G.8275.1 Telecom Profile

Precision Time Protocol (PTP) is a protocol for distributing precise time and frequency over packet networks. PTP is defined in the IEEE Standard 1588. It defines an exchange of timed messages

PTP allows for separate profiles to be defined in order to adapt PTP for use in different scenarios. A profile is a specific selection of PTP configuration options that are selected to meet the requirements of a particular application.

This recommendation allows for proper network operation for phase and time synchronization distribution when network equipment embedding a telecom boundary clock (T-BC) and a telecom time slave clock (T-TSC) is timed from another T-BC or a telecom grandmaster clock (T-GM). This recommendation addresses only the distribution of phase and time synchronization with the full timing support architecture as defined in ITU-T G.8275.

Effective from Cisco IOS Release 3.18.1SP, the Cisco ASR 901 Router supports G.8275.1 telecom profile.

Why G.8275.1?

The G.8275.1 profile is used in mobile cellular systems that require accurate synchronization of time and phase. For example, the fourth generation (4G) of mobile telecommunications technology.

The G.8275.1 profile is also used in telecom networks where phase or time-of-day synchronization is required and where each network device participates in the PTP protocol.

Because a boundary clock is used at every node in the chain between PTP Grandmaster and PTP Subordinate, there is reduction in time error accumulation through the network.

More About G.8275.1

The G.8275.1 must meet the following requirements:

Non-participant devices, that is, devices that only forward PTP packets, and PTP transparent clocks are not allowed.
The telecom grandmaster (T-GM) provides timing to all other devices on the network. It does not synchronize its local clock with any other network element other than the Primary Reference Time Clock (PRTC). T-GM in locked mode must have phase and frequency sources that are traceable to PRTC. T-GM in locked mode must always have QL-PRC/PRS frequency. T-GM can be in holdover when losing phase. In such case, its default clock class is defined based on the available frequency source quality.
The telecom time subordinate clock (T-TSC) synchronizes its local clock to another PTP clock (in most cases, the T-BC), and does not provide synchronization through PTP to any other device.
The telecom boundary clock (T-BC) synchronizes its local clock to a T-GM or an upstream T-BC, and provides timing information to downstream T-BCs or T-TSCs. If at a given point in time there are no higher-quality clocks available to a T-BC to synchronize to, it may act as a grandmaster.

The following figure describes a sample G.8275.1 topology.

PTP Domain

A PTP domain is a logical grouping of clocks that communicate with each other using the PTP protocol.

A single computer network can have multiple PTP domains operating separately, for example, one set of clocks synchronized to one time scale and another set of clocks synchronized to another time scale. PTP can run over either Ethernet or IP, so a domain can correspond to a local area network or it can extend across a wide area network.

The allowed domain numbers of PTP domains within a G.8275.1 network are between 24 and 43 (both inclusive).

PTP Messages and Transport

The following PTP transport parameters are defined:

For transmitting PTP packets, either the forwardable multicast MAC address (01-1B-19-00-00-00) or the non-forwardable multicast MAC address (01-80-C2-00-00-0E) must be used as the destination MAC address. The MAC address in use is selected on a per-port basis through the configuration. However, the non-forwardable multicast MAC address (01-80-C2-00-00-0E) will be used if no destination MAC is configured.

The source MAC address is the interface MAC address.

For receiving PTP packets, both multicast MAC addresses (01-80-C2-00-00-0E and 01-1B-19-00-00-00) are supported.
The packet rate for Announce messages is 8 packets-per-second. For Sync, Delay-Req, and Delay-Resp messages, the rate is 16 packets-per-second.
Signaling and management messages are not used.

PTP Modes

Two-Way Operation

To transport phase and time synchronization and to measure propagation delay, PTP operation must be two-way in this profile. Therefore, only two-way operation is allowed in this profile.

PTP Clocks

Two types of ordinary clocks and boundary clocks are used in this profile:

Ordinary Clock (OC)

OC that can only be a grandmaster clock (T-GM). In this case, one port will be used as master port.

The T-GM uses the frequency, 1PPS, and ToD input from an upstream grandmaster clock.

Note

The T-GM master port is a fixed master port.

OC that can only be a slave clock (T-TSC). In this case, only one PTP port is used for T-TSC, which in turn will have only one PTP master associated with it.

Figure 3. Ordinary Clock As Slave Clock (T-TSC)

Boundary Clock (T-BC)

T-GM in boundary clock is not supported.
T-BC that can become a master clock and can also be a slave clock to another PTP clock.

If the BMCA selects a port on the T-BC to be a slave port, all other ports are moved into the master role or a passive state.

PTP Ports

A port can be configured to perform either fixed master or slave role or can be configured to change its role dynamically. If no role is assigned to a port, it can dynamically assume a master, passive, or slave role based on the BMCA.

A master port provides the clock to its downstream peers.

A slave port receives clock from an upstream peer.

A dynamic port can work either as a master or a slave based on the BMCA decision.

In Cisco’s implementation of the G.8275.1:

OC clocks can support only fixed master or slave port.
One PTP port can communicate with only one PTP peer.
The maximum number of clock-ports on a T-BC is limited to the number of 1GE and 10GE interfaces.

Alternate BMCA

The BMCA implementation in G.8275.1 is different from that in the default PTP profile. The G.8275.1 implementation is called the Alternate BMCA. Each device uses the alternate BMCA to select a clock to synchronize to, and to decide the port states of its local ports.

Benefits

With upcoming technologies like LTE-TDD, LTE-A CoMP, LTE-MBSFN and Location-based services, eNodeBs (base station devices) are required to be accurately synchronized in phase and time. Having GNSS systems at each node is not only expensive, but also introduces vulnerabilities. The G.8275.1 profile meets the synchronization requirements of these new technologies.

Prerequisites for Using the G.8275.1 Profile

PTP over Multicast Ethernet must be used.
Cisco ASR 901 Router must be enabled using network-clock synchronization mode QL-enabled command on T-GM and T-BC.
Every node in the network must be PTP aware.
It is mandatory to have a stable physical layer frequency whilst using PTP to define the phase.
Multiple active grandmasters are recommended for redundancy.

Restrictions for Using the G.8275.1 Profile

PTP Transparent clocks are not permitted in this profile.
Changing PTP profile under an existing clock configuration is not allowed. Different ports under the same clock cannot have different profiles. You must remove clock configuration before changing the PTP profile. Only removing all the ports under a clock is not sufficient.
One PTP port is associated with only one physical port in this profile.
There is no support for BDI and VLAN.
Signaling and management messages are not used.
PTP message rates are not configurable.
Non-hybrid T-TSC and T-BC clock configurations are not supported.
SyncE is not compliant with G.8262 when G.8275.1 is enabled.
Virtual Port is not supported.

Configuring the G.8275.1 Profile

Note

To know more about the commands referenced in this module, see the Cisco IOS Interface and Hardware Component Command Reference or the Cisco IOS Master Command List .

Configuring Physical Frequency Source

For more information, see the Configuring Synchronous Ethernet ESMC and SSM section in the Clocking and Timing chapter of this book.

Creating a Master-Only Ordinary Clock


ptp clock ordinary domain 24 
clock-port master master profile g8275.1 
transport ethernet multicast interface Gig 0/0

Creating an Ordinary Slave


ptp clock ordinary domain 24 hybrid
clock-port slave-port slave profile g8275.1
transport ethernet multicast interface Gig 0/0

Creating Dynamic Ports

Note

Dynamic ports can be created when you do not specify whether a port is master or slave. In such cases, the BMCA dynamically chooses the role of the port.


ptp clock boundary domain 24 hybrid
clock-port bc-port-1 profile g8275.1 local-priority 1
transport ethernet multicast interface Gig 0/0
clock-port bc-port-2 profile g8275.1 local-priority 2
transport ethernet multicast interface Gig 0/1

Verifying the Local Priority of the PTP Clock


Router# show ptp clock dataset default
CLOCK [Boundary Clock, domain 24]
  Two Step Flag: No
  Clock Identity: 0x2A:0:0:0:58:67:F3:4
  Number Of Ports: 1
  Priority1: 128
  Priority2: 90
  Local Priority: 200
  Domain Number: 24
  Slave Only: No
  Clock Quality:
    Class: 224
    Accuracy: Unknown
    Offset (log variance): 4252

Verifying the Port Parameters


Router# show ptp port dataset port
PORT [SERVER]
  Clock Identity: 0x49:BD:D1:0:0:0:0:0
  Port Number: 0
  Port State: Unknown
  Min Delay Req Interval (log base 2): 42
  Peer Mean Path Delay: 648518346341351424
  Announce interval (log base 2): 0
  Announce Receipt Timeout: 2
  Sync Interval (log base 2): 0
  Delay Mechanism: End to End
  Peer Delay Request Interval (log base 2): 0
  PTP version: 2
  Local Priority: 1
  Not-slave: True

Verifying the Foreign Master Information


Router# show ptp clock dataset parent
CLOCK [Boundary Clock, domain 24]

  Parent Clock Identity: 0x3C:8:F6:FF:FE:79:3A:14
  Parent Port Number: 1
  Parent Stats: No
  Observed Parent Offset (log variance): 0
  Observed Parent Clock Phase Change Rate: 0

  Grandmaster Clock:
    Identity: 0x3C:8:F6:FF:FE:79:3A:14
    Priority1: 128
    Priority2: 128
    Clock Quality:
      Class: 58
      Accuracy: Within 25us
      Offset (log variance): 22272

G.8275.1 Deployment Scenario

The following example illustrates a possible configuration for a G.8275.1 network with two masters, a boundary clock and a slave. Let’s assume that master A is the primary master and B is the backup master.

Figure 5. Topology for a Configuration Example

The configuration on master clock A is:


ptp clock ordinary domain 24 
  clock-port master master profile g8275.1
    transport ethernet multicast interface GigabitEthernet 0/0

The configuration on master clock B is:


ptp clock ordinary domain 24 
  clock-port master master profile g8275.1
			transport ethernet multicast interface GigabitEthernet 0/1

The configuration on the boundary clock is:


ptp clock boundary domain 24 hybrid
  local-priority 3
  clock-port slave-port-a profile g8275.1 local-priority 1
    transport ethernet multicast interface Gig 0/0
  clock-port slave-port-b profile g8275.1 local-priority 2
    transport ethernet multicast interface Gig 0/1
  clock-port master-port profile g8275.1
    transport Ethernet multicast interface Gig 0/2

The configuration on the slave clock is:


ptp clock ordinary domain 24 hybrid
  clock-port slave-port slave profile g8275.1
    transport Ethernet multicast interface Gig 0/0

Additional References

Related Topic	Document Title
Cisco IOS commands	Cisco IOS Master Commands List, All Releases
Interface and Hardware Component commands	Cisco IOS Interface and Hardware Component Command Reference
Clocking and Timing	Clocking and Timing

Standards


Standard	Title
G.8275.1/Y.1369.1 (07/14)	SERIES G: TRANSMISSION SYSTEMS AND MEDIA, DIGITAL SYSTEMS AND NETWORKS Packet over Transport aspects – Synchronization, quality and availability targets

MIBs


MIB	MIBs Link
—	To locate and download MIBs for selected platforms, Cisco software releases, and feature sets, use Cisco MIB Locator found at the following URL: http://www.cisco.com/go/mibs

RFCs


RFC	Title
—	There are no new RFCs for this feature.

Cisco ASR 901 Series Aggregation Services Router Software Configuration Guide

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Cisco ASR 901 Series Aggregation Services Router Software Configuration Guide

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