What are VLANs?
VLANs or Virtual Local Area Networks are a set of virtually partitioned and isolated set of packets inside a network. VLANs allow network admins to group related traffic even if the packets do not share a physical switch. By virtually routing and grouping packets the network admin does not have to ensure that all the devices are physically on the same sub-net, thus simplifying designs and creating greater flexibility.
How do VLANs work?
VLANs work by applying an encoded header onto each packet. The header will differ depending on which standard is used, however all standards will include the VLAN ID and VLAN Priority. The VLAN ID will allow you to allocate which VLAN a certain packet type will be allocated to. The VLAN priority will distinguish the priority of each of the packet, and in the case that two packets need to be sent as the same time from a single node, the priority will determine which packet is sent first.
At Tekron the standard IEEE 802.1Q has been implemented as the VLAN standard and can be used to apply PTP and NTP Traffic to a VLAN.
This is done by applying the IEEE 802.1Q header into each packet. The IEEE 802.1Q header contains:
- Tag Protocol Identifier TPID (16 Bits) – A field set to identify the frame as an IEEE 802.1Q tagged Frame
- Priority Code Point PCP (3 Bits) - Class of service that maps to frame priority level
- Drop Eligible Indicator DEI (1 Bits) – Indicates frames eligible to be dropped in the presence of congestion
- VLAN Identifier VID (12 Bits) – Specifies where the frame belongs, 0x000 and 0xFFF are reserved, all other values may be used to specify a VLAN ID