Tuesday, 9 June 2015

iperf to multicast addresses

1.Add multicast addresses to all sta(AP and STA connected) as below
        route add -net dev eth0
2.At server
                Iperf –s –u –B –I 1
3.client side

                Iperf –c –u 1 1000 –T 5 –I 1

Ping - broadcast and multicast

The range of IP addresses is divided into "classes" based on the high order bits of a 32 bits IP address:

   Bit -->  0                           31            Address Range:
           |0|       Class A Address      | -
           |1 0|     Class B Address      | -
           |1 1 0|   Class C Address      | -
           |1 1 1 0|  MULTICAST Address   | -
           |1 1 1 1 0|     Reserved       | -

Every IP datagram whose destination address starts with "1110" is an IP Multicast datagram.
The remaining 28 bits identify the multicast "group" the datagram is sent to. You need to tune radio and kernel to receive packets sent to an specific multicast group. This is called joining a multicast group.

Some of the special (well known/reserved) multicast groups are:
· is the all-hosts group. If you ping that group, all multicast capable hosts on the network should answer, as every multicast capable host must join that group at start-up on all it's multicast capable interfaces.
· is the all-routers group. All multicast routers must join that group on all it's multicast capable interfaces.

Multicast ping: 
Get multicast address using "netstat -g" command

$ netstat -g
IPv6/IPv4 Group Memberships
Interface       RefCnt Group
--------------- ------ ---------------------
eth1            1
eth0            1

Ping to multicast address, you will get reply from multicast registered IPs.

$ ping -I eth0
PING ( from eth0: 56(84) bytes of data.
64 bytes from icmp_req=1 ttl=64 time=0.249 ms
64 bytes from icmp_req=1 ttl=64 time=0.257 ms (DUP!)

Ping to all-host multicast address.
$ ping -I eth0

Broadcast ping:

$ ping -I eth0 -b

Thursday, 4 June 2015


Data Link Layer Sub-Layers...

1. LLC (Logical link Control):- Name itself describes that it is responsible for handling layer-3 protocols (multiplexing/de-multiplexing), flow control and reliability. This is upper sub-layer of Data Link Layer.
2. MAC (Medium Access Control): It is responsible for controlling medium access, framing/de-framing, check-sum computation and verification.

Upper and Lower sub-layers of Data Link Layer

Packets, Frames...etc, what is it called at each layer....


MSDU: The MSDU is the service data unit that is received from the logical link control (LLC)
MPDU: The MPDU is a message (Protocol data unit) exchanged between media access control (MAC) entities in a communication system.



  1.  Ethernet is the native frame format
  2.  Ethernet header is much smaller than the 802.11 header
Hence multiple Ethernet frames can be combined to form a single A-MSDU.  
A-MSDU minimizes the MAC header overheads when the channel conditions are good.
Issue with AMSDU:
Probability of error increases with frame size. Since each AMSDU has one MAC frame and one CRC, in case of error whole frame needs to be re-transmitted (in most cases at lower rates). So it is better to use it in good channel conditions.

AMSDU Beacon:

AMSDU Packet exchange:


Multiple PDUs are aggregated, each with their own MAC header and CRCs. Hence, in the event of a failure, only those MPDUs can be retransmitted resulting in higher efficiency.

However, each MPDU in AMPDU has its own header and CRC, it adds to overhead but by using Block Ack mechanism this can be overridden.

Beacon of AMPDU enabled AP:

AMPDU Packet exchange:

Thursday, 21 May 2015

802.11n PHY and MAC Layer Improvements

Physical Layer Improvements
  1. Higher Modulation Rates – from 54 Mbps to 58.5 Mbps
  2. Improved Forward Error Correction – from 58.5 Mbps to 65 Mbps
  3. Short Guard Interval – from 65 Mbps to 72.2 Mbps
  4. Channel Bonding, 40 MHz Channels – from 72.2 Mbps to 150 Mbps
  5. MIMO – from 150Mbps to 600Mbps

MAC Layer Improvements
  1. Aggregation of Packets
  2. Block Acknowledge
  3. Lower Overhead: Reduced Interframe Space
  4. Backwards Compatibility
  5. Power Save Modes
  • Spatial Multiplexing (SM) power save – static and dynamic

  • Power Save Multi-Poll (PSMP)

Wednesday, 20 May 2015

802.11 Beamforming

Beamforming technology allows the antennas and controlling circuitry to focus the transmitted RF signal only where it is needed, unlike the omnidirectional antennas people are used to. Beamforming is PHY layer procedure.

Two types of beam forming:

1.    In implicit beamforming, the upstream wireless channel is measured by the beamformer, and the measurement used to derive the parameters for subsequent downstream beam formed transmission.

2.    Explicit beamforming, requires the downstream channel to be  measured at the receiver, or beamformee, and relayed back to the transmitter, or beamformer. The beamformer uses the measured channel information to derive the transmit beamforming parameters.

Implicit beamforming has the advantage that the beamformee does not need to measure and send the channel state information to the beam former. However, 11n standard implicit beamforming requires a calibration exchange between the beamformer and beamformee, which can complicate the transceiver design.

Since 802.11n allowed multiple forms of beamforming and both ends of a wireless link need to use the same beamforming method to produce real benefit.

Sounding frames are known pattern of RF symbols sent from each antenna.

Following is the sequence of events that take place as part of Transmit Beamforming to get reports from clients.

1.    AP sends an announcement that its going to send out a sounding frame containing data to be evaluated by client.
2.    AP sends the data in the Null data packet.
3.    All the clients supporting TxBF receive it as start responding to the report request by sending back a Compressed V-Matrix to the AP.
4.    Based on the feedback received by the AP from clients, it re-calibrates the phase shift for each of the transmitted signal from each antenna so that the signal strength reaches its maximum at client

Note: Images used are from various websites and copyright if any belongs to respective owners.

802.11n Vs 802.11ac

802.11n Versus 802.11ac
IEEE Standard
Jan-2014 (Wave-1)
Frequency Band
2.4GHz and 5 GHz
5 GHz
PHY Rates
65 Mbps – 600 Mbps
250 Mbps – 6.5 Gbps
Up to 64 QAM
Up to 256 QAM
Channel Widths
20, 40 MHz
20, 40, 80 MHz
160 MHz, 80+80 MHz
Spatial Streams
1 to 4
1 to 8 total
Up to 4 per client
Green Field
Implicit Beamforming
Explicit Beam Forming
Single Stream (1X1)
Max Client Data rate per radio
150 Mbps
450 Mbps
Three Stream (3X3)
Max Client Data rate per radio
450 Mbps
1.3 Gbps (80 MHz, Wave 1)
Max A-MPDU size
65,535 octets
1,048,575 octets