Dynamic bandwidth assignment

Written on 3:21 PM by ooe

Most access network systems, with the exception of point-to-point systems, use fixed, or static, bandwidth assignment. Most point-to-point systems allow static multiplexing and therefore the following section does not directly apply to them. With PON access network Systems, the situation is somewhat different.

A typical APON system would run at a bit rate of 155 Mbit/s, allowing a total customer bandwidth of around 130 Mbit/s. If this were statically multiplexed, the bandwidth per customer on a 32 way PON would be in the order of 5 Mbit/s. Now 5 Mbit/s is not a lot and could be provided by alternative technologies such as DSL.

Dynamic bandwidth assignment relies on the fact that it is uncommon for each user on a PON to require their full bandwidth at any one time. In fact each user can have potential access to bandwidths in excess of their mean bandwidths for extended periods of time and this is what dynamic bandwidth assignments would allow.

Instead of bandwidth being assigned on a per user basis, bandwidth is provided on demand, with the user unaware during normal operation that their share of the overall bandwidth ‘pot’ has increased.

On an APON system, it is not too difficult to arrange this as the speed of transmission over the PON is quite fast and therefore latency issues are kept to a minimum. The user is unaware of the precise mechanism by which the ATM packets are queued and then transmitted across the PON.

The principle recommendation from the ITU-T that deals with dynamic bandwidth assignment is G.983.4 (‘A broadband optical access system with increased service capability using dynamic bandwidth assignment’). Dynamic bandwidth assignment is also known as DBA.

In the DBA recommendation, G.983.1 is modified to allow for extra messages required in order to support the DBA mechanism. The DBA standard is designed to include:

• Performance objectives (for example, bandwidth assignment delay, maximum waiting time).

• Application functionality (for example, dynamic bandwidth assignment for bursty traffic and for ONU/ONT aggregated traffic composed of different traffic classes).

• Fairness criteria and protocols (for example, dynamic bandwidth assignment based on ONU/ONT status reporting, dynamic bandwidth assignment based on OLT monitoring, dynamic bandwidth assignment based on a combination of reporting and monitoring).

• Backwards compatibility.

As a background to understanding the dynamic bandwidth principles, it is essential to understand the way an APON works, at the cell level.

The basic APON standard is the ITU-T Recommendation G.983.1 which specifies a flexible access platform to provide broadband services through passive optical networks. In this Recommendation, the upstream traffic from the ONUs/ONTs to the OLT is transferred in a frame of 53 time-slots (cell slots). Each time-slot consists of three bytes of PON layer overhead and an ATM cell or a PLOAM cell. The upstream bandwidth is shared among the associated ONUs/ONTs. The OLT controls each upstream transmission from the ONUs/ONTs on a time-slot-by-timeslot basis. This is accomplished by sending data grants in downstream PLOAM cells.

PLOAM data grant cells are sent in the downstream direction to all ONUs/ONTs. The data grants are addressed to specific ONUs/ONTs and contain parameters that include the number of upstream data grants and the time-slots for the grants that are assigned to the individual ONU/ONT. Originally the grants were assigned in a static manner and only updated when a new connection is provisioned on a PON or an existing connection is removed from a PON. Once the bandwidth is provisioned, the OLT would continuously send the assigned grants to the associated ONUs/ONTs; subsequently, the OLT would receive corresponding user cells in the upstream. This current granting mechanism was efficient for real-time traffic but lacked any dynamic assignment capability.

However, the above mechanism is not efficient for non-real-time traffic, or where bandwidths are increased beyond the norm. For non-real-time traffic types, the ability to assign bandwidth dynamically is expected to provide higher efficiency than the original static granting mechanisms.

In ITU-T G.983.1, the OLT grant generation and distribution was updated when a new connection is provisioned on a PON or an existing connection was removed from a PON. Once the bandwidth is provisioned, the OLT would continuously send the assigned grants to the associated ONUs subsequently, the OLT would receive corresponding user cells in the upstream.

However, ITU-T G.983.1 was intended to enable a wide range of broadband services, including those that do not have a constant bit rate. For example, the Internet connects to many bursty traffic sources, which are best accommodated by ATM SBR Class 2 or GFR, which have less rigid requirements on cell transfer delay and cell delay variation. Mapping these non-real-time services into a fixed bandwidth channel prevents the ONUs on a PON from dynamically sharing the upstream PON bandwidth. For these non-real-time traffic types, the ability to assign bandwidth dynamically is expected to provide higher efficiency than the current static granting mechanisms.

The DBA protocol consists of three strategies, referred to as non status reporting, status reporting and hybrid types. The non status reporting type strategy is invoked by monitoring traffic in the OLT where lengthening buffer queues can be viewed as a requirement for extra bandwidth. The status reporting type strategy is invoked by status reports sent from ONUs to the OLT, where requests for more bandwidth are explicitly given. The hybrid strategy is invoked by both monitoring traffic in the OLT and processing in the OLT the status reports from ONUs. The recommendation does not specify detailed mechanisms or algorithms for these strategies, but rather message requirements and the required interfaces at specified reference points.

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