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What is multiplex?

Posted on 2002-03-06
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could any experts tell me waht is multiplex used in network?

Please explain as detail as possible!

Thanks!

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Question by:mandy9
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adowns earned 100 total points
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To combine multiple signals from possibly disparate sources, in order to transmit them over a single path.

Here is an article on NAT (Network Address Translation which describes how multiplex plays a roll for functionality.

NAT Operation
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The basic purpose of NAT is to multiplex traffic from the internal network and present it to the Internet as if it was coming from a single computer having only one IP address.

The TCP/IP protocols include a multiplexing facility so that any computer can maintain multiple simultaneous connections with a remote computer. It is this multiplexing facility that is the key to single address NAT.

To multiplex several connections to a single destination, client computers label all packets with unique "port numbers". Each IP packet starts with a header containing the source and destination addresses and port numbers:

Source address Source port Destination address Destination port



This combination of numbers completely defines a single TCP/IP connection. The addresses specify the two machines at each end, and the two port numbers ensure that each connection between this pair of machines can be uniquely identified.

Each separate connection is originated from a unique source port number in the client, and all reply packets from the remote server for this connection contain the same number as their destination port, so that the client can relate them back to its correct connection. In this way, for example, it is possible for a web browser to ask a web server for several images at once and to know how to put all the parts of all the responses back together.

A modern NAT gateway must change the Source address on every outgoing packet to be its single public address. It therefore also renumbers the Source Ports to be unique, so that it can keep track of each client connection. The NAT gateway uses a port mapping table to remember how it renumbered the ports for each client's outgoing packets. The port mapping table relates the client's real local IP address and source port plus its translated source port number to a destination address and port. The NAT gateway can therefore reverse the process for returning packets and route them back to the correct clients.

When any remote server responds to an NAT client, incoming packets arriving at the NAT gateway will all have the same Destination address, but the destination Port number will be the unique Source Port number that was assigned by the NAT. The NAT gateway looks in its port mapping table to determine which "real" client address and port number a packet is destined for, and replaces these numbers before passing the packet on to the local client.

This process is completely dynamic. When a packet is received from an internal client, NAT looks for the matching source address and port in the port mapping table. If the entry is not found, a new one is created, and a new mapping port allocated to the client:


Incoming packet received on non-NAT port
Look for source address, port in the mapping table
If found, replace source port with previously allocated mapping port
If not found, allocate a new mapping port
Replace source address with NAT address, source port with mapping port

Packets received on the NAT port undergo a reverse translation process:


Incoming packet received on NAT port
Look up destination port number in port mapping table
If found, replace destination address and port with entries from the mapping table
If not found, the packet is not for us and should be rejected

Each client has an idle time-out associated with it. Whenever new traffic is received for a client, its time-out is reset. When the time-out expires, the client is removed from the table. This ensures that the table is kept to a reasonable size. The length of the time-out varies, but taking into account traffic variations on the Internet should not go below 2-3 minutes. Most NAT implementations can also track TCP clients on a per-connection basis and remove them from the table as soon as the connection is closed. This is not possible for UDP traffic since it is not connection based.

Many higher-level TCP/IP protocols embed client addressing information in the packets. For example, during an "active" FTP transfer the client informs the server of its IP address & port number, and then waits for the server to open a connection to that address. NAT has to monitor these packets and modify them on the fly to replace the client's IP address (which is on the internal network) with the NAT address. Since this changes the length of the packet, the TCP sequence/acknowledge numbers must be modified as well. Most protocols can be supported within the NAT; some protocols, however, may require that the clients themselves are made aware of the NAT and that they participate in the address translation process. [Or the NAT must be protocol-sensitive so that it can monitor or modify the embedded address or port data]

Because the port mapping table relates complete connection information - source and destination address and port numbers - it is possible to validate any or all of this information before passing incoming packets back to the client. This checking helps to provide effective firewall protection against Internet-launched attacks on the private LAN.

Each IP packet also contain checksums that are calculated by the originator. They are recalculated and compared by the recipient to see if the packet has been corrupted in transit. The checksums depend on the contents of the packet. Since the NAT must modify the packet addresses and port numbers, it must also recalculate and replace the checksums. Careful design in the NAT software can ensure that this extra processing has a minimal effect on the gateway's throughput. Before doing so it must check for, and discard, any corrupt packets to avoid converting a bad packet into a good one.

Conclusion
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As the Internet continues to expand at an ever-increasing rate, Network Address Translation offers a fast and effective way to expand secure Internet access into existing and new private networks, without having to wait for a major new IP addressing structure. It offers greater administrative flexibility and performance than the alternative application-level proxies, and is becoming the de facto standard for shared access.

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by:SteveJ
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You should thank adowns for doing your homework.
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by:SunBow
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yeah. sounds impressive. but i had to use word prior to NAT, so it really depends.... on..
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by:SunBow
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Oh, well (<sigh>) There are some simple websites to bookmark for general info on simple terms. Among them WhatIs. Using it produces a networking perspective and details of another, more original kind (note that 'details' can clarify more or confuse more, depending on background and needs, and that there are other perspectives to same question):

"multiplexing

Multiplexing is sending multiple signals or streams of information on a carrier at the same time in the form of a single, complex signal and then recovering the separate signals at the receiving end. analog signals are commonly multiplexed using frequency-division multiplexing (FDM), in which the carrier bandwidth is divided into subchannels of different frequency widths, each carrying a signal at the same time in parallel. digital signals are commonly multiplexed using time-division multiplexing (TDM), in which the multiple signals are carried over the same channel in alternating time slots. In some optical fiber networks, multiple signals are carried together as separate wavelengths of light in a multiplexed signal using dense wavelength division multiplexing(DWDM). "
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by:adowns
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Sure you always have your classic dictionary defination for a technical term, though it may sound good as we all know 75% of the time it means nothing unless you can relate to how it actually works. Thats what I provided, a desription every person with a basic networking background could understand. There are many many other ways its used, but unless you understand WAN technologies and the OSI model its going to sound just as foreign as the definition. I hope its enough.
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by:CyberStretch
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Heh. Just to add to the turmoil:

I usually try to describe things in a "lowest common denominator" manner when talking with someone whom I do not know their level of experience. Therefore, for me, the simplest "definition" of multiplexing is Cable TV; something many people have and seem to have a very basic understanding of.

Think of home-based cable reception: The cable company comes in, installs a wire from the pole to your house, and Viola!, depending on the package you ordered you can have anywhere from just your local stations to upwards of 1000 tv stations delivered to your tv.

This is done, basically, by filling the cable with all of the tv signals at once, and having your tv (or tuner) "filter out" all of the rest of the channels so you only receive one channel at a time.

No networking inferences, no OSI model, no specific uses of multiplexing, etc. Just a very simple explanation of a potentially highly technical topic.
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by:SunBow
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;-) for easier approach, consider instead, demultiplexing
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by:adowns
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I was definately considering describing multiplexing with a differant technology for easy of understanding, but I wanted to follow the origional question of what multiplexing is used for in a network.
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by:SunBow
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?
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by:CyberStretch
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Sunbow,

If you are looking for the answer to demultiplexing, in my previous example considering cable TV, the TV is the demultiplexer.

Demultiplexing is basically taking a multiplexed signal - such as the cable TV channels - and separating them out into individual signals again. Following the previous example, when you select a channel on your TV, you are demultiplexing the multiple signals and only accepting the signals for that channel.

However, in true multiplexing/demultiplexing usage, all signals are used for one purpose or another, rather than in the TV example where you are basically filtering out one signal for use.
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by:SunBow
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CyberStretch, no, I not looking, but noted that the one (either) must imply the other, while it was seemingly neglected in this now unattended thread.

I think you've captured the premise quite visually and succinctly (eg: well done).
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by:SteveJ
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Just for the heck of it . . .

CyberStretch,

Why do you say that in "true" muxing/demuxing all signals are used? That would imply that every large circuit which isn't fully utilized isn't really being muxed or demuxed. In fact, I'd say your TV analogy isn't demuxing at all and you noted that fact yourself by saying that the signal is filtered . . . which is a totally different technology.

How about . . . the airport is the "mux / demux" and the airplane is the transport. The passengers are "muxed" onto the airplane (that is they arrive by independent means and paths at the airport) and they fly via common carrier to the "demux" facility, where they deplane and go by independent means and paths to a final destination. This might describe time division multiplexing.

For frequency division multiplexing, how about this analogy . . . Your mouth, teeth, and saliva are the "mux" and the other components of your digestive system, your stomach and digestive juices, are the "demux". You take bread and peanut butter and jelly and stuff them into your mouth where your teeth and saliva "mux" these components into a complex wave form, you swallow (common carrier) and the bolus -- I think that's what I remember from 8th grade Science -- ends up in your stomach where it is demuxed.

Wake up mandy9! It's a brand new day!

Steve
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by:CleanupPing
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mandy9:
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