SYNCHRONIZATION OVER PACKET

THƯC HIÊN : P.KTNV

Synchronization. Mobile RBS requirements TDM Base Synchronization Syn Ethernet Packet Base Synchronization (ntp/ptp)

NỘI DUNG TRÌNH BÀY

TA=1/fA

TB=1/fB

fA=fB

t

t

TA=1/fA

TB=1/fB

fA=fB

Frequency Synchronization

PhaseSynchronization

A

B

A

B

01:00:00

TA=1/fA

TB=1/fB

fA=fB

Time Synchronization

01:00:10

01:00:00 01:00:10

A

B

Synchronization

Aligning clocks with respect to frequency

Aligning clocks with respect to phase

Aligning clocks with respect to time. The two clocks must utilize the same epoch. Time synchronization implicitly includes phase and frequency synchronization

Frequency accuracy:

Mobile Base Station Requirements

Mobile Network Architecture Frequency Sync Time-of-day / Phase Sync

CDMA2000

GSM UMTS-FDD LTE-FDD UMTS-TDD LTE-FDD with MBMS-Single Freq. Network

LTE-TDD Mobile WiMAX TD-SCDMA

Requirements (air interface)

Mobile Base Station Requirements

• Frequency accuracy of 50ppb (for both FDD and TDD) over one time slot (2/3ms) and time accuracy of 2.5us (for TDD only)– 50 ppb : 50 ns per second in respect to nominal value or i.e. ±45Hz

error at 900 MHz carrier frequency– With TDD base stations need to be synchronized to each other at

frame level in same coverage area to avoid interference

• There is no standard specification on the network input frequency – It depends on vendors implementation of the base station PLL (e.g.,

oscillator choice, algorithm)

Basestation

RF circuits

RF output < ± 50ppb

RF input < ± 16ppb Basestation PLL

~ ~ ~ ~ ~

Network input Requirements

Mobile Base Station Requirements

• Impacts of bad synchronization on wireless network– Corrupt data– Dropped calls– Co-channel interferences– Slow handover between cells– Speech clipping (loss of speech segments)

F1+ f2

F1

T1 T2

+/- 50ppb

+/- 50ppb

Time

Mobile cannot lock to BTS2

and call is dropped

BTS2 drifts outside 50ppb window

BTS 2

BTS 1

Typical TDM-based synchronization architecture

RAN BSRAN NC

G.823/824 Compliant Clock

High accuracy Primary Reference Source

(PRC) clock

50ppbT1/E1

STM-1/OC-3nxT1/E1 SONET/SDH

• Constant bit rate- T1 is 1.544 Mbit/s rate

- E1 is 2.048 Mbit/s rate

• Strict sync requirements- To avoid “bit slips”

• TDM interface requirements- ITU G.823/G.824: Defines jitter and wander limits for traffic and synchronization interfaces

• Frequency synchronization only- Achieved via the physical TDM interface

- Phase/Time is not supported

- If needed, usually supported using GPS

Synchronous Ethernet-intermediate node support

• Native Ethernet today recovers the Rx clock, but does not use it for Tx• The Rx clock to be recovered and used an input to a G.813 equipment clock• The equipment clock to be used as the Tx clock• Point-to-point technology

– All intermediate node must support clock recovery scheme

PRC Traceable Reference Timing Signal on external clock port

• Packet based synchronization challenges:

- No end-to-end physical layer clock synchronization

- Packet delay variation – caused by queuing delays, routing changes, network technology

- Symmetric/asymmetric network

• Packet-based methods are adaptive in nature:

- No end-to-end physical layer clock synchronization

- No need for support of a network-wide synchronization reference

- Timing recovery process is based on (inter-)arrival time of the packets

- In-band (e.g.,ACR/RTP) or out-of-band (e.g.,1588-2008, NTP)

RAN BS

RAN NC

High accuracy Primary Reference Source (PRC) clock

50ppb T1/E1STM-1/OC-3

nxT1/E1 Carrier Ethernet/IP/

MPLS

IWF IWF

Packet-Based Synchronization Architecture

If there is no continuous physical layer carrying periodic eventsthen we must distribute frequency as information (data) The first method is Send a periodic stream of timing packets

Second method is insert into the packet a timestamp (NTP/PTP)

How to transfer Frequency over Packet

• Timing protocol of the Internet standardized by IETF• Pre-NTP era - Time Protocol IETF RFC 868 -1983• NTP developed by David Mills (University of Delaware)

– Synchronize hosts and routers over the Internet

– Independent of the physical layer– Transfer frequency and time of the day

• NTP Specifications– NTPv0 (RFC 958) – 1985– NTPv1 (RFC 1059) – 1988– NTPv2 (RFC 1119) – 1989

– NTPv3 (RFC1305) – 1992 - Current standard– NTPv4 (under development) – Stable but not yet formalized in an RFC– SNTPv4 (RFC2030) – revisions 1995 - 1996 - 2003

• Simple NTP client/server implementation

• Packet based synchronization mechanism– UDP/IP layers messaging (unicast and multicast)– Client and server

t2

t3

t4

t1

Master Clock Slave Clock

Req

Rsp

Network Time Protocol (NTP)

NTP HIERARCHICAL

IEEE 1588 PTP 2008

• Packet based synchronization mechanism– UDP/IP layers messaging (multicast and unicast) over Ethernet

• Client/server model– Master clock, slave clock (ordinary clock)

– Intermediary nodes may or may not support IEEE1588 PTP (unlike SyncE)

– On-path support mechanisms• Boundary clock

– It acts as a slave clock at port that connects to the grandmaster, and as a master to all other ports

– It isolates the “down stream” clocks from any delays and jitter within the switch/routers

• Transparent clock– It measures residence time of PTP events

ITU SYN RECOMMENDATION

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