Logical reach is the maximum distance between ONU/ONT and OLT except for the limitation ofthe physical layer. In GPON, the maximum logical reach is defined as 60 km.
Extending the reach of GPON
Many operators are currently considering CO consolidation to reduce the access network OPEX. Thus there are proposals for reach-extended GPONs. These technologies may allow much longer distances and potentially also higher splits.
There are basically two ways of doing reach-extended GPONs. The first is to increase the optical power budget by introducing optical amplifiers in the OLT or somewhere along fiber link (called mid-span extension). This allows for distances up to 60 km, which is the logical reach limit of GPON [1]. Work is currently under way in FSAN/ITU-T with G.984.re to standardize this use of optical amplifiers or alternatively opto-electrical for GPON reach-extension.
To circumvent the 60 km limitation, another approach called remote protocol termination (RPT) can be used. The basic idea of RPT is to extend the backplane of an OLT with a 10 Gigabit Ethernet optical link to the GPON capable part. This way, reach-extension of up to 100 km can be achieved. The benefits of the RPT are considerable:
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Long-reach not logically limited to the GPON protocol, i.e. a decoupling of reach from delay
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Conventional optical transport technologies such as WDM and optical amplifiers can be used to reduce the fiber count in uplink network
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The uplink and RPT is flexible in terms of future services and protocols
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Uplink protection is straightforward: well-known Ethernet protection mechanisms can be used such as link aggregation (LAG) or rapid spanning tree (RSTP)
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Simple management of the RPT: Since it is an extension of the OLT backplane, the RPT is basically managed as a GPON card in the OLT
时间: 2009-9-20 09:04
作者: bestfly
楼上很专业.
时间: 2009-9-21 09:32
作者: 招聘专用ID
我自己找到了答案。
G.984.2：
8.2.8.4 Maximum logical reach
The maximum logical reach is defined as the maximum length that can be achieved for a particular transmission system independently from optical budget. It is measured in km and it is not limited by PMD parameters, but rather TC layer and implementation issues.
不受限于PMD，受限于TC和应用层
This is the maximum distance managed by the higher layers of the system (MAC, TC, Ranging), in view of a future PMD specification.
PMD=Physical medium dependant layer parameters of ODN。物理媒介从属层参数。TC=Transmission Convergence 传输会聚
考虑到今后 PMD的规范，这个最大距离受系统的更高层(MAC、TC、测距)控制。