What is SDH,POS,STM

What is POS interface in a Cisco Router?

POS stands for Packet over Sonet

It is a point to point serial interface .

It is NOT a general type of serial port. It is especially for sending ip packets over sonet/sdh frames.So POS is for pure ip backbones.

 

Eventhough POS interface stands for Packet over Sonet, it can be configured for packet over sdh.

The command is like this:

Router(config-if)#POs framing [sdh|sonet] Here. sonet is default

 

So actually pos stands for packet over sonet/sdh

 

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Encapsulation used POS. The ip packet sent out of pos interface can be encapsulated with either hdlc or ppp. when encapsulated with ppp, actually the ip is encapsulated into ppp and ppp is again encapsulated into sonet.

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what is sdh?

Earlier, sync was done by pdh (plesiochronous digital hierarchy). It was replaced by sdh.

The main diff between sdh and pdh : The speed is tightly synchronized across the entire network, using atomic clocks in case of sdh.

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What is STM?

STM stands for synchronous Transport Module.

STM1 speed is 155.52 mbps.

STM is the frame format for SDH.

STM1 is the BASIC fram format for sdh.

STM is associated with sdh. in the same way OC or STS is assocated with sonet.

STM1 is equal to OC3 in its speed.

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All about STM-1 Frame:

The is the basic frame for sdh.

STM1 sends one frame in 125 micro second.

Therefore, it sends 8000 frames per second.

Each frame contains 9 rows

each row contains 270 bytes.

 

Hence 270 bytes* 9rows * 8 bits * 8000 frames=155.52 mbps

 

Eventhough each row contains 270 bytes, only 261 bytes are the actual data, remaining 9 bytes are used for overhead and pointers. Hence actual data rate is about 150mbps.

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difference between sonet and sdh

 

sonet stands Synchronous Optical Network. SDH stands for Synchrnous Digital Hierarchy

 

The purpose of these two are same. but Sonet is used in usa and sdh is used in europe.

The internal staructure of sonet and sdh is slightly different.

The sonet link is called as sonet optical carrier ie OC.

 

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It is a very high speed interface. So normally used in netowrk backbones.

Note:backbone means : between large cities.