Understanding Stratum Levels

Before learning about stratum levels, it is important to understand the term Network Time Protocol (NTP). An NTP is a protocol that is used to synchronize the clocks of the computer to some time reference. NTP helps network engineers synchronize time across multiple network devices. Now, the question is, why there's a need for synchronization? If the network devices are out of sync by even a few milliseconds, it can be very difficult for network administrators to determine the sequence of events taking place within the network. Time synchronization is a crucial part of monitoring a network and resolving issues within it. NTP allows network devices to synchronize their clocks with a central source clock. An NTP network usually gets its time from an authoritative time source, such as an atomic clock or a radio clock attached to a time server. NTP then distributes this time across the network.

In Network Time Protocol, time is distributed from a high precision time device such as radio clock or atomic clocks, hierarchically, with the primary servers at the top keeping the master time, and distributing the time down to the secondary servers and so on. Stratum is defined as the distance from an authoritative time source to the network devices. 

Each level of this hierarchy is assigned a number, starting with zero (0) for the reference clock at the top. Stratum 0 means that a device is directly connected to the atomic clock e.g., a GPS antenna. Stratum 1 devices have a direct connection with the primary time source, and they provide that time to clients on stratum 2 via a network connection. Stratum 2 devices can function like a server by providing time to clients on stratum 3, and Stratum 3 can provide it to those on stratum 4, and so on. A total of 15 synchronized stratum levels are possible, but each one introduces another layer of network delay, causing accuracy to decrease.

NTP Hierarchy

The different Stratum levels have been explained below:

Stratum 1: Stratum 1 is a completely autonomous source of timing, which has no other input. The usual source of Stratum 1 timing is an atomic standard (Cesium Beam or Hydrogen Maser) or reference oscillator (OCXO).

Stratum 2: Stratum 2 receives time from stratum 1 servers and then act as a server for stratum 3. Typical examples of Stratum 2 clocks are Rubidium Standards and Double Oven OCXO’s.

Requirements of Stratum 2 Systems include:

• 1.6 × 10^-8 minimum adjustment/tracking range
• Less than 1.6 × 10^-8 drift with no input reference in 1 year
• Less than 1 × 10^-10 short term drift in 24 hours

Stratum 3: Stratum 3 is defined as a clock system that tracks input from Stratum 2. Some Stratum 3 clock equipment is not adequate to time SONET network elements. SONET is a communication protocol that is used to transmit a large amount of data over relatively large distances using optical fiber.

Requirements of Stratum 3 Systems include:

• Minimum tracking range of 4.6 × 10^-6. 
• Less than 3.7 × 10^-7 short term drift in 24 hours. This is about 255 frame slips in a 24 hours period.

Stratum 3E: Stratum 3E is a new standard created to support SONET equipment requirements, which was not stated in stratum 3.

Stratum 4: Stratum 4 is driven by stratum 3 clock, but it is not recommended as a source of timing for any other clock system. Additionally, Stratum 4 clock systems don't have holdover stability and runs free within the adjustment range limits without a reference. Holdover is the period required to keep the network sync-stabilized when the source of sync is disrupted or temporarily unavailable.

Stratum 4E: Stratum 4E is a proposed new clock standard that would account for a holdover characteristic that isn't free running. This new level would be intended for customer provided equipment by expanding their networks.

Stratum 1 clock can control stratum 2, 3E, 3, 4E, or 4 clocks. A Stratum 2 clock can drive stratum 2, 3E, 3, 4E, or 4 clocks. A Stratum 3E clock can drive stratum 3E, 3, 4E or 4 clocks. A Stratum 3 clock can drive stratum 3, 4E or 4 clocks. A Stratum 4E or 4 clock is not recommended as a source of timing for the next stratum level, since the accuracy by then has been decreased.