Ethernet passive optical networks (EPONs) are a combination of point-to-multipoint (P2MP) topology and Ethernet framework. So they may be called "P2MP LANs." An EPON operates point-to-point in the optical-network unit (ONU) to optical-line-terminal (OLT) direction and point-to-multipoint in the OLT-ONU direction. Furthermore, EPON is asymmetrical, with the OLT assuming the role of master and ONU the role of slave.
But to date, IEEE 802.3 has defined only two kinds of LAN: point-to-point (P2P) and shared. P2P LAN is the special case of only two stations in a LAN; a shared LAN may also be called a "multipoint-to-multipoint LAN" where three or more stations are connected. In both these LAN types, any frame transmitted by any station would reach all other stations on the same LAN. Existing IEEE 802.1D-based routers, hosts, and bridges know only about P2P and shared LANs. They do not know anything about P2MP LANs and certainly not about EPONs. Additionally, an EPON operating in its native P2MP mode is incompatible with Ethernet's spanning tree protocol (STP). Using STP on a P2MP LAN can result in either delivering multiple copies of data frames or no data frames at all to portions of the bridged network.*
IEEE 802.1D-based bridges have been applied widely in enterprise campus and residential environments. Thus a LAN type that cannot fit easily into the existing standards base is clearly handicapped in its bid to become widely accepted. Therefore, EPON must have the ability to emulate P2P or shared LANs if the technology is to succeed. There is a design scheme for EPON equipment that realizes a reflection function in the OLT and a filtering function in the ONU for P2P LAN emulation (P2PE) and shared LAN emulation (SE).
P2PE is emulation of P2P communication between two end stations, with one outside the EPON and the other inside the EPON (e.g., the ONU) or between two ONUs in an EPON; SE is emulation of multicast or broadcast communication among all ONUs in an EPON. According to these definitions, three problems must be solved:
- EPON is a multicast medium, very suitable for single-copy broadcasting (SCB) but not for P2P communication. To offer a P2PE service between the OLT and ONU, a virtual P2P link (logic link) must be established between the OLT and one ONU. The logic link is identified by LLID.
- There are no direct links between ONUs, so the communication between ONUs must be transferred through the OLTs. Therefore, OLTs must have the ability to automatically reflect all frames received from any ONU back down to other ONUs, except the originating ONU.
- To take maximum advantage of the broadcast nature of the downstream channel by sending a single copy of a frame received by all ONUs, a general broadcast_LLID is also defined. It is necessary to distinguish the broadcast frame sent by an ONU inside the EPON from the one sent by an outside station. The broadcast frame from outside the EPON can be received by all ONUs in an EPON, and it can be identified by broadcast_LLID. Meanwhile, the broadcast frame from an ONU can't be received by the original ONU, so it can't use broadcast_LLID. Therefore, an additional mode bit must be defined that indicates whether the reflected frame is unicast or broadcast.
We propose a design scheme of EPON equipment (OLT and ONU) for P2PE and SE, as shown in Figure 1. The EPON physical layer (PHY) module provides a 1000Base-PX optical interface conforming to IEEE 802.3ah Draft 1.3. Frame receive (FR) and frame transmit (FT) modules do the same work as a standard Ethernet full-duplex-mode media access control (MAC). The FR removes the preamble of the received frame, checks CRC (cyclic redundancy check), then delivers it to parser/multiplexer module; the FT adds preamble and CRC to the frame, then delivers it to the PHY for transmission. The parser module identifies the received frame, then delivers it to different circuits. The multiplexer module schedules frames of different types to transmit. Both the network interface and user interface can be considered a normal Ethernet Layer 2 or Layer 3 switch. The OLT's network interface can provide a 1000Base-X or 1000Base-T interface for the OLT to connect with an outside Internet. The ONU's user interface can provide several 10/100-Mbit/sec Ethernet interfaces to access users.
The functions of these modules are prerequisites for transmitting Ethernet packets between an OLT and ONUs; thus, we call them basic function modules. The discovery module (DM), address learning and reflection module (ALRM), reflected data buffer, registration module (RM), and filter module are key for ONU registration, LLID assignment, frame reflection, and filtering.
Discovery and registration modules. The main functions of the DM and RM are ONU registration and LLID assignment. The upstream channel of an EPON is shared by several ONUs in time-division multiple access (TDMA) mode. To avoid data collisions and increase efficiency, ONU transmissions are arbitrated by the OLT. The OLT will register all online ONUs and allocate transmission windows to them. An ONU without grant cannot transmit any data. So a newly connected ONU shall first complete the discovery and registration progress that is also the process for establishing the logic link and assigning an LLID. Once a logic link is established, the ONU will be able to share the upstream bandwidth resource within a designated time. Four multipoint control protocol (MPCP) messages will have been applied during the discovery and registration process: GATE, REGISTER_REQ, REGISTER, and REGISTER_ACK.
The DM in an OLT should periodically or non-periodically open the discovery window (i.e., broadcast the discovery GATE messages) to provide newly joined ONUs with registration chances. When an unregistered ONU receives the discovery GATE message, its RM will respond with a REGISTER_REQ message to the OLT within the discovery window. Because all unregistered ONUs may transmit messages to the OLT within one discovery window, a collision may be caused. Therefore, to reduce collision probability and shorten successful registration time, the ONU's RM should have a random delay before sending a REGISTER_REQ message.
After sending the discovery GATE, the OLT's DM may successfully receive several REGISTER_REQ messages from several ONUs and will process them in the sequence of first come/first served. Once the OLT decides to accept one ONU's register requirement, the DM will assign an LLID to the ONU and deliver the LLID to the ONU through a REGISTER message. The LLID will be stored by the ONU's RM. Thus the logic link between this ONU and OLT has been established. A REGISTER_ACK message will then be sent from the ONU to the OLT via this logic link. After the OLT's DM receives this message, the registration process for this ONU will be completed and the OLT will schedule grants for this ONU.
Address learning and reflection module. The ALRM is installed only in the OLT. It has three functions: MAC address and LLID learning, reflection frame check, and LLID lookup for downstream frames.
The purpose of learning is to realize the reflection of upstream frames; therefore, the ALRM learns only the upstream data frames' source address (SA) and LLID. The downstream frames' SA and LLID are not learned. The learning results are stored in the address table, which has two entries corresponding the SA with the LLID of one station. Because one ONU can be connected with many users' devices, several MAC addresses may correspond with one LLID.
Another function of the ALRM is to discover frames that need to be reflected from received upstream data frames and request the parse module to put them into the reflecting data buffer to realize the communications between ONUs. A received upstream frame's destination address (DA) will be checked by ALRM. If the DA is unicast and can be found in the address table, meaning the destination is inside the EPON, this frame shall be reflected back; the ALRM will notify the parser module to put it into the reflecting data buffer. If the DA is a broadcast one, it shall be put into both the upstream data buffer and the reflecting data buffer.
There are two kinds of downstream frame storage: in downstream data buffer and reflecting data buffer, respectively. The frames in the downstream data buffer are sent by stations outside the EPON, while the frames in the reflecting data buffer are reflected upstream frames. These two types of frames are treated differently by the ALRM (see Figure 2). For the frame in the downstream data buffer, if it is unicast, the ALRM will look up the address table using the DA as an index to search for the corresponding LLID. If the DA exists in the address table, the corresponding LLID value will be inserted into the preamble, and the mode bit should be unicast; if not, or if the DA is a broadcast one, a broadcast_LLID should be used for this frame and the mode bit should be broadcast.
For the frame in the reflecting data frame buffer, if it is unicast, the same process is followed; if it is a broadcast frame, then a search will be made for the LLID in the address table using the SA as an index, and the mode bit should be broadcast.
Filter module in ONU. As mentioned, EPON is a broadcast medium in the downstream direction, and a frame sent by the OLT can arrive at all ONUs. Therefore, the ONU filters out the packets not destined for it. The filtering method is shown in Figure 3. When a new downstream frame arrives, the filter module will check its LLID and mode bit. If the frame is unicast and has a matched LLID, then the module will accept it and deliver it to the FR module; otherwise, the module will discard it. If the frame is broadcast and has a broadcast_LLID or an unmatched unicast LLID, then the module will also accept this frame. If the frame is broadcast but the LLID equals the receiving ONU's LLID (which means the source of that frame was the receiving ONU), then the module will discard this frame.
To interoperate with the existing and widely applied routers and bridges, etc., an EPON must have the ability to provide P2PE and SE service. Our proposed system design scheme for realizing the reflection filtering not only makes EPON compatible with the existing routers and bridges, etc., but also utilizes the downstream broadcast capacity of EPON for SCB.
Liu Yang works at the access network products department of Fiberhome Telecommunication Technologies (Wuhan, China) for her doctoral paper; she's receiving her doctoral training at the Department of Electronics & Information Engineering, HUST (Wuhan). Qian Mao works at the Wuhan Research Institute of Post and Telecommunications.
*N. Finn, Cisco Systems, "Spanning Trees and IEEE 802.3ah EPONs," May 2002, http://www.ieee802.org/3/efm/public/may02/finn_2_0502.pdf, and "Two Models for IEEE 802.3ah EPONs," May 2002, http://www.ieee802.org/3/efm/public/may02/finn_3_0502.pdf.