IEEE 802.17 MAC Layer
At the MAC level, the primary objective is to provide enhanced services for the transmission of Ethernet packets over a ring-based interconnect topology. This implies a simple mapping from the Ethernet frame format to the RPR frame format.
In addition, the MAC protocol provides a flexible QoS capability to support various classes of real-time and nonreal-time traffic. Two major classes are supported:
Synchronous for low-latency requirements, voice, and video
Asynchronous for high-bandwidth cost-effective LAN support.
The asynchronous class provides latency-insensitive subclasses such as the following:
Unfair. High-priority traffic, such as prioritized HTTP traffic.
Fair. Lower-priority traffic, such as management messages.
Bulk. Opportunistic traffic, such as file transfers.
Key features of the MAC protocol include the following:
Each node on the ring has a unique MAC address. As in other IEEE 802 MAC protocols, each MAC frame includes a destination address and a source address.
For a unicast frame (only one destination), it's the responsibility of the destination to remove the frame. The effect is that multiple frames can be circulating on the ring at any one time, providing efficient spatial reuse of the fiber capacity.
For a broadcast or multicast frame, each destination copies the frame as it goes by, and the source removes the frame after a complete circulation.
The frame header includes a class-of-service indicator that can be used to implement priority handling of frames.
Unlike FDDI and the IEEE 802.5 token ring, RPR doesn't use a token to control access to the medium. With the high data rates and need for flexible QoS, a token-passing medium access scheme would be inadequate. Instead, IEEE 802.17 relies on a capacity-management scheme based on channels. Although the full details of the channel architecture are not yet in place, the following general characteristics apply to RPR:
A channel is a reserved portion of the capacity of the link from a sender to one or several receivers. Channels are multiplexed on the fiber or frequency band when WDM equipment is used.
A channel is a dynamic resource that is managed in quantum steps, up to the full capacity of the link. The bandwidth allocated to the channel can be changed dynamically during its lifetime.
A channel can have multiple receivers, enabling true multicast operations (as well as unicast and broadcast).
Channels are simplex, making it possible to guarantee resources both upstream and downstream, and thus provide inherent support for asymmetric traffic patterns with high bandwidth utilization.