High-bandwidth QoS sensitive services such as large scale video surveillance generally depend on provisioned capacity delivered by circuit-switched technology such as SONET/SDH. Yet development in layer 2 protocol sets and manageability extensions to Ethernet standards propose layer 2 packet switching technology as a viable, cheaper alternative to SONET/SDH. Layer 2 switched networks traditionally offer more complex topologies; in this paper we explain general QoS issues with layer 2 switching and show the impact of topology choice on service performance. Keywords: layer 2 switching, quality of service, topology.
INTRODUCTIONElectronic circuit-switching technology has existed for quite some time, and was originally envisioned for telephony services. Circuit-switching has culminated into SONET/SDH technology, which offers a wide range of features that facilitate manageability, resilience and quality of service. As bandwidth needs grew larger, optical transmission was used in the physical layer. At the same time, required bandwidth of services grew larger as well. Of course one such service consists of best-effort TCP/IP packet transmission for internet connectivity. More interestingly, digital transmission also became popular for services that traditionally were carried over an analog encoding. Given the guaranteed high bandwidth needs and QoS constraints of such services such as video streaming, SONET/SDH is the obvious digital transmission technology.On the other hand, the packet switched TCP/IP layer model defines a packet (or frame) switched transport layer (layer 2). Ethernet and related IEEE defined physical layers are the de facto standard for TCP/IP transport. Yet development in layer 2 protocol sets and manageability extensions to Ethernet standards propose layer 2 packet switching technology as a viable, cheaper alternative to SONET/SDH. Furthermore, native optical transmission is available for higher (1 Gbit/s and up) bandwidth Ethernet standards.While circuit-switched technology is essentially connection-oriented by design, Ethernet is basically a connectionless protocol. Similar to how TCP/IP can be transformed into a connection-oriented network through the use of MPLS[1] (adding a shim header to TCP/IP packets), connection-oriented functionality can be added to Ethernet by adding a forwarding label to the Ethernet frame. The ongoing design of MPLS TP[2] (transport profile) is a result of a joint effort between ITU-T and IETF. MPLS TP removes features from MPLS not needed for pure connection-oriented operation.This paper looks at layer 2 switching technology -specifically Ethernet-as a viable alternative to SONET/SDH. Carrier Ethernet[3] offers functionality (such as QoS) typically only associated with circuitswitched carrier-grade transport. Where SONET/SDH offers deterministic multiplexing behavior because of fixed time slotting, frame-based Ethernet uses statistical multiplexing, which influences QoS performance; this is detailed in Section 2. The impact of more complex topology constructi...