Technical data

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Switch functions
Prioritisation (QoS, Quality of Service)
Switches that support prioritisation have two or more queues connected to respec-
tive ports to handle data (QoS). Prioritisation can take place on different levels and
using different techniques.
There are a number of techniques, the switch can send a predetermined number of
packets from a high priority queue before is sends a low priority packet (Round-robin).
Or with strict prioritisation, where all prioritised traffic has preference over low priori-
ty traffic.
Layer 2 priority
A layer 2 switch can prioritise data on a MAC-level based on:
… MAC-address, both the destination and source address can be used to priori-
tise data. The switch must be managed in order to utilise this, so that it is possi-
ble to set the priority on the MAC-addresses.
… Ethernet port (layer 1), one or more ports can be configured for high priority
data. All traffic to these ports is handled as high priority data.
… Priority assigned with tags, IEEE 802.1 p (and 802.1Q) the Ethernet packet is
supplemented with a field designated Tag Control Info (TCI). This field is posi-
tioned between the source address and the type field. The field results in the
length of the packet increasing from 1518 byte to 1522 byte. 3 bits are used by
the “tag information” to set the priority. This makes it possible to set priority on
8 levels.
Destination Source
0x8100 XXX X 0xXXXX
Tag Type
12-bit 802.1Q VLAN Identifier
Canonical – 1 bit
Layer 2 prority with 802.1p
3-bit Priority Field (802.1p)
Tagged frame
Type Interpretation – 16 bit
FCS
B A C K