Supporting Multimedia with Time-driven Priority


Yoram Ofek
IBM T. J. Watson Research Cente
P.O.Box 218, Route 134
Yorktown Heights, NY 10598

We consider real-time traffic in a heterogeneous internetworking environment with IP routers, MAC bridges, Hubs, Switched LANs etc. We assume that the current routing protocols remain unchanged. However, in this environment, in order to provide quality of service (QoS): bandwidth, delay, constant-bounded jitter and no-loss due to congestion, we suggest a new flow control function called time-driven priority, which is an internal traffic shaping mechanism. It is constructed by three basic components: (i) GPS-based inter-switch synchronization, (ii) non-destructive preemptive priority, and (iii) "RISC-like" packet forwarding scheme.

We show how it supports two classes of connections: constant bit rate (CBR) with deterministic guarantees, and variable bit rate (VBR) with statistical multiplexing. The mechanism does not require to identify and separate the packet flows of different real-time sessions/connections inside the network. As a result, it achieves lower switching complexity when compared with other internal traffic shaping methods. As consequences of the time-driven priority mechanism we further achieve: (1) QoS parameters which are independent of the connection bandwidth, (2) QoS parameters which are independent of the existing heterogeneous internetworking asynchronous data traffic, (3) the capability for policing and securing the network QoS.

In heterogeneous internetworking arbitrary amount of fixed delay is required by every switch for protocol processing in software. The forwarding and synchronization embedded in our solution imply that adding such predefined constant delays can be done without increasing the end-to-end delay jitter.

The time-driven priority mechanism is part of the physical and link layers, and thus, its operation is transparent to the network layer routing protocols, mechanisms for ``best effort'' (non real-time) traffic, and other protocols in higher layers. Inside the network, time-driven priority does not need to separate the real-time connections into different queues with per packet scheduling which is a function of the connection rate. Therefore, our solution has lower switching and buffering complexity compared to other internal shaping methods.