Saturday, November 14, 2009

Cisco Unified Wireless QoS

Cisco Unified Wireless QoS

fixed network delay – encoding/decoding time
variable network delay – queuing/congestion

Radio downstream is most common deployment.
Radio client upstream QoS depends on client implementation.

When providing only radio downstream QoS from the AP, radio upstream client traffic is treated as best effort.

Data frames in 802.11 are sent using distributed coordination function (DCF)
     interframe spaces – (SIFS, PIFS, DIFS)
     random backoff (contention window) DCF is used in 802.11 networks to manage access to the medium.

Interframe spaces (base timing in 802.11b listed below – 802.11a/g are different)
     SIFS = 10us
     (PCF) PIFS = SIFS +1 x slot time = 30us
     (DCF) DIFS = 50us + 2 x slot time = 50us

Random backoff
When a data frame using DCF is ready to be sent it goes through the following steps:
     1. generates a random backoff number between 0 and a minimum contention window (CWmin)
     2. waits until the channel is free for a DIFS interval
     3. if the channel is still free, begins to decrement the random backoff number for every slot time (20us) that the channel remains free.
     4. if the channel becomes busy, the decrement stops & steps 2 – 4 are repeated
     5. if the channel remains free until the random backoff reaches zero, the frame can be sent.

Contention window is defined by aCWmin & aCWmax
The random number is between zero and aCWmin

If the initial random backoff expires without successfully sending the frame, the STA or AP increments the retry counter & doubles the value random backoff window size. The doubling continues in size until the size equals aCWmax. The retries continue until the max retries or TTL is reached.
      This process is referred to as binary exponential backoff

WMM is primarily the implementation of the EDCA component of 802.11e

WMM uses the 802.11p classification scheme developed by the IEEE (now part of the 802.11d spec)

A U-APSD client remains listening to the AP until it receives a frame from the AP with an end of service period (EOSP) bit set. This bit tells the client it can now go back into power save mode. The use of U-APSD allows the use of long DTIM intervals to maximize standby time without sacrificing call quality.

TSpec admission control – allows an 802.11e client to signal its traffic requirements to the AP. a TSpec request can be used to control the use of the various access categories in EDCA.
      - 7920 does not support TSpec
      - 7921G does support TSpec

Add Traffic Stream (ADDTS) – it’s how a WLAN client performs an admission request to an AP. Signaling it’s TSpec request, an admission request is in one of two forms:

ADDTS action frame – when a phone call is originated or terminated by a client associated to the AP

Association & Re-association message – the association message might contain one or more TSpecs & one TSRS (traffic stream rate set) IE if the STA wants to establish the traffic stream as part of the association.

WAN QoS & H-REAP – for locally switched WLANs with WMM traffic, the AP marks the dot1p value in the dot1q VLAN tag for upstream traffic. This occurs only on tagged VLANs; not on native VLANs.

With multicast enabled, any kind of multicast packet received on the VLAN from the first hop router is transmitted over the wireless including HSRP hellos, all router, EIGRP, and PIM multicast packets.

AP sends heartbeat messages to the WLC once every 30 seconds. If one response is missed, the AP sends 5 successive heartbeats (one per second) to determine whether connectivity still exists.

Up to 8 H-REAP APs needs at least 128kbps. PKC capable clients that roam between H-REAP APs undergo full 802.1x authentication.

If a WLAN is configured for IPSEC, CRANITE or FORTRESS, that WLAN cannot be configured for local switching on an H-REAP AP.

If dynamic DHCP is used, establish more aggressive NAT translation entry timeouts for ports 12222 & 12223 to 20-25 seconds.

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