What is ArcNet?
Attached Resource Computer Network is a communication protocol widely used in build automation. This protocol is very efficient in Local Area Network, wherein multiple devices want to share information at faster phase. Core functionality of this protocol relies on token passing between nodes. Better analogy could be, consider network as a shared object and token as the key to the object.
What are other features?
Data speeds up to 10Mpbs, Variable packet lengths, Supports Bus or star topology, Uni-cast and broadcast messages.
Let’s dive into each message types in the protocol.
RECON – Reconfiguration
This is another important message or feature to keep track of the network changes. This message can be triggered in following cases:
- When a node joins the network
- When a node leaves the network
- For high performance, it can be triggered every 20 – 30ms
ITT: Invitation to Transmit
Token is the identifier for the node or the device in the network. When node enters the network it sends a RECON message. This message will inform the network about change in configuration of the network and re-start the token passing.
Let’s take a simple example of a network.
Now node 1 starts the token passing, it will send out ITT message to destination nodes starting from next address of its current address. In our case it would be 0x0B. It keeps on sending ITT until it receives the response from any node in the network.
Now as you can see, 0x02 node has responded to the request from 0x0a. Now the token is with 0x02 node, so it starts it token parsing to find out another node in the network.
When ITT message reached the destination as 0x05, node three responds takes on the task of token parsing. Then it identifies the another node that is i.e 0x0a. So token passing happens until all nodes in the network are identified. For our setup the intial token passing is completed we have a network 0x0A <-> 0x02 <-> 0x05 <-> 0x0a.
After accomplishing the network and identifying the nodes in the network. The token passing keeps on continuing as shown below for efficient data transfer.
How? say for example 0x0a wants to send some data to 0x05. It can send the data only when it has the token. As I said earlier, token is kind of key to the network infrastructure and resources.
FBE – Free Buffer Enquiry
Before a nodes a sends packet to another node, it needs to know if the receiver can accept incoming packets.
When the node has the token, it would send a FBE request to receiving node. The receiver can chose to send ACK or NAK.
FBE with NAK/ACK is shown below
This is the message protocol to send data frames to the receiving node. The data could be anything.
The frame contains Source address node, destination node, length and the actual data. The receiver can ACK or NAK the transmission.
PAC sequence is shown below:
Sent by receiving node to the sender about accepting the information sent
NAK: Negative Acknowledgement
Sent by receiving node to the sender about rejecting the information sent
No, marriages are not made in heaven. Or hell. They’re made on earth. You and your partner can decide whether to MAKE a marriage a heaven or a hell
I was fresh out of college and joined a prestigious company in Mysore as a trainee. Life went on smooth and I was so into career development forgot about other life aspects. Sooner most of my friends had girl friends, their first kiss and carnal pleasures.
Some of my close friends started to get hitched knowingly or unknowingly :). I had two sisters, so i did not feel the pressure from my family. Years passed by then i realized some thing was missing, may be girl friend or lover to chat ? Nah. My dad and mom started to bring in the topic of marriage now and then. So I decided to take plunge into online matrimonial sites.
I registered in several matrimonial sites, diligently I wrote a intro about myself. Sites started to give me potential/preferred matches based on my settings. And I started clicking on sending invitation to all matches. It was like speed dating or blind date just to look at the categories and match. Next day I received several message on my inbox, 90% of them where rejections. I could not understand the reason, because website wont give any.
Months passed by without any luck, probability of rejection increased to 100%. One fine day like a dream i got a invitation from a prospect through online site. We chatted for a while and Family liked the profile they wanted have a face-time with me. I prepped myself for the interview online. The interviewers started questioning, about bible, Jesus, David’s Sins. I answered all of their question as I had completed by Biblical courses. The Next day morning I got rejection mail from the Interviewers. I was puzzled, so I just wanted to get their feed back. They told me I quote “We did not like your answers on David’s Sin”.
My Next Interview was a few days later, I told my colleagues to pray for the interview. I was nervous after my first encounter. This time they asked me to play song on guitar and sing along. The very next day i got a mail about their rejection. I did not get any feed back from them, I think they did not like my chord progression.
Meanwhile, I had few accepted candidates but they got married before I could get a shot at the interview. Few weeks back I received a message from a girl through online matrimony. She texts me only when she is drunk. After a few rounds of chatting over whats-app, she called and said i quote “My parents wants a guys close to my home and earns more money than you”.
Most of the candidates on online proposals have high expectation about their partners on categories like Money, Citizenship, Green card, Height, Caste, Color. Being a single male for these years without any girl friend and relationship, I find this search ridiculous and unholy. I strongly feel it should be based on the personnel connection and interests not on monetary benefits.
After making sure that my resumes are updated and well according to the standards. I scanned through the job portal for amazon jobs, after carefully reading the job description; i decided to apply during mid last year. I almost forgot about the application, out of the blue i got call from the recruiter asking whether you’re still available in market. I replied positively to the recruiter. They sent my profile to the hiring manager and asked me time slots for the interview. Got the response for the interview slots.
I prepared myself for the interview for about a week, brushed programming skills on data structures/algorithms, I took courses with MIT open course ware. Finally i am at peace before the interview, all the topics i covered was running in my mind like a dream. ok enough of story, let’s go through the interview questions.
The interviewer was very kind enough to explain about his role in the company and its business values. He asked me about my roles and responsibility in the team, following are the questions in order.
a. Tell me about your responsibilities and teams
b. Tell me about the challenging situation with the team and how do you handle it.
c. What proactive ideas you gave to the client and what is the response from them. And to follow-up this he asked me questions about the solutions.
d. Tell me about the situation where you took hard decision and its impacts. Specifically he was asking about breaching the dead line.
e. How did you handle changes in requirement and how did it impact the team.
f. How did you manage the conflicts in the team.
I answered the questions with utmost diligence. I complimented the team all along, it was the truth. Hey No cribbing about the management. After that he asked to open http://collabedit.com for programming. He started to write the problem statement. Following is the problem statement. Create alogrithm for the dependency list of the package installer.
I looked at the diagram and got a bright idea to use undirected graph. He asked me why undirected graph, I though for a while and decided to use directed. Since there is no reverse dependency i.e core is not dependent on Apache so it is always one direction. He asked me to write the program and i started to create a graph class. Following is the solution i provided, i will just write in sudo code.
a. Created a class called “graph” which has a list of int, strings, and variable to store the max number of the vertices. Integer list has the index for the string in the string list, example string “Root Package” has index “0”.
b. A function to iterate through the graph and update if it is visited in boolean list. I chose Depth First algorithm for iteration.
c. In Public specifier; I added constructor, Add edge function, Get list function.
I was not silent in the call but kept on telling him what i was thinking, just to engage him in conversation. You may think why list of vertices, I will give you a pictorial representation of the list.
This list has the path to trace back to the parent, because he asked me to check for cycle. Anyways the coding went well and he said it is good. May be you have a better idea for this problem, but this is what I can come up during the interview session.
Code is available in my git hub page
I have been searching for while to add a progress indicator in excel. It is so elegant and easier for convening the percentage completion to the reader. I will walk you through the steps and settings. I used office 2010 professional.
We need to create a new rule for the cell. So select the cell for which you want progress indicator. Then click on “New Rules” from Home->Conditional formatting.
A popup window will appear, select the following options
After selecting the options mentioned above, click on “OK” to apply the rule for the cell selected.
Then go back to your excel sheet, click on the cell and update the number you wish for the progress bar in the formula bar.
You can copy this formatting to other cell and update the number in formula bar for each cell separately.
I am going to put forth my ideas based on my experience. Software designing needs lots of patience and need to be open to ideas or changes. The first few phase are very trivial for software design. I had an opportunity of designing three software from conceptual phase.
1.Gathering the requirements
May be this sounds easier, but believe me it needs your time. For the first software which i designed, it took me two months for requirement elicitation. First step is to find out whether the product is for the internal or external customers. Both customers are very unique, internal customers are critics and external customers are consumers.
After determining the customers, bring the stake holder in for a discussion and understand the following:
- What is the end goal of the product?
- What is scope of adding features in future ?
- Is it aligned to the market trend
- What is the targeted time frame ?
These discussions will give you a fair idea about the software/product and it will be easier for you to communicate in their language. Create a database to store all the meeting minutes, requirements documents. Go through the requirements document and create an understanding document based on your understanding about the requirement and get it reviewed by the stake holders.
Initial tough phase is completed.
2. Finalize the technology
Now you have a fair idea about the product. Do a market research about the softwares that can be used, just to give an example.
|Technology||Difficulty level||Is it easier to maintain||License||Is support available|
|C||Medium||yes||GPL||Yes, online forums|
This phase is also very trivial since this is going to determine path for rest of your project.
3. Proof of concept
According to me, this is where your software gets life. This phase can eliminate the unnecessary complication on platform, technology. In my first project i ignored this phase and it lead to major complications. We were to design a cross platform software for a Embedded product. My fellow engineers chose wxWidgets just based on the initial market research. The development went fine on one platform until time came to test on windows, the entire software broke. We ended up debugging the software to create work around.
So don’t do this mistake. This phase will not resolve any un foreseen issue, but it will at least give you an heads up.
4. Decide on the design pattern
Here comes the dilemma, there are more design patterns available than anticipated. In phase 3 you might get some idea about the working of the software in very high level. Use that understanding to zero in on design patterns. A piece of advise, have two or three design patterns as fall back position.
5. Decide on building blocks/modules
The design should be structured, easier to maintain and expendable. Create building blocks of the software and find a common blocks which can be used by all modules. Blocks not the very low level of coding, I am just talking about the modules.
I will take an example of GSM messaging, Layer 3 messaging RR,CC, SMS etc,
I created base block for the all the layers to be able to control completely if any changes in future.
6. Communication channel between modules
We have decided on the modules, number of building blocks and common blocks. Now we need to finalize on the mode of communication between these blocks. In my first project the softwares includes four services, a. Main GUI or tracing b. Configuration management c. Clock d. Simulation components. So How did i decide on the communication mechanism, its simple. First draw a map about who’s is going use those modules, is there any updates required during run time.
So in my case the configuration server holds all the information about the current running instance and needs to be update in runtime and broad casted. So i decided to go for a client server design on sockets, one for updating data and another for broadcast.
In your case it could be just a function call.
7. Designing individual modules
If you have reached this phase, you have complete understanding of the software by now. The modules are tap points for the application layers. So make sure you make it very generic so that it can be completely controlled by the application layer. For instance we will take the GUI wants to connect to a server
Instead of this you can have
function_connect_to_server(url, password,username… etc);
8. Design document
You may be an architect but in order to make your design rolling, you need to document. Keep in mind that the engineer does not have extensive knowledge, since he/she is not involved from conceptual phase. In first project i got this wrong and assumed the engineer knows.
After you document the design, make sure to get the design review by SME’s, stake holders. This will give a chance to correct the design, if you have missed any trivial part. This does not mean that you have got all requirement correct 🙂
Before closing up this discussion. The above mentioned points are purely based on my experience. I always strive to make the design make very generic and to embrace changes based on the customer experience.
Good luck in your software design.