Why T1 bandwidth shows 1536 for T1 interface on Cisco router?

You might see bandwidth indication on your T1 link shows 1536Mbps instead of 1544Mbps. The number implies data portion of frames.

As you know T1 is divided into 8000 frames per second and each frame has 192bits for data frames + 1bit for framing bit. So without framing bit, data portion of frames will be 8000(pe second) x 192bits = 1536000 BPS.

T1 speed  = 1.544Mbps = 193bits x 8000 (frames)

Without framing bits(Payload) = 1.536Mbps = 192bits x 8000(frames)

 

Also, you might see bandwidth of DS3 is shown as 44210 Kbps

DS3 bandwidth  = 44,736,000 bps – 526,306 bps(overhead) = 44,209,694 bps = 44210 kbps

 

Extra >

There are three different types of T1s

1. Unchannelized T1
2. Channelized T1 and
3. ISDN PRI

A single T1 frame format = 193 bits = 192 bits(24 x 8 bits) + 1 framing bit
** 24 x DS0, each DS0 has 8 bits.

Unchannelized T1 = 1.544Mbps = 193 bits x 8000 frames

Channelized T1 = 1.544Mbps = [ 64Kbps x 24(time slot) ] + 8000 (framing bit)

64Kbps = 56Kbps + 8kbps (signaling)
** 56Kbps = 7 bit (payload) x 8000 frames