Difference between POE-PD and POE-PSE, POE vs POE+

• POE stands for power over ethernet
• A POE system consists of two components - a POE-PSE (Power sourcing equipment) the device which provides the power, and a POE-PD (Power delivered) equipment which receives the power over ethernet.
• PSE equipment examples - POE switches, POE injectors, POE media convertors [1]
• PD equipment examples - APs, IP cameras, other devices.
• PSE has 4 types based on PSE, PD power ratings with a 90W PSE max allowing a 71W PD draw on a port [2].
• PD ratings are only based on the power draw limit - max of 71W - class 8 PD [2]
• Power delivery limits are based on negotiations determined by the 802.3 standard. 802.3at (POE) delivers 15.4W, 802.3af (POE+)  delivers 25.5W. This is the difference between POE and POE+.

labels: POE, ethernet, difference, solution, networking, IP networks, TLDR

Reference

[1] FS community blog

[2] POE - PSE and PD types

[3] POE Standards and history

(C++): Difference between angle bracket < > and double quotes “ ” while including header files in C?

From stackoverflow:

It's compiler dependent. That said, in general using " prioritizes headers in the current working directory over system headers. <> usually is used for system headers. From to the spec (Section 6.10.2):

 

Wireless: Difference between WiMAX and LTE Physical and MAC layers

This is an old post based on excerpts I read from the web. Adding it back.

This post will cover the technological differences between these two technologies and contains a brief bullet point description as to why LTE took off and WiMAX did not:

Slot times: LTE uses much smaller slot times - 1ms as opposed to 4ms which gives much worse delay performance with multiple users and does not scale just as well.

Uplink modulation: LTE introduced SC-FDMA which dramatically improved uplink performance for cellular systems. This modulation technique combines the advantages of low peak to average ratio of traditional systems (such as GSM) and multipath resistance of newer modulation schemes (such as OFDM). SC-FDMA also provides savings for the mobile users (on their uplink).

Timing: WiMAX was the first to start off. Hence most of the experiments were performed on this and LTE could learn from their experience and MISTAKES. WiMAX was initially designed for fixed systems rather than mobile systems and they were not able to adapt well for usage with cellular providers.

Other non MAC/PHY differences:

WiMAX is based on IEEE standards (specifically, the 802.16 family), and then managed by the WiMAX Forum. LTE is defined by 3GPP.  

WiMAX was originally designed for fixed networks and has gradually evolved into a mobile network. But this has resulted in some changes not being  made correctly. LTE was designed as a mobile network from the first day. This particularly impacts the power at the receiver (handhelds). WiMAX handhelds are slated to consume more power as compared to LTE.

WiFi: Difference between single and multiple protection settings in the duration ID

A single protection setting is used to protect the transmission until the end of any following data management or response frame. In multiple protection setting the NAV protects until the estimated end of multiple sequence of frames.

WiFi: One line difference between delivery enabled and trigger enabled in U-APSD

Excerpt from wikipedia:

Queues may be configured to be trigger enabled, (i.e. a receipt of a data frame corresponding to the queue acts as trigger), and delivery enabled, (i.e. data stored at those queues will be released upon receipt of a frame). Queues refer to the four ACs defined for WMM.

How to use IS_ERR and PTR_ERR? What do they mean?

From the kernel definition there are three macros:

1. IS_ERR - used to check, Returns non-0 value if the ptr is an error. Otherwise 0 if it’s not an error.
2. PTR_ERR - used to print. Current value of the pointer.
3. IS_ERR_VALUE - is explained a little bit more detail here¹.

I find this the most useful for kernel space programming. Used as follows- if ptr is the pointer you want to check then use it as follows:

if (IS_ERR(ptr))
     printk("Error here: %ld", PTR_ERR(ptr));

Which Linux Kernel timing or delay APIs to use for what?

I classify the waiting or timing API in the Linux kernel in two categories:
1. Which blocks the current thread of execution.
2. Something which has to be scheduled for later but we want the current thread to continue.
In most cases, the distinction between 1 and 2 is clear, but the techniques used to implement 2 can also be manipulated to behave like 1.

1. Blocking current thread of execution (Inline delays)
The API for 1. in the above case are:

WiFi: A Short Introduction to 802.11ax

Citation info:
Gautam Bhanage, "A Short Introduction to 802.11ax", Technical report GDB2018-001, published at bhanage.com, 2018

Download PDF

802.11ax support for Uplink MU-MIMO

Who should read these?
At the time of writing this blog post the 802.11ax standard is in the process of getting ratified and we are at the version 2.0. Are you intrigued about what makes this standard different from the previous 802.11ac standard?

Download PDF of the TR.

Disclosure:
Slides from a talk I recently presented on 802.11ax technology.
These slides are purely based on my reading of the standards and other free documents available on the internet.
Copyright for the images are held by the owners. I have only used the ones which were labeled for reuse.

Tech report: GDB2018-001

Linux: Difference between livelocks and deadlocks

Everyone knows what a deadlock [2] is!

Wait for graph as described on wikipedia

Defn Deadlock:
Two or more entities vying to get two or more locks in out of order. This leads to each entity waiting for the other entity to release the lock. Since no one releases their held lock the system is stuck and there is no easy recovery.

What is a livelock then?

Difference between Active and passive beamforming mechanisms

Active beamforming is the mechanism used in WiFi access points and clients where the beamforming is done with different chains on a transmitting radio (usually using active components like transistors). Passive beamforming on the other hand is achieved by using passive components (resistors, capacitors, and inductors) to introduce phase and amplitude variations across different path such that we are able to achieve constructive addition in some cases which results in beamforming.

The main differences between these two mechanisms: 

• Butler matrices are a passive beamforming and channel model rendering mechanism.
• Beamforming can be useful against jamming attacks to increase SNR at client.
• Passive beamforming networks are better because active components are more susceptible to change in temperature and other environmental factors.
• Compared to active beamforming networks, passive beamformers can be made electrically stable and reliable across a wide range of operating conditions, although a great deal of flexibility in the physical layout may be needed to accommodate a large variety of connector interfaces for different antenna arrays.

References

* Miscellaneous articles on butler matrices

* Understanding Passive Beamforming Networks http://defenseelectronicsmag.com/military-defense-electronics/understanding-passive-beamforming-networks

WiFi: What is beacon bursting? What are the tradeoffs?

Beacon bursting is a mechanism by which beacon frames for all the BSSs (Virtual APs) running on that radio are sent out one after the other instead of spreading them out equally in time over the beacon interval. The other approach to sending beacons is to spread them out equally over the beacon interval.

For example, consider the beacon interval is 100msec and you have 5 VAPs beaconing, then, with bursting all the beacons are sent back to back after the beacon interrupt. However, if you are not using bursting, beacons are spread across the 100msecs. Hence, every 20msec the radio will send out the beacon of a different VAP.