Ethernet in Metro and Long Haul Networks
(continued)

VLAN Technical Details

The IEEE VLAN standard, 802.1Q, is simple in concept. First, we add four bytes to each Ethernet frame so that the frame is identified as belonging to a specific subscriber on the Ethernet network.  It is the responsibility of the Ethernet switches to look at those four bytes and determine where the frame is to be delivered.  A tiny part of that four-byte header - three-bits to be exact--is used to specify the priority of the frame.  The details of this are spelled out in a standard called 802.1P. You should note that the 802.1Q header contains the 802.1P field, so you must have 802.1Q in order to have 802.1P.  

Figure 2

Now let’s take a closer look at those four 802.1Q bytes. The first two bytes (TPI) identify the Ethernet frame as an 802.1Q frame. These bytes never change. When an Ethernet switch sees these bytes, it knows that the frame is an 802.1Q frame.  The second two bytes, or 16 bits if you prefer, are carved into three fields. The first field is a three-bit field used to identify a priority level for this packet.  The second field, Canonical Format Indicator (CFI), usually indicates whether bit order is canonical or non-canonical, and can have other significance depending on MAC protocol. The third field is a 12-bit field that defines which customer this particular frame belongs.  The 12-bit field restricts us to 4096 VLAN customers. This is fine for most enterprises but not big enough for carrier applications. The IEEE plans to modify 802.1Q specification to define millions of VLANs and maybe that will hold us for a while. Vendors like Nortel claim that they can support millions of different VLANs on their OPTera 3500 platform right now.

Ethernet delivers CoS, not QoS

Ethernet delivers Class of Service (CoS), not Quality of Service (QoS).  Without going into a lot of detail regarding QoS vs. CoS religious issues, consider this: QoS means that we establish some kind of path through the network and then make every device on that path promise to deliver a certain level of service at a specified bit rate. This means that QoS gives every device that will be part of that path a chance to reject the connection (think fast busy tone).  CoS, on the other hand, does not set up any specific path, so it never gives any network device the chance to refuse a high priority connection.   CoS simply allows the sender to mark a frame with a certain priority level.  With CoS, the network administrator must ensure that the network will not be over-committed with high priority traffic.  If you have a 1 Gbps link and only allow 100 Mbps of priority traffic on the link, CoS is just fine.  

Figure 3

Remember the three priority bits in the 802.1Q header?  The meaning of those three bits is spelled out in the IEE 802.1P standard.  If we borrow an airline analogy, those three bits work out like this: 000 means stand by (or best effort), 001 means baggage class, 010 means coach class, 011 means business class, 100 is first class, and so on. This means you now have a way to mark a frame according to a certain priority level, giving Ethernet the power to prioritize voice and video frames.

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