Cisco Systems 1000 Series Manual De Usuario

 

After the master-slave clock hierarchy is established, the clock synchronization process starts. The 
message exchange occurs in this sequence: 

The master clock sends a Sync message. The time at which the Sync message leaves the master is 
time-stamped as t

1

.

The slave clock receives the Sync message and is time-stamped as t

2.

.

The slave sends the Delay_Req message, which is time-stamped as t

3

 when it leaves the slave, and 

as t

4

 when the master receives it.

The master responds with a Delay_Resp message that contains the time stamp t

4

.

The clock offset is the difference between the master clock and the slave clock, and is calculated as 
follows:

Offset = t

2

 - t

1 

- meanPathDelay

IEEE1588 assumes that the path delay between the master clock and the slave clock is symmetrical, and 
hence, the mean path delay is calculated as follows:

meanPathDelay = ((t

2

 - t

1

) + (t

4

 - t

3

))/2

All PTP communication is performed through message exchange. The two sets of messages defined by 
IEEE1588v2 are General messages and Event messages.

General messages--These messages do not require accurate time stamps, and are classified as 
Announce, Follow_Up, Delay_Resp, Pdelay_Resp_Follow_Up, Management, and Signaling.

Event messages--These messages require accurate time stamping, and are classified as Sync, 
Delay_Req, Pdelay_Req, and Pdelay_Resp. 

Timestamps

known by slave

Slave

Time

Master

Time

t

1

t

1

t

2

t

2

t

1

,t

2

,t

3

t

1

,t

2

,t

3

,t

4

t

1

,t

2

t

2

Sync

Follow_Up

Delay_Resp

Delay_Req

t-ms

t-ms

372

8

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