архив прош.сесий / архив прош.сесий / sinkhr_v_paket_setyakh
.pdf
Частотная синхронизация
Referencetiming signal |
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to systemA |
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Referencetiming signal |
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to system B |
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System B |
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θ = phase offset of signal from system B |
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relative to signal from systemA |
Часы двух NE синхронизированы по частоте (frequency synchronized), если у частот есть общий знаменатель .
Разница между фронтами синхросигнала называется фазовым сдвигом (phase offset)
Часы двух NE, разница фаз которых постоянна находятся в состоянии «phaselocked» и как следствие «frequency synchronized».
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Распространение частоты
Распространение частоты на физическом уровне или в пакетной среде
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EEC |
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SONET/S |
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SyncE/E |
SyncE/E |
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DH/SSM |
DH/SSM |
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SMC |
SMC |
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Physical Layer Frequency Distribution |
Recovered Clock |
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IEEE1588PTP Exchange |
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Packet Layer Frequency Distribution
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Физический уровень
t(n-1) t(n) t(n+1) time (t)
NE clock
Rate: 8 kHz to 100’s of MHz
CDR
Синхросигнал зачастую реализуется в виде цифрового периодического сигнала.
CDR … Clock Data Recovery
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Физический уровень – significant instance
Синхронизация и шум
Rate: 8 kHz to 100’s of MHz
Timing signal |
Significant instants |
Timing jitter and |
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Time |
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wander |
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Source: |
G. |
_F04 |
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ITU-T8260(10)G. |
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Example : 25 MHz signal
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Частота в пакетной среде
Adaptive Clock Recovery (ACR) или IEEE1588 Precision Time Protocol (PTP)
Reference |
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Recovered |
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Clock |
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Clock |
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Packet |
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Packet |
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master |
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slave |
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Packet-based timing Flow
Три ключевых шага:
‒Генерация: из физической среды в пакетную сеть
‒Передача: транспорт пакетов или фреймов по пакетной сети
‒ Восстановление:из пакетной сети в физический сигнал
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Пакетная среда - significant Instants
Источник: ITU-T G.8260 (201007)
Significant instants
Rate : typical 1-64 Hz
F |
Payload |
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Payload |
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Payload |
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Payload |
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Event packets |
Packet delay variation |
Time |
(header, payload and footer) |
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G.8260(10)_F05 |
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Сигнал синхронизации может быть
‒периодическим (e.g., CES) или
‒апериодическим (e.g., NTP, PTP) с
Дополнительной информацией (например, timestamps) определяя точное положение во времени синхроимпульса по временной шкале источника
CES … Circuit Emulation Service;NTP … Network Time Protocol
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Типовая архитектура Маршрутизатора / Коммутатора
Egress queuing, |
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Ingress |
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classification, |
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scheduling, |
Forwarding |
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policing, |
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shaping |
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decision |
queuing |
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Ingress classification,
Egress queuing,
policing,
scheduling,
queuing
shaping
Packet Delay Variation (PDV): prop_time_pkt#n != prop_time_pkt#m
Ассиметрия: prop_time != prop_time 
PDV … PacketDelay Variation
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Packet Delay Variation
Нагрузка и QoS
Каждый тип оборудования обладает своим профилем PDV (и ассиметрии)
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80% |
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background |
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20% |
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w/strict priority |
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80% |
background |
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w/strict priority |
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background |
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without |
with |
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strict |
strict |
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priority |
priority |
20% background |
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Idle |
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with strict priority |
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X-axis: number of observed packets
Y-axis: packet delay from the minimum delay observed during measurement
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Восстановление в пакетной сети
Три основных задачи
1.Классификация пакетов
2.Выбор «хороших» пакетов
3.Обработка «хороших» пакетов и
восстановление временного сигнала
XO |
PLL |
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Localreference |
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Packet |
Servo |
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selection |
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Packet |
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Packet |
master |
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Slave |
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Reference |
Recovered |
Clock |
Clock |
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Packet-based timing Flow |
XO … Oscillator; PLL … Phase LockedLoop |
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Выбор источника синхронизации в NE
SETS - Synchronous Equipment Timing Source
Network Element (NE)
PTP 
SETG … Synchronous EquipmentTiming Generator (PLL - Phase Locked Loop)
Three Selector table to control
‒System Frequency
‒Output Interface Frequency
SETS … SynchronousEquipmentTiming Source;SETG … SynchronousEquipmentTiming Generator
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