nvnuAbstract: In the competitive broadband access environment, providing access to “triple play” services (voice, video, and high-speed data) is an important way for both telephone and CATV network providers to increase their revenue. Pas-sive Optical Networks (PONs) are a cost-effective, flexible, and future-proof medium for providing triple play services. The 1 Gbit/s IEEE 802.3ah Ethernet PON (EPON) and ITU-T G.984 2.5 Gbit/ s PON (GPON) are currently being deployed for triple play service access. However, high defini-tion switched digital video service such as IPTV will require more bandwi圆跳动怎么测量
dth. This paper provides a tutorial overview of the IEEE 802.3av 10Gbit/s Ethernet PON (10G EPON) standard, including the ways in which it differs from EPON.
i. introduCtion
Passive optical networks (PONs) have become popular as a way to reduce the number of optical transceivers and fibers in access networks by mov-ing the splitters closer to the subscriber[2]. The most popular PON protocols being deployed today are the IEEE 802.3ah Ethernet PON (EPON) and ITU-T Gigabit PON (GPON)[3]. The emerging IEEE 802.3av 10G EPON [1] standard is the latest and highest speed PON protocol using time divi-sion multiple access (TDMA). At least two broad factors are already at work to push a migration to 10G EPON. The first is the increased bandwidth of home networks. Both the IEEE 802.11 wire-less networks and the wireline home networks are increasing in capacity beyond 100 Mbit/s. Part of this factor is the decreasing cost and increasing availability of 802.11n and 1 Gbit/s Ethernet inter-faces on new personal computers. The other fac-tor is the expectation of increased customer desire for more on-demand digital video delivery. While the current generation of PON systems can satisfy some of this demand, the migration to HD video will require higher speed PONs.
After a brief introduction to the concepts of TDMA PON protocols, this paper describes the 10G EPON protocol and how it differs from the EPON protocol.
ii. introduCtion to Pon
As illustrated in Figure 1, a PON system uses a single optical transceiver at the optical line termi-nal (OLT) to serve multiple subscribers over a fiber tree/bus network constructed with passive optical signal splitters. The 10G EPON protocol uses time
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division multiple access (TDMA) in which the OLT broadcasts the downstream data and synchro-nization information to all optical network units (ONUs). The ONUs extract their downstream data based on packet address information. In the upstream direction, the OLT assigns (grants) each ONU time slots in which to transmit its upstream data.
A guard time is required between the upstream burst transmissions of different ONUs so that their transmissions don’t overlap at the OLU receiver. Note that the ONUs must turn their lasers off when they are not transmitting in order to prevent spon-taneous emission noise from ONUs closer to the O
LT from interfering with data transmissions from ONUs further from the OLT. In order to minimize this guard time, the OLT uses a protocol to deter-mine the round trip delay time between itself and each ONU and takes it into account when assign-ing the ONU upstream transmission times.
iii. iEEE 802.3av 10Gbit/s EthErnEt-basEd Pon (10G EPon)
10G EPON shares much of its protocol with EPON. A combination of coarse wave division multiplexing (CWDM) and time division multi-plexing (TDM) is used in order to allow EPON and 10G EPON systems to co-exist on the same PON. As with EPON, 10G EPON relies on V oIP for car-rying voice traffic and circuit emulation service (CES) for carrying other TDM client signals.
3.1 10G EPON Physical Layer
The downstream data rate of 10G EPON is 10 Gbit/s. Both 1 Gbit/s and 10 Gbit/s rates are sup-ported in the upstream direction. The 64B/66B block line code is used for all of the 10Gbit/s sig-nals with a resulting signal line rate of 10.3125 Gbit/s. The 1 Gbit/s upstream uses the same 8B/10B block line code as EPON, giving a line rate of 1.25 Gbit/s.
The downstream and upstream data is trans-mitted over a single PON fiber, using WDM to separate
the upstream and downstream signals. The wavelengths used by the different upstream and downstream signals are shown in Figure 2. Since there are many ONUs on the PON and only a single OLT, the wavelength bands were chosen to allow using less expensive lasers at the ONUs. Lasers that operate in the 1270nm and 1310nm re-gions are less expensive than those operating in the 1500-1600nm range due both to the technology re-quired for their fabrication and the relative market volumes for the lasers.
For 1 Gbit/s upstream operation, 10G EPON uses the same 1310nm wavelength as the EPON upstream signal. This allows the OLT to use the same receiver for all 1 Gbit/s signals. The 10 Gbit/ s upstream signals use a separate wavelength band, however since it overlaps with the 1 Gbit/s upstream wavelength band, the upstream transmis-sion is time shared between 1 and 10 Gbit/s ONUs.
A dynamic bandwidth allocation (DBA) algorithm allocates the bandwidth of the upstream signal be-tween the EPON and 10G EPON ONUs.
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方案The advantages to allowing 10G EPON to oper-ate over the same PON optical distribution network that is already being used for EPON include: Allowing customers to use the most cost-effec-tive ONU for the desired service Allowing a network to migrate from EPON to 10G EPON by upgrading the OLT then migrating the ONUs as needed
Continued operation of the existing network and services during the upgrade of the network
Figure 3. illustrates a network where an OLT supports a mix of EPON ONUs, ONUs with 10Gbit/s downstream and 1Gbit/s upstream, and ONUs with 10Gbit/s upstream and downstream. For convenience, the wavelength color key in Figure 3. is consistent with the key for Figure 2. Note that WDM is used to separate the 1Gbit/s and 10Gbit/s traffic in the downstream direction, and a combination of WDM and TDM is used in the upstream direction. The ONU discovery and other prot
ocol extensions to support the co-existence of EPON and 10G EPON ONUs are discussed in the appropriate sections below.
As its reference for the optical link loss budgets, the 802.3av specification uses a split ratio of either 1:16 (i.e., 16 ONUs on a single PON connecting to one OLT interface) or 1:32. In practice, larger split ratios such as 1:64 or 1:128 can be used if the oth-er optical losses (e.g., the length of the fiber) are constrained to offset the additional 3dB loss that is incurred when the split ratio is doubled.(Note that the practical limits on the split ratio are a function of the combination of the optical parameters, such as loss budget, and the desired per-ONU band-width.) All of the interfaces are specified to oper-ate at an uncorrected bit error rate no worse than (BER) 10-3. After FEC correction, the bit error rate will be no worse than 10-12. The nomenclature adopted to identify the different optical interface options may be summarized as follows:
PRX interfaces use 10 Gbit/s downstream and 1 Gbit/s upstream transmission
PR interfaces use 10 Gbit/s for both downstream and upstream transmission
PR-D n and PRX-D n (n = 10, 20, 30) refer to the OLT optical interface specification
PR-U n and PRX-U n (n = 10, 20, 30) refer to the ONU optical interface specification
PR10 and PRX10 specifies an optical channel insertion loss of ≤20 dB for ≥10 km reach with 1:16旧唐书李白传
肉桂酸split ratio
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调查报告PR20 and PRX20 specifies an optical channel insertion loss of ≤24 dB for ≥20 km reach with a 1:16 split ratio or ≥10 km reach with a 1:32 split
ratio
PR30 and PRX30 specifies an optical channel insertion loss of ≤29 dB for ≥20 km reach with a 1:32 split ratio
As with EPON, the 1550-1560nm-wavelength band is reserved for downstream video transmis-sion.
Following the same approach as EPON, the up-stream burst timing is relaxed for 10G EPON in order to allow using existing off-the-shelf compo-nents. The standard has mechanisms to allow for future tighter timing to be implemented with better components for increased bandwidth efficiency.
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Dual-rate operation
Dual-rate operation refers to an OLT that simul-taneously receives upstream signals from ONUs using 1 Gbit/s and 10 Gbit/s rates. The received 1 Gbit/s and 10 Gbit/s streams can either be sepa-rated in the optical domain or electrical domain. Unfortunately, since both signals time-share the same upstream wavelength band, it is not pos-sible to use WDM filters to isolate them in the optical domain. Separating the signals in the opti-cal domain involves using a 1:2 optical splitter. Each of the two splitter outputs goes to its own photodetector followed by a receiver with a filter optimized for its signal rate in order to maximize the receiver’s sensitivity. The drawback with this approach is the roughly 3dB additional optical loss introduced by the 1:2 optical splitter. If this ad-ditional loss cannot be tolerated, a low-gain optical amplifier must be used in the receiver. Splitting in the electrical domain allows using a single photo-detector and introduces no additional optical signal loss. There are different approaches to recovering the 1 and 10 Gbit/s signals in the electrical domain with tradeoffs between performance and complex-ity.
3.2 Signal formats and Media Access Control (MAC) protocol
3.2.1 Signal formats
With the exception of the added forward error cor-rection (FEC) coding, the downstream signal is simply a stream of Ethernet frames and Idle char-acters, as with a point-to-point 10 Gbit/s Ethernet signal. The upstream signal is also essentially an Ethernet stream except that, as discussed above, a TDMA burst format is used. The upstream signal also uses FEC.
The format of an upstream burst is illustrated in Figure 4. The synchronization patterns at the beginning of an upstream transmission burst allow the OLT to synchronize its receiver to new burst from an ONU. The Burst Delimiter pattern is used by the OLT to determine the start of 66B block transmission and the FEC codeword alignment. The 66-bit value of the Burst Delimiter is 0x 4 97 BA C4 69 F0 4C 88 FD (which results in a trans-mission bit sequence of 10 11101001 01011101 00100011 10010110 00001111 00110010 00010001 10111111). The FEC codeword align-ment can be achieved in the presence of transmis-sion errors. This burst delimiter is followed by two 66-bit blocks containing Idle characters. These Idle characters allow the OLT to synchronize its descrambler and delineate the start of the actual data frame. The first two blocks of Idle characters are included in the initial FEC codeword.
The preamble and start of frame delimiter (SFD) are modified for EPON and 10G EPON from their normal values for Ethernet. Specifically, the preamble bytes are replaced by the transmitting MAC’s MODE and LLID variables. While the Ethernet 8-character preamble/SFD consists of/ S/, 0x55, 0x55, 0x55, 0x55, 0x55, 0x55, and 0xd5, the EPON and 10G EPON preamble/SFD consists of 0x55, 0x55, SLD, 0x55, 0x55, 2-octet LLID, and CRC-8. The SLD is the Start of LLID Delim-iter, and has the value 0xd5. The LLID is the two-octet logical_link_id field that uniquely identifies the ONU MAC. The MSB of the two octets that contain the LLID is the MODE indication bit. As discussed in section 3.4, the LLID is assigned to the ONU by the OLT during the registration phase of the discovery process. The CRC-8 covers the SLD through the LLID octets, and uses the genera-tor polynomial x8 +x2 + x + 1.
The upstream transmission ends with a burst terminator pattern comprised of three 66-bit blocks of alternating zeros and ones (1010 … 10) after the last FEC codeword of the burst. The ONU turns off its laser at the beginning of the burst terminator pattern, which insures that it will be completely off by the end of the burst.
Each ONU has one unique Logical Link Identi-fier (LLID) that the OLT associates to the ONU for uni-cast traffic. In other words, these MAC instances are used to emulate a point-to-point con-nection b
etween an ONU and the OLT over the PON. In addition, the OLT has two Single Copy Broadcast (SCB) MAC instances that are used as
an efficient mechanism to broadcast downstream traffic to the ONUs. Such a broadcast is used for broadcast data or for when the OLT must commu-nicate with unregistered ONUs. In the upstream direction, an SCB MAC is only used for client registration. The LLID value of 7F-FF is associ-ated with the SCB MAC for 1 Gbit/s downstream operation and the LLID value of 7F-FE is associ-ated with the SCB MAC for 10 Gbit/s downstream operation. An ONU can use higher layer network-ing processing, such as VLAN filtering and IGMP snooping, to narrow the amount of received multi-cast traffic that is passed to applications. It is pos-sible that these higher layers may require addition multicast MAC instances at the OLT, in which case an OLT can have more MACs than two plus the number of ONUs.
Multi-Point Control Protocol PDUs (MPCPDUs) are control frames used by the ONUs to make their requests for bandwidth, and by the OLT to assign it. As illustrated in Figure 5, the MPCPDU1 frame is a basic 802.3 MAC control frame containing a 4-byte timestamp and a 40-byte field filled with data and padding as needed. MPCPDU messages are also used for the discovery and ranging proc-esses, as discussed in sections 4.3 and 4.4. MPCP-DUs are layered below the data interface, and h
ave higher priority than any data packet. This ensures that the bandwidth requests and grants are sent in a timely manner.
3.2.2 MAC-layer control protocol The 10G EPON MAC-layer control protocol is based on the protocol for EPON and includes enhancements for management of 10G FEC and inter-burst overhead. This MAC protocol oper-ates on the basis of the ONUs informing the OLT of their upstream bandwidth requirements, and the OLT scheduling and granting bandwidth to the ONUs to transmit their upstream data. The details of the MAC protocol are described in this section and are illustrated in Figure 6 with the downstream and upstream data flows.
3.2.3 GATE messages for upstream bandwidth
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