ITU-T G.8275.1 Phase/Time Profile
ITU-T G.8275.1 defines the “Precision time protocol telecom profile for phase/time synchronization with full timing support from the network”. It defines the options and attributes from IEEE 1588 to be used to deliver phase/time synchronization to the end application.
ITU-T G.8275.1 targets accurate time and phase distribution, and requires T-BC (Telecom Boundary Clocks) at every node in the network. Because a boundary clock is used at every node in the chain between the T-GM (Telecom Grandmaster) and the T-TSC (Telecom Time Slave Clock), there is a reduction in time error accumulation through the network, in order to do that the use of two-way time transfer mode is mandatory.
ITU-T G.8275.1 can also be used in networks where phase and ToD (Time-of-Day) synchronization is required, and where each network device participates in the PTP protocol.
In order to synchronize a network using the ITU-T G.8275.1, primarily it is necessary to have all devices synchronized in frequency. According to ITU-T Recommendation G.8275.1/Y.1369.1 it is mandatory the use of SyncE (Synchronous Ethernet) standard for frequency synchronization since it is fast and accurate. Therefore, all devices must support this capability. The phase alignment is accomplished by utilizing the PTP protocol. The device having the best clock source is usually configured as the network T-GM.
Multicast messages are used for this profile with the PTP mappings defined in IEEE 1588 Annex F. Both the non-forwardable 01:80:C2:00:00:0E and forwardable multicast addresses 01:1B:19:00:00:00 are permitted. The multicast address is set by configuration on a per-PTP port basis, but both non-forwardable and forwardable multicast addresses must be accepted by the PTP port. The domain numbers can be set between 24 and 43 with the default being 24.
The messages types and message rates used in this profile are:
- Sync/Follow up - Fixed nominal rate of 16 packets per second.
- Delay Request/Delay Response - Fixed nominal rate of 16 packets per second.
- Announce - Fixed nominal rate of 8 packets per second.
ITU-T G.8275.1 uses an alternate BMCA that is based on the IEEE 1588 default BMCA. The alternate BMCA differs as follows:
- Multiple T-GM to be active simultaneously.
- The clock attribute priority1 is static.
- The clock attribute priority2 is configurable.
- The clock attributes clockAccuracy and offsetScaledLogVariance must be set to specific values defined in ITU-T G.8275.1
- Per-port Boolean attribute notSlave. If notSlave is set to TRUE, then the port is never placed in the SLAVE state. If notSlave is set to FALSE, then the port can be placed in the SLAVE state
- ITU-T G.8275.1 also specifies the clockClass values to be used.
Demonstrating: ITU-T G.8275.1 using DATACOM Switch DM2302
The DM2302 have the following timing ports:
- 1 PPS - Input and Output
- 10 MHz - Input and Output
- ToD – Input or Output
- Building Integrated Timing Supply (BITS)
The ToD and 1PPS ports on DM2302 can be used to exchange ToD clocking. When DM2302 is configured as T-GM, it can receive ToD clocking from an external GPS unit; DM2302 requires a ToD and 10 MHz connection to the GPS External unit. In this case ToD port should be explicitly configured in input mode.
When DM2302 is configured as T-BC or T-TSC, it can recover ToD and 1PPS information from a PTP session and repeat the signal on ToD and 1PPS ports. In this case, ToD port should be explicitly configured in output mode.
ToD in input mode and 1 PPS input ports cannot be used at the same time.
The following set of commands will configure the T-GM previously shown on the scenario with:
- Frequency source is received in 10 MHz input interface (Derived from GPS External Unit) with SSM QL set as PRC.
- The Phase/Time source is received through the ToD (Derived from GPS External unit).
- VLAN 1588 configured to send PTP information to the T-BC and T-TSC devices through the network.
The configuration for the T-BC or T-TSC will be similar but with:
- Frecuency is recovered through GigabitEthernet interfaces using SyncE.
- Phase/Time is recovered from the PTP session and repeated on ToD and 1PPS ports.
T-GM DM2302
!
vlan 1588
name G8275-1
!
ptp 0 mode master onestep ethernet twoway vid 1588 7 profile g8275.1
ptp 0 priority2 10
ptp system-time get
ptp tod rx ser
!
interface GigabitEthernet 1/8
no loop-protect
switchport trunk allowed vlan 1588
switchport mode trunk
switchport forbidden vlan add 1
qos trust tag
qos trust dscp
no spanning-tree
ptp 0
ptp 0 announce rate 8pps timeout 3
ptp 0 sync rate 16pps
ptp 0 delay-req rate 16pps
!
sync-source
wait-to-restore 0
hierarchy 3 quality-level-overwrite prc
hierarchy 3 freq-clock-source 10MHz
!
synce
quality-level-enable interface GigabitEthernet 1/8
!
T-BCs DM2302
!
vlan 1588
name G8275-1
!
ptp 0 mode boundary onestep ethernet twoway vid 1588 7 profile g8275.1
ptp system-time set
!
interface GigabitEthernet 1/7-8
no loop-protect
switchport trunk allowed vlan 1588
switchport mode trunk
switchport forbidden vlan add 1
qos trust tag
qos trust dscp
no spanning-tree
ptp 0
ptp 0 announce rate 8pps timeout 3
ptp 0 sync rate 16pps
ptp 0 delay-req rate 16pps
!
sync-source
wait-to-restore 0
hierarchy 1 freq-clock-source synce interface GigabitEthernet 1/7
hierarchy 2 freq-clock-source synce interface GigabitEthernet 1/8
!
synce
quality-level-enable interface GigabitEthernet 1/7
quality-level-enable interface GigabitEthernet 1/8
!
T-TSC DM2302
!
vlan 1588
name G8275-1
!
ptp 0 mode slave onestep ethernet twoway vid 1588 7 profile g8275.1
ptp system-time set
!
interface GigabitEthernet 1/8
no loop-protect
switchport trunk allowed vlan 1588
switchport mode trunk
switchport forbidden vlan add 1
qos trust tag
qos trust dscp
no spanning-tree
ptp 0
ptp 0 announce rate 8pps timeout 3
ptp 0 sync rate 16pps
ptp 0 delay-req rate 16pps
!
sync-source
wait-to-restore 0
hierarchy 1 freq-clock-source synce interface GigabitEthernet 1/8
!
synce
quality-level-enable interface GigabitEthernet 1/8
!
Once devices are properly configured with ITU-T G.8275.1 and SyncE, interfaces Clock Status should be FREQ_LOCKING or LOCKED on the T-GM, on the T-BC and on the T- TSC as shown below:
On the T-GM
Verifying the frequency synchronization source
DM2302-T-GM# show sync-source status
Synchronization Source Status:
Status Detail: Locked
Selected Hierarchy: 3
Clock Source: 10MHz
Hierarchy Alarm Status:
Hierarchy Level: 1 2 3
LOCS: TRUE TRUE FALSE
QL: FALSE FALSE FALSE
WTR: FALSE FALSE FALSE
Global Alarm Status:
LOL: FALSE
DHOLD: FALSE
SyncE Quality Level Status:
Interface Tx QL Rx QL ANEG Mode
GigabitEthernet 1/8 QL_PRC QL_DNU N/A
Verifying the ToD, 1PPS and clocking derived from the GPS external unit
DM2302-T-GM# show ptp tod
PTP RS422 clock mode: ToD_Rx, delay : 0, Protocol: Serial, port: 1
DM2302-T-GM# show ptp pps-bnc status
1PPS signal status: Ok
DM2302-T-GM# show clock
System Time : 2017-11-15T08:00:01+00:00
Verifying the parameters configured for ITU-T G.8275.1
DM2302-T-GM# show ptp 0 default
ClockInst DeviceType 2StepFlag ClockIdentity Domain
--------- ---------- --------- --------------------------- ------
0 MasterOnly False 00:04:df:ff:fe:40:43:1a 24
ClockQuality Pri1 Pri2
------------------------------ ---- ----
Cl:104 Ac:Unknwn Va:65535 128 10
Protocol One-Way VLAN Tag Enable VID PCP
------------- --------- ----------------- ----- ---
Ethernet False False 1588 7
In a T-BC near T-TSC
Verifying the frequency synchronization source
DM2302-T-BC# show sync-source status
Synchronization Source Status:
Status Detail: Locked
Selected Hierarchy: 2
Clock Source: SyncE GigabitEthernet 1/8
Hierarchy Alarm Status:
Hierarchy Level: 1 2 3
LOCS: TRUE FALSE TRUE
QL: FALSE FALSE FALSE
WTR: FALSE FALSE FALSE
Global Alarm Status:
LOL: FALSE
DHOLD: FALSE
SyncE Quality Level Status:
Interface Tx QL Rx QL ANEG Mode
GigabitEthernet 1/6 QL_PRC QL_DNU N/A
GigabitEthernet 1/7 QL_PRC QL_DNU N/A
GigabitEthernet 1/8 QL_DNU QL_PRC N/A
Verifying the ITU-T G.8275.1 status and time synchronization
DM2302-T-BC# show ptp 0 slave
Slave port Slave state Holdover(ppb)
---------- ------------- -------------
8 LOCKED 0.0
DM2302-T-BC# show clock
System Time : 2017-11-15T08:00:01+00:00
Verifying the parameters that were announced by T-GM
DM2302-T-BC# show ptp 0 parent
ParentClockIdentity Port ParentStatsComputed Variance PhaseChangeRate
---------------------- ---- ------------------- -------- ---------------
00:04:df:ff:fe:40:43:a9 8 False 0 0
Grandmaster clock information:
Identity ClockQuality Pri1 Pri2
----------------------- ------------------------- ---- ----
00:04:df:ff:fe:40:43:1a Cl:104 Ac:Unknwn Va:65535 128 10
On the T-TSC
Verifying the frequency synchronization source
DM2302-T-TSC# show sync-source status
Synchronization Source Status:
Status Detail: Locked
Selected Hierarchy: 1
Clock Source: SyncE GigabitEthernet 1/8
Hierarchy Alarm Status:
Hierarchy Level: 1 2 3
LOCS: FALSE TRUE TRUE
QL: FALSE FALSE FALSE
WTR: FALSE FALSE FALSE
Global Alarm Status:
LOL: FALSE
DHOLD: FALSE
SyncE Quality Level Status:
Interface Tx QL Rx QL ANEG Mode
GigabitEthernet 1/8 QL_DNU QL_PRC N/A
Verifying the ITU-T G.8275.1 status, ToD and time synchronization
DM2302-T-TSC# show ptp 0 slave
Slave port Slave state Holdover(ppb)
---------- ------------- -------------
8 LOCKED 0.0
DM2302-T-TSC# show ptp tod
PTP RS422 clock mode: ToD_Tx, delay : 0, Protocol: Serial, port: 1
DM2302-T-TSC# show clock
System Time : 2017-11-15T08:00:01+00:00
Verifying the parameters that were announced by T-GM
DM2302-T-TSC# show ptp 0 parent
ParentClockIdentity Port ParentStatsComputed Variance PhaseChangeRate
---------------------- ---- ------------------- -------- ---------------
00:04:df:ff:fe:40:43:02 8 False 0 0
Grandmaster clock information:
Identity ClockQuality Pri1 Pri2
----------------------- ------------------------- ---- ----
00:04:df:ff:fe:40:43:1a Cl:104 Ac:Unknwn Va:65535 128 10
Verifying the number of clocks traversed from T-GM to the T-TSC, the time difference between T-GM and T-TSC measured in ns, and the average propagation time:
DM2302-T-TSC# show ptp 0 current
StepsToGrandmaster OffsetFromMaster MeanPathDelay
------------------ ------------------ ------------------
4 0.000,000,012 0.000,000,020
Note: By default, the ITU-T G.8275.1 is not available in DM2302, a test firmware was used to understand how the profile works.
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References