Specifications

Engineering Guidelines
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The COS values run from 0 to 7. Typically 7 is the highest value, 0 the lowest. However, newer
standards and switches define a COS 2 as “best effort” with 0 and 1 as lower. Also, the default
setting on some switches might place COS 5 into the expedite queue, potentially giving this
higher priority than 6 and 7. Check these settings on the switch to ensure correct and expected
operation.
Use of subnets and subnet size
Creating a flat network appears to speed up transactions due to the high link speed, but Layer-
3 switches are hardware-oriented, and perform equally as well as their Layer 2 counterparts.
In the Layer 2 switch environment, data can be addressed directly to a specific port, thereby
reducing loading on links not used. However, if the Layer 2 devices cannot identify an address
or port location to use, additional protocols are needed to get the information. The additional
protocols broadcast data to every port and device, causing the loading on the network to be
almost back to that of a shared environment. The Layer 2 devices maintain a list of addresses
and port locations in internal memory. If the memory and list are small, the level of broadcasts
can also increase, since new information is rapidly aged out of the list.
A large flat network can potentially grind to halt, not because of genuine traffic loading, but
simply due to the amount of broadcast traffic that is required. Using subnets helps by segmenting
broadcast domains. The Layer 2 devices subsequently need to hold less information, and so
broadcast less often. Smaller subnets are preferable to reduce the level of broadcast traffic
within a particular network domain.
Including Layer 3 devices improves speed within communities of interest and the overall
network, and reduces the burden on the system to all broadcast traffic. It is also a requirement
for VLANs to operate correctly and provide the voice priority required when using dual-port
phones.
A subnet with more than 1024 (/22 or mask 255.255.252.0) addresses is not recommended.
Typical and recommended subnet sizes are /24 (mask 255.255.255.0) and /23 (mask
255.255.254.0).
Full Duplex and Half Duplex Settings
It is recommended that all LAN connections use full duplex settings. This ensures maximum
bandwidth and minimum delay. WAN links are typically specified as full duplex.
Full duplex network basics
Even though speech may be half duplex or full duplex to the user, the internal voice codecs
are receiving and sending data all the time via the LAN connection.
Note: The terms “full duplex” and “half duplex” are often used at the phone to describe
the hands-free operation. This has nothing to do with the LAN connection. The terms,
when used for hands-free operation, refer to whether only one party (half a conversation)
can speak or whether it is possible for both parties (full conversation) to speak at the
same time.