Saturday, November 14, 2009

VoWLAN Design Guide 4.1

VoWLAN Design Guide 4.1

U-APSD – the primary benefit is that it allows the voice client to synchronize the transmit and receive of voice frames with the AP, thereby allowing the client to go into power-save mode between the transmit and receive of each voice frame tuple.

The use of U-APSD allows the use of long DTIM intervals to maximize standby time without sacrificing call quality (also increases call capacity)

Client in power save detects data waiting for it at the AP via the presence of a TIM in the AP beacon. The client PS-polls the AP to get the data

     There are two major problems with this:
        1. Requiring PS-polls on top of the normal data exchange to go through the standard access delays associated with DCF.
        2. Retrieving the buffered data is dependent on the DTIM, which is a multiple of the beacon interval. Standard beacon intervals are 100ms, DTIM intervals can be integer multiples of this.

The AP sends data to the client as a TXOP burst where only the first frame has the EDCF access delay.
      - timing of the polling is controlled via the client. Jitter introduced is symmetric rather than N x 100ms

TSpec allows an 802.11e client to signal its traffic requirements to the AP. It can also be used to control the use of various ACs in EDCF. The use of EDCF ACs rather than HCCA to meet TSpec requests is possible in many cases because the traffic parameters are sufficiently simple to allow them to be met by allocating capacity rather than creating a specific TXOP to meet the application requirements.

QoS Basis Service Set (QBSS) is an IE element advertised by the Cisco AP. The load field indicates the portion of available bandwidth currently used to transmit data on the AP.

1 octet 1 octet 4 bytes

There are three QBSS IEs that need to be supported in certain situations
  • Old QBSS (draft 6 pre-standard)
  • New QBSS (draft 13 IEEE 802.11e (standard))
  • New distributed cac load IE (a Cisco IE)
**The various QBSS IEs use the same ID, and therefore the three QBSSs are mutually exclusive. For example, the beacons and probe responses can contain only one QBSS IE.

The QBSS used depends on the WMM & 7920 phone settings on the WLAN.
      Client CAC limit – supports legacy 7920 code pre v2.01
      AP CAC limit -7920 settings are learned from the WLAN advertisement

Enabling admission control determines how much AP bandwidth will be set aside for voice clients attempting to roam to the AP. This is based on the APs capacity, not factoring in the possible channel loading impact of other APs in the area. Load Based ac uses channel load in capacity calculations.

TSpec admission control is used to protect high priority resources, not to deny clients access to the WLAN.

L2 LWAPP does not effectively support QoS because the AP does not send the 802.1p/q tags and in L2 LWAPP there is not an outer DSCP on which to fall back because the AP no longer uses NULL VLAN ID
      - APs carry out EDCF-like queuing on radio egress
      - APs do FIFO on the Ethernet egress

Call capacity assuming no competing high priority WLAN traffic & normal background noise:
      - 14 simultaneous call son 2.4ghz
      - 20 simultaneous call son 5ghz

WLAN QoS is performed AFTER the CCA.

Standard office environment – AP client RADIUS of 43 feet, co-channel interference RADIUS of 150 feet using 2db gain antennas and a power output of 16dbm (40mw)

VoWLAN & 5ghz – generally recommended to use the lower 4 channels & upper 4 channels. These channels do not require the use of DFS or TPC.
      UNII-1: 36, 40, 44, 48
      UNII-2: 52, 56, 60, 64, 100, 104, 108, 112, 116, 120, 124, 128, 132, 136, 140
      UNII-3: 149, 153, 157, 161

show {IEEE 802.11a | IEEE 802.11bg} L2 roam RF-params
show {IEEE 802.11a | IEEE 802.11bg} L2 roam statistics [AP-MAC]
show client roam-history [client-MAC]

EIGRP – two main route summarization ways in campus test architecture = route summarization & stub
      reduce EIGRP hello/dead timers to 1 & 3 seconds (hello = 1 & dead = 3) rather than the default timers of 5 & 15

QoS policing – check on classification using port trust state & policy maps being mutually exclusive in 2970, 3560, and 3750s

Deployment models for Cisco Unified Communication Manager
number of call processing agent clusters
number of IP phones
location of the call processing clusters & IP phones

For voice traffic, it is better to deny network access under congestion conditions that to allow traffic to be dropped or delayed.

QoS protects voice from being overrun by data
CAC protects voice from voice
CAC topology unaware – based on static configuration
CAC topology aware – based on communication between call processing agent & the network about available resources – uses Resource Reservation Protocol (RSVP)


  1. hi
    is there any software like gns3 for ccie wireless practice.

  2. I'm not aware of any simulation software for the wireless practice - this is why it is going to be so difficult to put together a useful lab environment.

  3. Are you studying for the written or the lab?

  4. I'm studying for the lab - my date is 02/02/2010

  5. Cool, I took it last week. It is tough.

  6. I saw that - I started following your blog so I won't miss your next post..