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:
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Non-participant devices, that is, devices that only forward PTP packets, and PTP transparent clocks are not allowed.
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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.
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The telecom time client/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.
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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.
One-Step and Two-Step Clock Mode
Both one-step and two-step clock modes are supported in the G.8275.1 profile.
A subordinate port must be capable of receiving and processing messages from both one-step clocks and two-step clocks, without any particular configuration. However, the primary clock supports only one-step mode.
PTP Clocks
Two types of ordinary clocks and boundary clocks are used in this profile:
Ordinary Clock (OC)
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OC that can only be a grandmaster clock (T-GM). In this case, one or more PTP ports will be used as server ports.
The T-GM uses the frequency, 1PPS, and ToD input from an upstream grandmaster clock.
Note
The T-GM server port is a fixed server port.
Figure 2. Ordinary Clock As T-GM 
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OC that can only be a subordinate/client clock (T-TSC). In this case, only one PTP port is used for T-TSC, which in turn will have only one PTP server clock associated with it.
Boundary Clock (T-BC)
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T-BC that can only be a grandmaster clock (T-GM).
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T-BC that can become a server clock and can also be a server clock to another PTP clock.
If the BMCA selects a port on the T-BC to be a server port, all other ports are moved into the server role or a passive state.
PTP Ports
A port can be configured to perform either fixed server or client role or can be configured to change its role dynamically. If no role is assigned to a port, it can dynamically assume a server, passive, or client role based on the BMCA.
A server port provides the clock to its downstream peers.
A client port receives clock from an upstream peer.
A dynamic port can work either as a server or a client clock based on the BMCA decision.
In Cisco’s implementation of the G.8275.1:
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OC clocks can support only fixed server or client port.
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One PTP port can communicate with only one PTP peer.
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BC can have a maximum of 64 ports. Fixed client ports are not supported on the BC.
Virtual Port Support on T-BC
G.8275.1 introduces the concept of a virtual port on the T-BC. A virtual port is an external frequency, phase and time input interface on a T-BC, which can participate in the source selection.
Alternate Best Primary Clock Algorithm
Note |
The BMCA is referred to as the Best Primary Clock Algorithm in this document. |
The Best Primary Clock Algorithm implementation in G.8275.1 is different from that in the default PTP profile. The G.8275.1 implementation is called the Alternate Best Primary Clock Algorithm. Each device uses the alternate Best Primary Clock Algorithm 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.
- 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
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PTP Transparent clocks aren’t permitted in this profile.
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Changing the PTP profile under an existing clock configuration isn’t allowed. Different ports under the same clock can’t have different profiles. You must remove the clock configuration before changing the PTP profile. Only removing all the ports under a clock isn’t sufficient.
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One PTP port is associated with only one physical port in this profile.
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There’s no support for BDI and VLAN.
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Signaling and management messages aren’t used.
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PTP message rates aren’t configurable.
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Non-hybrid T-TSC and T-BC clock configurations aren’t supported.
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ITU-T G.8273.2 requires that standardized performance testing for boundary clocks to be conducted using perfect inputs, which means zero traffic load. When 1GE interfaces experience high utilization (for example, over 40-50%), the resulting time error may exceed the G.8273.2 Class A limits. If your application demands the highest level of time accuracy, we suggest configuring PTP and data traffic on separate interfaces. This is because a hardware limitation causes packet delay variations on the following Cisco ASR 920 Series Routers when configured with the G8275.1 profile on 1GE interfaces: ASR-920-12CZ-A/D, ASR-920-4SZ-A/D, ASR-920-24SZ-IM/M, ASR-920-24TZ-M, ASR-920-12SZ-IM.
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