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Advantage of LAN synchronization

• No Synchronization levels needs to be configured.
• When a base station fails (Not the LAN Master) it will not have any impact on the other base stations.

• Fewer base stations required as the overlapping area of the base stations is smaller. The overlapping area for handset handover can be smaller, because neighbored base stations do not need to receive each other in stable error free quality, but they must still be able to detect each other for the process of dynamic channel selection.

Disadvantage of LAN synchronization

• Base stations must be in the same network segment.
• Precision Time Protocol (PTPv2), deviation must be lower then 500 ns (rms).
• High reliable LAN network is needed, see below for the requirements.

Here an example how PTP synchronisation works, using the two-step method.

1. The LAN sync master clock is at 11:00:00
2. The slave clock is at 11:00:03, there is a 3 seconds offset
   
   We imagine a one-second delay to transfer the package from the LAN sync master to the slave
   
   
3. Master sends a sync message at exactly 11:00:00 (T1 value)
4. After the sync message, a follow-up message is send immediately (with T1 value)
5. The T1 value is not in the sync message
6. The follow-up message means, “The sync message you received was sent at the time specified in the T1 value”
7. The slave stored the time it received the sync message as the T2 value. The local clock believes is it 11:00:04. There is a 4 second time difference. But as it does not know how long it takes to receive a message from the Master, it can not update the local clock.
   
   To correct itself, two values needs to be created:
   - The delay, how long does it take to receive the PTP messages from the master
   - The offset to update its clock
   
   delay = ((T2 - T1) + (T4 - T3)) / 2
   
   offset = ((T2 - T1) - (T4 - T3)) / 2
   
   
8. After receiving the Follow-up message, the slave sends a Delay_Request message. The slave stores the time as T3 value
   
   Because of the one-second delay, the master receives it at 11:00:03, this is stored as T4 value
9. The master replies immediately with a Delay_Response message that has the T4 value.
   
   The slave clock has now four timestamps

• T1: 

Included in the follow-up message from the master clock: 11:00:00.

• T2:

The time that the slave clock received the sync message: 11:00:04

• T3:

The time that the slave clock send the Delay_Request message: 11:00:05

• T4:

The time the master receives the Delay_Request message: 11:00:03

Precision Time Protocol (PTPv2), deviation must be lower then 500 ns (rms)

Accuracy: How accurate the slave clock is to the master. Every clock, even an atomic clock, has some deviation from the actual time. The lower, the better.

The DECT measures the PTP deviation and shows/counts if the value > 500ns on the device status page: o-thr-exc  

Base stations must be in the same network segment

PTP messages are forwarded in a VLAN but not between VLANs

The less switch hops. the lower the transmission delay and its jitter will be.

Each switch in the network has to forward the PTP messages, this takes time and adds delay. (Indicated with the blue arrows)

The higher the bandwidth or quality of used switches is regarding packet delay and its jitter, the lower the packet delay and the lower the packet delay jitter will be.

Enhanced packet processing logics (like L3 switching or packet inspection) could have significant negative impact on the resulting packet delay jitter. If possible, they should be deactivated for the N870 base stations connected switch ports.

Significantly increased traffic load on a switch, in the range of the maximum throughput, could have significant negative impact on the packet delay jitter.

VLAN based prioritisation of LAN packets could be a fruitful measure to minimize packet delay and its jitter for N870 base stations.

VLAN disconnection from other network devices is mandatory.

Switch port

The DECT system must be connected to an switch port with:

• Minimum 100 Mbit/s full-duplex speed
• Allow multicast / broadcast messages
• PoE IEEE 802.3af < 3.8 W (Class 1)

• VLAN disconnection from other network devices is mandatory.

• Supports VLAN tagging

• Activation of Quality of Service mechanisms is mandatory

• DECT Manager and all base stations in the same layer 2 segment

• The whole Layer 2 network in which the BS/DMs are operating have to IP/Ethernet Multicast enabled in case of Synchronisation via LAN

DECT LAN Sync (DLS) long term statistics:

• ptp-d: ptp clock delay. ptp clock offset to master clock. This value should not exceed 500ns for a longer time
• dls-d: Measured offset of the local DECT-SYPO signal to the master DECT-SYPO signal. This is the DECT-SYNC criteria and should not exceed 1000ns for a longer time
• ptp-pd: ptp path delay. This is an indication of the transmission path quality. The account of switches and their processing speed and queue delay's influence this value. Jitter should be as less as possible.

DECT LAN Sync (DLS) statistics overview table

The latest LAN sync output for more detailed information see this wiki page.

DECT LAN Sync (DLS) log

LAN sync log messages.

DECT synchronisation

Distance between DECT base is measured with the SPK and should be -65 / -70 dBm

LAN synchronisation

The DECT base must be able to see the neighbor base via the DECT protocol.

The visualization tool can be used to check this.