An access network is a telecommunication network allowing to connect one or more end users to a core network, which can be either a circuit-switched network or a packet-switched network.
More particularly, an optical access network is an access network implemented through optical fibers and either active optical devices (amplifiers, regenerators, switches, etc.) or passive optical devices (couplers, splitters, etc.).
A particularly advantageous type of optical access network is the so-called “passive optical network”, or briefly PON. A PON comprises an optical distribution network which is mainly built up of passive optical components forming a tree structure.
The root of such an optical distribution network is connected to an optical line termination, which interfaces the PON with the core network. Further, each leaf of the tree structure is connected to a network termination, which is typically located at the customer premises (e.g. cabinet, building, home), and which interfaces the PON with one or more end users.
Different types of PONs are known in the art, such as ATM PONs, Broadband PONs, Ethernet PONs and Gigabit-capable PONs (or, briefly, GPONs). Each type of PON is adapted to carry different kinds of traffic at different speeds. For instance, a GPON allows to transport different kinds of traffic such as ATM traffic, Ethernet traffic, TDM traffic, etc., at speeds up to 2.5 Gb/s.
In a PON, traffic may be transmitted either in downstream direction (i.e. from the optical line termination to the network terminations) or in upstream direction (i.e. from a network termination to the optical line termination).
Downstream traffic is typically transmitted as a flow of downstream frames generated by the optical line termination. Each downstream frame typically has a header and a payload, which payload comprises bytes of the downstream traffic. Each downstream frame is received by all the network terminations of the PON. Each network termination processes the downstream frame header, thus determining whether the downstream frame comprises bytes of downstream traffic addressed to it. In the negative, the network termination discards the downstream frame. In the affirmative, the network termination forwards the bytes to the end user(s) connected to it.
Upstream traffic generated by each network termination is transmitted according to the known time division multiple access technique.
More particularly, a network termination typically is in charge of transmitting on a PON different upstream traffic flows. Such different upstream traffic flows may be for instance generated by different end users, or by a single end user enjoying different services (e.g. telephone service, video-on-demand service, Internet, etc.) supported by the PON. Typically, each upstream traffic flow traveling in a PON has an identifier. For instance, in a GPON each upstream traffic flow is associated to a Transmission Container (T-CONT), identified by a so-called Alloc-ID, which comprises a sequence of 12 bits.
Typically, the optical line termination of a PON allocates to each upstream traffic flow a respective upstream transmission time, during which bytes of the upstream traffic flow can be transmitted in the PON. For avoiding collisions, upstream transmission times of different upstream traffic flows do not overlap. For instance, in a GPON, the optical line termination allocates to each Alloc-ID a respective upstream transmission time. Further, in a GPON the optical line termination inserts in the header of each downstream frame a field which is termed Upstream Bandwidth Map. The Upstream Bandwidth Map comprises, for each Alloc-ID, a respective sub-field which is termed “grant”. Each grant typically comprises the Alloc-ID, an upstream transmission start time and an upstream transmission stop time. Upon reception of a downstream frame, each network termination processes the Upstream Bandwidth Map comprised in the downstream frame header, and it reads grants relative to Alloc-IDs identifying upstream traffic flows which the network termination is in charge of transmitting.
In the following description and in the claims, the expression “allocating upstream bandwidth” will designate an operation of allocating a respective upstream transmission time to each upstream traffic flow traveling in a PON.
Allocation of upstream bandwidth may be either static or dynamic. In case of static allocation, each upstream traffic flow always has allocated a same upstream transmission time, independently of traffic conditions in the PON. On the contrary, in case of dynamic allocation, upstream transmission times allocated to each upstream traffic flow may vary as a function of traffic conditions in the PON. The Recommendation ITU-T G.983.4, November 2001, paragraph 1.2.3, proposes to provide the optical line termination of a BPON with a logic adapted to perform dynamic allocation of the upstream bandwidth to different Alloc-IDs.
To this purpose, the ITU-T G.983.4, November 2001, paragraph 1.4 divides the total upstream bandwidth in four different parts, which are termed: fixed bandwidth, assured bandwidth, non-assured bandwidth and best effort bandwidth. Each part of upstream bandwidth is associated to upstream traffic flows according to a different criterion.
More particularly, the fixed bandwidth is entirely reserved and cyclically allocated in order to achieve a low cell transfer delay. Therefore, if a traffic flow is provisioned with fixed bandwidth and has no cells to send, idle cells will be sent upstream from the network termination to the optical line termination. Assured bandwidth is bandwidth that is always available to the network termination if the traffic buffer is expected to have cells to transmit. If the traffic flow does not have cells to transmit, the assured bandwidth may be allocated to other traffic flows. Non-assured bandwidth is a high priority variation of additional bandwidth that is assigned to traffic flows with assured bandwidth. Finally, best effort bandwidth is bandwidth that a traffic flow may be able to use if no higher-priority traffic consumes the bandwidth; there is no assurance or guarantee that the bandwidth will be available.
Each upstream traffic flow may have upstream bandwidth allocated according to at least one of the above cited criteria. More particularly, the ITU-T G.983.4, November 2001, paragraph 8.3.5.10.2 defines four different types of “transmission container” to which an upstream traffic flow may belong. Upstream traffic flows belonging to a first type of transmission container (T-CONT1) may have only fixed bandwidth allocated. Upstream traffic flows belonging to a second type of transmission container (T-CONT2) may have only assured bandwidth allocated. Upstream traffic flows belonging to a third type of transmission container (T-CONT3) may have both assured bandwidth and non-assured bandwidth allocated. Finally, upstream traffic flows belonging to a fourth type of transmission container (T-CONT4) may have only best effort bandwidth allocated.
Further, the ITU-T G.983.4, November 2001, paragraph 1.2.4 discloses that, for dynamically allocating upstream bandwidth to Alloc-IDs, each network termination of a BPON may send to the optical line termination the status of its buffers. The OLT reassigns the bandwidth according to these reports.
In GPONs, each network termination may send a DBRu (Dynamic Bandwidth Report upstream) relative to an Alloc-ID, indicating the number of bytes of that Alloc-ID which are waiting to be transmitted in a queue at the network termination.
The ITU-T G.984.3 (paragraphs 3.20, 5.3, 7.7, 8.2.5, 8.4.2) proposes that dynamic allocation of upstream bandwidth in GPONs could be advantageously performed by taking into account both transmission containers and Dynamic Bandwidth Report upstream of the various Alloc-IDs. However, the ITU-T G.984.3 (paragraphs 3.20, 5.3, 7.7, 8.2.5, 8.4.2) does not propose any algorithm for implementing dynamic allocation of upstream bandwidth based on transmission containers and Dynamic Bandwidth Report upstream.
The paper “A GPON MAC controller adapting to varying number of active users, traffic volume and QoS” of J. D. Angelopoulos et al., International Conference on Computer, Communications and Control Technologies 2003, Orlando Fla., Jul. 31, Aug. 1-2, 2003, describes a method for dynamically allocating upstream bandwidth to Alloc-IDs in a GPON network. In particular, the allocation is decided as a function of the QoS parameters (negotiated at flow activation phase) and the temporal properties of this flow. The QoS parameters are passed to a MAC controller of the optical line termination, which keeps them in a bandwidth allocation parameter matrix. On the other hand, the temporal traffic properties are reflected in buffer length variations that are announced to the MAC controller through special report messages. Hence, the MAC controller maintains a matrix reflecting the buffer fill levels. The dynamically changing bandwidth distribution is calculated by the MAC FPGA. In particular, a guaranteed part of the bandwidth is assigned to flows of T-CONT1, T-CONT2 and T-CONT3, implying that they are allocated a fixed number of bytes with a minimum frequency. One timer per traffic flow is implemented and decreased by one every frame. Each time that a timer expires, the corresponding flow has to be assigned the contracted number of bytes, kept in the bandwidth allocation parameter matrix. Then, the surplus bandwidth allocation mechanism is triggered, leading to assignment of bandwidth to flows of T-CONT3 and T-CONT4. Both the guaranteed and the surplus bandwidth assignment require using timers.