# General Networking/Lan/Wan/circuit switching using TDM

Question
QUESTION: Hello Mr.McQueen,
I apologize in advance about my English because I'm not a native speaker.

I'm confused about a specific issue in circuit switching using TDM, which is in the following example from the book(computer networking top down approach, Kurose and Ross ):

Suppose users share a 1 Mbps link. Also suppose that each user alternates between periods of activity (when it generates data at a constant rate of 100Kbits/sec) and periods of inactivity (when it generates no data).Suppose further that a user is active only 10% of the time (and is idle drinking coffee during the remaining 90% of the time). With circuit-switching, 100 Kbps must be reserved for each user at all times. For example, for circuit switched TDM, if one-second frame divided into 10 time slots of 100 ms each, then each user would be allocated one time slot per frame. Thus, the circuit switched link can support only 10(=1Mbps/100kbps) simultaneous users

my  problem lies in the last line why did the author divide 1Mbps/100kbps while each user will transmit data in time slot= 100 ms which means the user won't be able to complete 100kb but will only generate 10 kb (100*0.1ms) at its first time slot

Your English is very, very good by the way.

As I understand TDM the 100Kbps * 10 = 1Mbps thus
allowing 10 users 100Kbps allocation.
The 1000ms time frame is divided into 10 = 100ms.

Can I ask you why you stated (100 * 0.1ms) ..i.e. you
have stated 0.1ms as opposed to 1ms.

Best Regards,

Scott McQueen.

---------- FOLLOW-UP ----------

QUESTION: Thanks for replaying

sorry I meant 10kb=(100*0.1s).However, My question was:
Will the first user who uses the first time slot be able to transmit 100kb in his .1 second although its data generation rate is 100kbpSECOND?

Thinking about it now. I guess that I built my question on doubtful rule, So I'll ask to correct this rule first:

I thought that the data generation rate is an independent property of a user. Is that true?

or does it depend on the circuit bandwidth (i.e. the user can generate and transmit as much as the  bandwidth allows it to transmit in the current moment)?

regards

Hi Huda

Yes, the data rate is independent of users.

There are various models within TDM.
A device can burst to a certain restricted bandwidth
or all available bandwidth in the current moment.

The most common use is to only grant the bandwidth when
needed. Statistical TDM does not reserve a time slot for
each terminal, it assigns a slot when the terminal
is requiring data to be sent or received.

Regards

Scott McQueen.
Questioner's Rating
 Rating(1-10) Knowledgeability = 10 Clarity of Response = 10 Politeness = 10 Comment Thanks a lot

General Networking/Lan/Wan

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#### Scott McQueen

##### Expertise

All Networking and Internetworking questions. Specialist in Cisco Systems equipment.

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17 years experience in the Networking industry working for the likes of Siemens and Kingston Communications.

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