"This Recommendation concerns 10 gigabit-capable symmetric passive optical network (XG(S)-PON) systems with optical link budgets up to the logical limits of the transmission convergence (TC) layer. It extends the capabilities of [b-ITU-T G.987.4]@ limited to asymmetric XG-PON@ to XG(S)-PON@ including its dual rate working capacity. The increased optical capability@ which includes both increased overall fibre length and increased overall splitting ratio@ is referred to in this Recommendation as ""reach extension"". The primary concerns addressed are the increase of the loss budget and the management of optical impairments. This Recommendation considers mid-span extension@ which uses an active extension node placed in the middle of the optical network. The recommended parameters for the optical distribution network(s) [ODN(s)] involved in this scheme are specified. Single-sided extension is considered as an improvement to the optical line terminal (OLT) interface. The systems considered here must remain compatible with existing optical network units (ONUs). Furthermore@ the approaches should maintain compatibility with existing OLTs to the maximum extent possible. It is recognized that some modification of the OLT equipment may be necessary@ but this should be minimized. Two system architectures are considered: optical amplification and opto-electronic regeneration. These can be viewed as providing reach extension at the physical layer. The key interfaces and functional blocks in each of these architectures are identified and specified. Optionally@ optical-electronic-optical (OEO) type reach extender (RE) units may support a burst mode (BM) to continuous mode (CM) conversion function@ as specified in clause 6.2. In this case@ it is also necessary to introduce a CM to BM conversion function before the S/R interface at the OLT. Optionally@ both optical amplifier (OA) and OEO type RE units may support the wavelength conversion (WC) function@ as specified in clause 6.4. In case of WC-enabled RE units@ it is also necessary to introduce a WC-function block before the S/R interface. Optionally@ combination RE units may support all the functions described in this Recommendation@ and the G-PON RE functions described in [ITU-T G.984.6]. The optional support of a type B protection in RE units is described."