generally there is no 256kbps Link standardization in terms of Telecommunications, the provider will assign a T1 Link (but only a 256kbps pipe) and the WIC you need to use will also be a T1 WIC, however you need to ask the Service Provider what all Time-Slots you are being provided and you can configure the WIC accordngly.
3. Whenever ISDN PRI is subscribed its always on Channelized T1 however over here, and when you configure it, its divided into 23B Channels and 1 D Channel.
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