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Mobile Backhaul by Esa Metsala, Juha Salmelin

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6.3 Frequency Synchronization in Packet Networks

Since the dawn of internet protocol (IP), IP packets have been transported over telecom networks. Until the end of the last century, the physical layer of inter-office connections was one of the traditional telecom technologies, analog, PDH, or SDH/SONET. However, the introduction of optical Ethernet interfaces towards the end of the 1990s started to change the scene. Now, the most common interface in telecom routers is Ethernet. Mobile networks long remained the last fortress of legacy transport since the bandwidth requirement of cellular base stations remained low. However, then HSPA (high-speed packet access) combined with fixed monthly fees exploded the bandwidth demand and PDH access did not scale up. Ethernet came as the saviour in the price per bit problem – but associated with another problem: synchronization.

Ethernet has been designed as a low-cost plug and play technology. Combined with IP friendly adaptation to higher layers, Ethernet has become the dominant link layer and physical layer technology in packet networks. Within the Ethernet scene bridges have almost completely overcome repeaters. Correspondingly, Ethernet itself spans just a single link, terminating the clock at the receiving port. The idea of connecting the received clock to outputs (as in SDH/SONET) had been around since the early years of the past decade and standardizing Synchronous Ethernet began mid-way through. However, it takes a long time from starting ...

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