Networking help?

[BuzzKillington]

I'm having a hell of a time in networking. I'm terrible at learning from books so an online networking course probably wasn't the best idea. Still, 200 bucks invested in the class would be a terrible thing to waste so maybe someone could help me with my project in lamens?

http://img9.imageshack.us/img9/455/26783111bb6.jpg

[DaveW]

Ok, well you could just have googled for the answers so I'm guessing you want to actually understand this. All the same, posting the question and asking for help is too vague for me to help you sort out the problem areas.

I can give you an over-view if you like, and then maybe you can read up about the answers. The 7-layer OSI model represents how data is sent along a network, like the internet. At each stage, the data that you want to send it transformed or packaged in some way; the first layer is the application layer, where you put the information into the network. Ok, I'm taking the following information from the book "Computer Networks; 4th Edition", by A.S. Tanenbaum.

Each layer is designed to:
1) Represent a different level of abstraction
2) Describe a well-defined function
3) Layer boundaries chosen to minimise information flow across boundaries (in other words, layers should not be required to strictly communicate with each other)
4) Should be enough layers to represent each distinct function

Hence, the OSI model. What that ideally means is that the OSI model...take an analogy. A company makes batteries. They want their batteries to be universally accepted, so they make them to standard sizes. However, they don't know who uses the batteries; they just make them. Similarly, the company that ships the batteries just wants the boxes and the addresses; they don't care what's in the boxes. Then, the toy company doesn't care who makes or ships the batteries; all it cares is that it gets them. That's sort of the thinking behind the OSI model; each layer serves a single, distinct, and well defined function.

The top layer is application, where you type a message on TBCS, for example. Protocols here include things like HTTP, FTP, POP3, etc. This is the user end; when you type a message, the browser uses the HTTP protocol to pass the information to the presentation layer.

The presentation layer concerns syntax (http://en.wikipedia.org/wiki/Syntax) and information semantics (http://en.wikipedia.org/wiki/Semantics). If computers represent data in different ways-for example, Linux and Windows use different new-line characters-this can be taken into account at this stage. This also allows abstract data to be exchanged, such as records and binary, without damage. It then passes the data to the Session layer.

The session layer is where we consider and manage connections between two systems. At this stage, a connection is abstracted, so it's considered like a physical wire between two computers. At this level, things get a little complex, because the OSI model and the TCP/IP model don't really fit together all that well. But you can find that out later, once you understand how this works. This layer provides the path for the prepared data to travel to the destination. This is all passed down to the transport layer.

The transport layer does the bulk of the work. Here, the data is split up if required and sent along the network, and is responsible for rebuilding the information that arrives. At both ends, the transport layer is responsible for ensuring a successful transfer. There are two main protocols in use, and they're very different; TCP, and UDP. TCP stands for Transfer Control Protocol (http://en.wikipedia.org/wiki/Transmission_Control_Protocol), and offers guaranteed delivery if possible. It's used for things like MSN and the internet. The other, UDP, is User Datagram Protocol (http://en.wikipedia.org/wiki/User_Datagram_Protocol). This gives a best effort with no guarantee of delivery. This is used for things like skype, where retransmitting a lost packet would be rather silly; it's more important to get some packets now than all packets at an indeterminable time. Note that the session manager is only partially aware of the underlying protocol. Ok, now we're at a physical level; the packets hit the Network layer.

Ok, the network layer is where we try to get the packets to the destination. At this level, you have to imagine the network as a giant net. The network layer is partially aware of the network, and when it's given a packet, it looks at the destination and tries to get it there, like the delivery driver in my battery analogy. Routing is pretty complex, so i'm not going to go into that here. You just need to know that the network layer handles congestion control. It also restructures packets to allow them to travel through a different network, if said network uses different addressing schemes or a lower packet size. The network layer protocol will then send the packet towards it's destination via the Data Link Layer.

The data link layer considers sending the data along a physical wire. The packet is at one end, and we need to know how to send it along the wire to the other. The packet is split up into frames, which are transmitted along the wire and rebuilt at the other side. Frames are not the same as packets. One of the main functions of this level is to manage sharing the physical line; who gets to send, when to receive, etc. In a reliable connection, acknowledgement packets called acks are sent when frames are received. When a frame is sent, it is passed to the final layer, the physical layer.

The physical layer considers the actual sending of bits along the physical link. It considers things such as timing, voltage, and other mechanical and electrical issues.

So, that's how the 7 layer OSI model works. Unfortunately, in real life things don't quite work that way; the TCP/IP model that the internet is based on doesn't have a presentation or a session layer in their model.

I've given you enough information and links here to solve your questions, but I'll understand if you have any questions of find something confusing. You can easily pull the protocols from wikipedia, but you'll know that already. If you get stuck, we're here.

-Dave

[XcOM]

couldn't have put it better myself dave,

I hated the network section doing my cource, was a pain in the arse learning tcp/ip and osi

[Drum Thumper]

I'm currently studying for Network+. I've found that everything else is relatively easy once you get past understanding the 7 layers.

[SgtM]

Please Do Not Throw Sausage Pizza Away

[si-skyline]

we also have from the other way round,

All People Some Times Need Data Processing

very useful for when writing it down on paper :)

[DaveW]

Well Buzz, did that help?

-Dave

[BuzzKillington]

Very much so although it's still hard for me to get the correct order when it comes to testing on this stuff! So far there's been a lot of trick questions or very specific ones. I need to step up my studying and start triple reading the chapters otherwise I'm going to start slipping in the class. X.X

I appreciate all of the help guys. It helps to get input from other people rather than just from a book as the book doesn't really give any helpful tips or tricks.

[simon275]

If you want to be taken seriously in the networking world ccna is a minimum.

APSTNDP

[Crazy Buddhist]

I appreciate all of the help guys. It helps to get input from other people rather than just from a book as the book doesn't really give any helpful tips or tricks.

http://www.semsim.com/ccna/learn.html

That is a good resource and they have free versions of network modelling tools downloadable.

Matthew

[DaveW]

If you want to be taken seriously in the networking world ccna is a minimum.

APSTNDP

It seems to me that this is valid everywhere in the world but the UK. Here, they advertise all over the place. They have a heavy TV presence, and try to tell people that if they get MS Certification they are guaranteed a job. However, I've got reasonable contact with people in academia and the industry and they really scorn these qualifications (in many cases, I feel quite unfairly). They're only interested in degrees or experience, it seems. However, I may be mistaken-could someone clarify what sort of jobs you would aim to get with such qualifications? If memory serves, SgtM managed to get a good job after completing such a course; I remember thinking "Wow, in the UK they would only ever accept graduates for that". This has been something I've wanted to ask for a while, so anyone who knows more on this (or even better, someone who disagrees with me entirely) please let me know! Buzz, sorry if this is highjacking your thread-if you want, I can split them in half.

Please note: I don't have much faith in paper qualifications. I'm willing to bet that the best modders on this website have no formal qualifications in Computer Hardware, Science, or Engineering. I believe our very own Crimson Sky for example is a registered Master Craftsman who started as a cabinet maker.

Also, CB, good linkage, +Rep.

-Dave

[Crazy Buddhist]

Dave

CCNA is Cisco Certified Network Analyst (I believe)
CCNE is Cisco Certified Network Engineer

These are not scorned - they are well recognised in the UK and if you want to work in networking at the physical implementation level or planning physical networks they are qualifications that are recognised as industry standard. With CCNE, some relevant experience and some chutzpa you can be looking at £26 - 28K to start and we're talking about laying cables. Hard work and CCNA, 2 years later you can be a project manager at £50K.

MS certification has never meant much in the programming world here except certain sectors. Let's face it, we all know Microsoft can't code properly, so basically, aside from someone coding a strap-on system around MS office, the last thing you want is MS certification - much better to have a few thousand lines of code in your bag and be able to show a working skill-set.

Matthew