ITU-T G.8265.1 Frequency Profile
ITU-T G.8265.1 defines the “Precision time protocol telecom profile for frequency synchronization”. It defines the options and attributes from IEEE 1588 to be used to deliver frequency synchronization to the end application. It’s developed for telecommunications, but has been used in other systems as well, for example power grids, which have traditionally made use of telecom network technology to connect all of the sub-stations to the control centers.
This profile doesn’t use any assistance from the network between the GM (GrandMaster) and the SOOC (Slave Only Ordinary Clock), such as Boundary Clocks or Transparent Clocks as defined in IEEE 1588, and therefore the performance is dependent of the network. Due to the shared resources nature of packet networks, the packets will experience variable delays. Depending on the network condition each packet will experience its own propagation delay from the GM to the SOOC and vice-versa. The difference between propagation delays of the same type of packets is called PDV (Packet Delay Variation) and greatly affects the quality of the recovered clock.
Unicast messages are used for this profile. The PTP mappings defined in IEEE 1588 Annex D and E, IPv4 and IPv6 respectively, are used. Also, the unicast message is used to establish a PTP domain. The domain numbers can be set between 4 and 23 with the default being 4.
The messages types and message rates used in this profile are:
- Sync/Follow up – min rate: 1 packet every 16 seconds, max rate: 128 packets per second, default: 64 packets per seconds.
- Delay Request/Delay Response – min rate: 1 packet every 16 seconds, max rate: 128 packets per second, default: 64 packets per seconds.
- Announce – min rate: 1 packet every 16 seconds, max rate: 8 packets per second, default: 1 packet every 2 seconds
Note that the message rates are much higher than is normally employed in PTP networks. The reason is the assumption that connecting devices are switches and routers which do not have PTP support. In such devices sync messages and delay request messages often sit in queues.
The Best Master Clock Algorithm BMCA used in ITU-T G.8265.1 is static and each master is isolated by a separated PTP domain that is done through the Unicast communication and the algorithm has the following characteristics:
- Grandmasters do not exchange Announce messages.
- Grandmasters are always active.
- Slaves are always Slave Only Ordinary SOOC clocks
The Grandmaster selection process is based on the SSM (Synchronization Status Message) QL (Quality Level) according with ITU-T G.781. The Grandmaster selection process also takes in consideration the PTSF (Packet Timing Signal Fail) and the Grandmaster priority.
The Clock Class attribute in the Announce messages in PTP is used to carry the SSM QL value. The master with the highest QL that is not experiencing a PTSF failure condition will be selected as the Grandmaster. In case of masters with similar QL values that are not experiencing a PTSF failure condition, the master with the highest priority is selected. The priority is set for each Grandmaster, and it is locally maintained in the Slave Only.
There are different PTSF signals defined in ITU-T G.8265.1:
- PTSF lossSync : Signals the lack of sync messages being received by the slave from the master.
- PTSF lossAnnounce : Signals the lack of Announce messages being received by the slave from the master.
- PTSF unusable : Signals the excessive PDV noise at the input of the slave.
Demonstrating: ITU-T G.8265.1 using DATACOM Switch DM2302
The following set of commands will configure the Grandmaster 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.
- VLAN 1588 configured with IPv4 to send frequency information to the SOOC devices through the interface GigabitEthernet 1/2.
The configuration for the SOOC will be similar but in other network segment to receive frequency information of the GrandMaster device
GrandMaster DM2302
!
vlan 1588
!
interface vlan 1588
ip address 10.15.88.1 255.255.255.0
!
ip route 10.82.65.0 255.255.255.0 10.15.88.2
ip route 192.168.0.0 255.255.0.0 10.15.88.2
!
ptp 0 mode master onestep ip4uni oneway vid 1588 0 profile g8265.1
!
!
interface GigabitEthernet 1/2
switchport trunk allowed vlan 1588
switchport mode trunk
qos trust tag
qos trust dscp
no spanning-tree
ptp 0
ptp 0 announce rate 0.5pps timeout 2
ptp 0 sync rate 64pps
ptp 0 delay-req rate 64pps
!
!
sync-source
wait-to-restore 0
hierarchy 3 quality-level-overwrite prc
hierarchy 3 freq-clock-source 10MHz
!
Slave DM2302
!
vlan 1588
!
interface vlan 1588
ip address 10.82.65.100 255.255.255.0
!
ip route 10.15.88.0 255.255.255.0 10.82.65.1
ip route 192.168.0.0 255.255.0.0 10.82.65.1
!
ptp 0 mode slave onestep ip4uni oneway vid 1588 0 profile g8265.1
ptp 0 uni 0 duration 100 10.15.88.1
!
!
interface GigabitEthernet 1/2
switchport trunk allowed vlan 1588
switchport mode trunk
qos trust tag
qos trust dscp
no spanning-tree
ptp 0
ptp 0 announce rate 0.5pps timeout 2
ptp 0 sync rate 64pps
ptp 0 delay-req rate 64pps
!
!
sync-source
wait-to-restore 0
hierarchy 1 freq-clock-source ptp 0
!
Once devices are properly configured with ITU-T G.8265.1 Frequency Profile, The Frequency Synchronization Status should be Locked on the GrandMaster, and on the Slave should be Acquiring Frequency Lock and after several minutes should get Locked.
Verifying the GrandMaster
DM2302# 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
PTP Status (G.8265.1):
Instance Rx QL PTSF
-------- ------- ------------
0 QL_FAIL lossAnnounce
The SOOC devices connected on the GrandMaster
DM2302# show ptp 0 master-table-unicast
IP Address MAC Address Port Announce Sync
--------------- ----------------- ---- -------- -----
10.82.65.100 00-04-df-32-0e-ee 2 0.5pps 64pps
DM2302# ping ip 10.82.65.100
PING server 10.82.65.100, 56 bytes of data.
64 bytes from 10.82.65.100: icmp_seq=0, time=0ms
64 bytes from 10.82.65.100: icmp_seq=1, time=0ms
64 bytes from 10.82.65.100: icmp_seq=2, time=0ms
64 bytes from 10.82.65.100: icmp_seq=3, time=0ms
64 bytes from 10.82.65.100: icmp_seq=4, time=0ms
Sent 5 packets, received 5 OK, 0 bad
Verifying the Slave-Only OC
DM2302# show sync-source status
Synchronization Source Status:
Status Detail: Acquiring Frequency Lock
Selected Hierarchy: 1
Clock Source: PTP 0
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: TRUE
SyncE Quality Level Status:
Interface Tx QL Rx QL ANEG Mode
PTP Status (G.8265.1):
Instance Rx QL PTSF
-------- ------- ------------
0 QL_EEC1 none
Verifying the parameters that were announced by Grandmaster, as well as, connectivity
DM2302# show ptp 0 parent
ParentClockIdentity Port ParentStatsComputed Variance PhaseChangeRate
---------------------- ---- ------------------- -------- ---------------
00:04:df:ff:fe:c4:1d:e1 2 False 0 -808
Grandmaster clock information:
Identity ClockQuality Pri1 Pri2
----------------------- ------------------------- ---- ----
00:04:df:ff:fe:c4:1d:e1 Cl:104 Ac:Unknwn Va:65535 128 128
DM2302# show ptp 0 slave-table-unicast
Master index 0:
IP Address State MAC Address Sync Rate
----------- ----- ----------------- ---------
10.15.88.1 SYNC 00-04-df-cc-55-b7 64pps
Master Clock ID Slave Port Master Port
----------------------- ----------- ------------
00:04:df:ff:fe:c4:1d:e1 2 2
DM2302# ping ip 10.15.88.1
PING server 10.15.88.1, 56 bytes of data.
64 bytes from 10.15.88.1: icmp_seq=0, time=10ms
64 bytes from 10.15.88.1: icmp_seq=1, time=0ms
64 bytes from 10.15.88.1: icmp_seq=2, time=0ms
64 bytes from 10.15.88.1: icmp_seq=3, time=0ms
64 bytes from 10.15.88.1: icmp_seq=4, time=0ms
Sent 5 packets, received 5 OK, 0 bad
Verifying the PTP Status
DM2302# show ptp 0 slave
Slave port Slave state Holdover(ppb)
---------- ------------- -------------
2 FREQ_LOCKING N.A.
Verifying the number of PTP clocks traversed from Grandmaster to the slave, the time difference between Grandmaster and slave measured in ns, and the average propagation time
DM2302# show ptp 0 current
StepsToGrandmaster OffsetFromMaster MeanPathDelay
------------------ ------------------ ------------------
1 -0.068,885,273 0.000,000,000
After several minutes the frequency is aligned, and the Frequency Synchronization status should get Locked, as well as, the PTP status
DM2302# show sync-source status
Synchronization Source Status:
Status Detail: Locked
Selected Hierarchy: 1
Clock Source: PTP 0
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: TRUE
SyncE Quality Level Status:
Interface Tx QL Rx QL ANEG Mode
PTP Status (G.8265.1):
Instance Rx QL PTSF
-------- ------- ------------
0 QL_EEC1 none
DM2302# show ptp 0 slave
Slave port Slave state Holdover(ppb)
---------- ------------- -------------
2 LOCKED N.A.
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References