Skipper,
You missed a few layers in that data-cake. :-)
I'm sure that you intentionally generalized. I like your posts. Since I have 30 minutes, I will expand:
You start with the ASCII data in your PC. This gets packaged in a V.34 wrapper (digital to analog, AKA modulation), which then gets packaged in a PCM wrapper (analog to digital) and sent through the network. The network is carrying a digital representation of an analog representation of digital data.
7)You first start at your Application (lets use a WWW brouser as an example). The application has a set of rules (html) that define how the ASCI characters are represented on your screen.
6)There is a layer of "glue" (Session layer) that is within your program that determines the data representation of the Application Layer rule sets.
5)Your ASCI data is just that in this layer, DATA. No meaning, no means to get to the destination. It is a tunnel between the above WWW application and the comunications media (logical). This is the Session layer. The data is passed on down the line
4)This is where the ASCI data is packetized (in TCP packets...connection oriented packets...see my previous post). Each packet contains header information pertaining to the order of the information that is to be passed on to this particular session. Packets don't always arive in order. But they must be sent to the session in order. Also the packets must be identified to a specific session. You can have multiple WWW application running on your client. This is the Transort layer.
3)Now that the raw ASCI data has been packetized, it must be given a destination WWW server to be passed on to on the Network. This is where the IP address (TCP/IP network address and host address) and IP information is encapsulated around the above TCP. This tells each packet the destination address of the WWW packet, as well as tags the packet with your source address, so the packet can return. Other information is included as well identifying the transport packet type (TCP, ICMP, UDP, OSPF, BGP, etc.). Other information is contained here as well, such as TOS field (synonomous to Quality of Service for ATM).
2)Once the IP Packet is complete, it also is encapsulated to form the data-link packet. This usually starts out on the client end as something call Ethernet and is changed as your physical media changes. It is the address unique to your hardware (NIC card) by an address called a MAC address. Once this packet leaves your client, it hits a LAN segment, or connects directly into a modem device. This layer is often stipped when it goes from Ethernet, to a WAN (Wide area network segment, such as Frame Relay, X.25, ATM, SONET, etc.) where it picks up a MAC address common to the new media. But along the way to the destination the IP packet does not change (in most cases).
1)After all is said in done you have a complete data-link packet that is much like an onion, having many layers.....Application,presentaion,session,transport,network,data-link. Now the packet is ready to be released into a network device that supports the above packet. In the case of a modem a PPP data-link connection has been established. A physical wire is used to carry the data to the destination, and as expect, this last layer is called the physical layer.
As far as your WWW connection is concerned, for a normal (upload) session, layers 2-7 are formed on your PC and do not change. Layers 3-4 are 1st established on your PC and modem as digital information and will remain that way, and layer 1 is 1st your phone line. Layer 2 is a PPP connection to the provider. Layer 1-2 can change to allow for media conversions from analog to digital and back to analog. Layer 1-2 then changes to 10BaseTx (twisted pair Ethernet) where you are connected to an authenticating host system of your provider. Layer 1-2 can change many times as your packet travels through thier network of routers and switches until your reach your final destinaion. From here your session is authenticated, and you are allowed free access to the internet (in most cases). Then you client www packet attempts to esablish a connection to the host WWW server, where layers 1-3 will change as the packet is routed through the internet many times. The reverse path is generally true as well, accept that your provider will establish (you have already explained the medias and protocols used...I will followup if my research finds different) a digital connection to your local phone carrier, where an analog downloaded packet is sent back to the client modem that is not limited to the upload 4Mhz modulated sine-wave signal as is the upload path is. This greatly increases the data capacity available to the end user for downloading.
PRB |
