Horizon Compact Plus Release 1.0.1
Wireless Ethernet Product User Manual
– Volume 2
Appendix E - 802.1P Priority Tagging Overview
IEEE 802.1P The Institute of Electrical and Electronics Engineering (IEEE) 802.1P signaling method is
used for traffic prioritization at OSI Reference Model Layer 2. 802.1p is a spin-off of the 802.1Q (Vlans)
standard. Network adapters and switches route traffic based on the priority level for best-effort Quality of
Service (QoS).
The 802.1Q VLAN standard specifies a VLAN tag that appends to a MAC frame. The VLAN tag has two
parts: The VLAN ID (12-bit) and Prioritization (3-bit). The prioritization field was not defined in the VLAN
standard and the 802.1P implementation defines this prioritization field.
To be compliant with 802.1p, Layer 2 switches must be capable of grouping incoming LAN packets into
separate traffic classes.
Eight classes are defined by 802.1p. Although network managers must determine actual mappings, IEEE
has made broad recommendations. The highest priority is seven, which might go to network-critical traffic
such as interactive video and voice. Data classes four through one range from controlled-load
applications such as streaming multimedia and business-critical traffic - carrying voice traffic, for instance
- down to "loss eligible" traffic. The zero value is used as a best-effort default, invoked automatically when
no other value has been set.
IP protocols can efficiently transport various data types over the same network resources. IP traffic is
“bursty” in nature and requires flow control, buffering, and other mechanisms to deal with this “bursty”
traffic when networks are heavily loaded. The performance attributes of time-sensitive traffic streams,
such as voice and video conferencing, are of particular concern when implementing IP networks. The
majority of time-sensitive traffic streams (VoIP, TDM over Ethernet, etc) do not have control protocols to
negotiate speeds or re-transmits. Traffic is sent assuming delivery and ordering is unchanged.
Ethernet-based architectures require buffering capacity to absorb typical IP bursty traffic and to prevent
packet loss to maintain Service Level Agreements (SLAs).
802.1P COS/QOS is used to accommodate bursty IP traffic
CoS vs QoS
What Is Quality of Service?
Quality of Service (QoS) is a traffic management scheme that allows you to create differentiated services
for network traffic, thereby providing better service for selected network traffic.
QoS works by slowing down unimportant packets, or discarding those packets under high load. It
therefore delivers the important packets, but at the expense of the unimportant packets.
QoS primarily comes into play when the amount of traffic through an interface is greater than the
interface’s bandwidth.
When the traffic through an interface exceeds the bandwidth, packets from one or more Queues from
which the device selects the next packet to send. By setting the queuing property on a device or interface,
you can control how the Queues are serviced, thus determining the priority of the traffic.
What is Class of Service?
Class of Service (CoS) is an algorithm that tags packets then classifies those packets in order to assign
them to Queues of differing priority. Unlike Quality of Service (QoS) traffic management, CoS does not
ensure network performance or guarantee priority in delivering packets.
In summary:
CoS = assigning priority values to data streams
QoS = traffic engineering to process data according to the priority values
