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TOPIC: some questions about CSMA/CD,hubs and full/half duplex

some questions about CSMA/CD,hubs and full/half duplex 11 years 4 months ago #9328

  • x1ng
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Hi guys i really have some questions that confusing me and i need a explanation

1) If CSMA/CD is to prevent collision .if CSMA/CD will listen to make sure that is no other device sending data so, why the collision can occur?

2) If there are devices sharing the same domain (10base2 or 10base5) if one device sent a packet so this packet will broadcast to every device on the segment, I want to know if the broadcast include the sender. So the sender will receive the packet that he sent?

3) I need to know what exactly the uses of the hub and what is the deference that hubs made. Is to allow use of UTP that are cheaper than coax and easy to install, and to be in star topology?
Or there is any other deference or there is deference in the throughput and efficiency?

4) I really don't understand the deference between passive, active
And intelligent hubs. I need to know is there any big deference?


That's it, thanks
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Re: some questions about CSMA/CD,hubs and full/half duplex 11 years 4 months ago #9329

  • LooseCannon
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1) If CSMA/CD is to prevent collision .if CSMA/CD will listen to make sure that is no other device sending data so, why the collision can occur?

There is always the chance that two devices will transmit at the exact same time and if this happens CSMA/CD will detect it and use its random backoff algorithm.
2) If there are devices sharing the same domain (10base2 or 10base5) if one device sent a packet so this packet will broadcast to every device on the segment, I want to know if the broadcast include the sender. So the sender will receive the packet that he sent?

Broadcasts are received by every device on the subnet including the sender.

As for the rest I recommend you read the excellent documents that the firewall.cx team has written in the menus at the top. There are good explanations of hubs, broadcasts and the CSMA/CD technique under the Networking menu.
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Re: some questions about CSMA/CD,hubs and full/half duplex 11 years 4 months ago #9334

  • x1ng
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thanks for answer

Another thing came to my mind .if broadcasting is include the sender so what is the purpose of the loop back (the NIC loops the sent frame onto it's receive pair internally on the card)
So the frame will not come to the sender.
I need an explain Please!
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Re: some questions about CSMA/CD,hubs and full/half duplex 11 years 4 months ago #9347

  • MezzUp
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4) I really don't understand the deference between passive, active
And intelligent hubs. I need to know is there any big deference?
Passive hub: A hub that takes the electrical signal in one port and lets it "flow" to other ports. Doesn't do any signal regeneration.
Active hub: Just like a passive hub with repeaters at every port, that is; "boost's" the signal to other ports. Probably the most common form of hubs.
Intelligent hub: An active hub with some administration functions through f.ex. a telnet or a web interface. (like a managed switch)
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Re: some questions about CSMA/CD,hubs and full/half duplex 11 years 4 months ago #9352

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thank you mezzup for the good answer about hubs.

but there is another thing that i still dont understand the "NIC loop back" if any body here can explain it please

thanks
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Re: some questions about CSMA/CD,hubs and full/half duplex 11 years 4 months ago #9364

  • nske
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if broadcasting is include the sender so what is the purpose of the loop back (the NIC loops the sent frame onto it's receive pair internally on the card)

Unless you are refering to something else that I am not aware of, the loopback interface is not associated to any hardware. In fact, datagrams targeted to the loopback interface, never reach the data link layer, they are instead looped back directly to the associated address at the network layer.

The loop-back interface is a virtual device introduced to the OS through a special driver. It's existense doesn't serve any critical specific purpose and practically, most systems could be fine without it. But it can also be valuable in many cases, such as to test that the TCP/IP stack is working fine and to be able to utilize TCP/IP services locally, without the requirement or the interference of a hardware adapter. A very simple example of it's use, is for a webmaster to set an httpd & mysql database server at a system and apply anything before he implements it publically. Appart from such obvious applications, some times software (not necessarily network-oriented) uses the loopback interface for internal communication between some of it's underlying systems.

The loopback interface, as stated by the RFC, should always be associated with 127.0.0.1 IPv4 and ::1 IPv6 addresses.
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