Cirrus Logic CobraNet - Manual (Page 9)

CobraNet Programmer’s Reference
Overview
DS651PM25
©
Copyright 2006 Cirrus Logic, Inc. 9
1.2 Protocol
The CobraNet protocol operates at the Data Link Layer also referred to as OSI Layer 2 or
MAC layer. CobraNet uses four basic packet types described below. All CobraNet packets
are identified with a unique Ethernet protocol identifier (0x8819) assigned to Cirrus Logic.
As CobraNet is a Local Area Network (LAN) technology and not a Wide Area Network
(WAN) technology, CobraNet does not utilize Internet Protocol (IP) to transport audio.
Packet Bridge packets are generic packets and are not identified as CobraNet packets.
Packet bridging is discussed in more detail in
Section 2.2 "Packet Bridge" on page 13 .
1.2.1 Beat Packet
A multicast packet with an address of 01:60:2B:FF:FF:00 and which contains network
operating parameters, clock and transmission permissions. The beat packet is
transmitted 750 times per second from a single CobraNet device on the network (the
conductor) and indicates the start of the main isochronous cycle. Since the beat packet
carries the clock for the network, it is sensitive to delay variation. If the delay variation
specification shown in
Table 1 on page 10 is not met, CobraNet devices may not be able
to lock their local sample clocks to the network clock. The beat packet is typically small
(on the order of 100 bytes) but can be large on a network with numerous active bundles.
1.2.2 Isochronous Data Packet (or Bundle)
A multicast or unicast packet. A stream of isochronous data packets carries an audio
bundle. Though the size of the packet is dependent on the number and format of the
audio channels contained in the bundle, isochronous data packets are typically large. A
bundle carrying 8 channels of 20-bit audio at 48kHz sample rate in the standard latency
mode is approximately 1300 octets.
Isochronous data packets are transmitted in response to receipt of the beat packet.
However when low-latency modes are utilized, isochronous data packets are transmitted
twice or four times per isochronous cycle. The first transmission is in response to beat
packet receipt, the other transmissions are made at evenly timed intervals thereafter.
Because CobraNet devices buffer isochronous data packets, out-of-order delivery of data
packets is acceptable as long as the packet forwarding delay specifications in
Table 1 on
page 10 are not exceeded.
1.2.3 Reservation Packet
A multicast packet with an address of 01:60:2B:FF:FF:01 used by the CobraNet protocol
to allocate bandwidth and establish connections between CobraNet interfaces. CobraNet
devices transmit reservation packes as needed or typically once per second at minimum.
1.2.4 Serial Bridge Packet
A multicast or unicast packet used to bridge asynchronous serial data between CobraNet
interfaces. Serial bridging is discussed in more detail in
Section 2.1 "Serial Bridge" on
page 12 .