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by Dinesh Thakur Category: Computer Network

To access Internet Through a Cable TV. Computer Network requires a cable Modem. It has two interfaces on it one for computer  and other for Cable Network The Computer side interface is usually is a USB Interface or 10Mbps Ethernet And The Cable Network is common cable wire interface.

This Modem makes a connection when it is turned on. Cable modems are always retaining the connection(unless they are switched off) because the cable operator does not charge for the duration of connection. 

When a cable Modem is switched on It scans the downstream channel looking for a special packet periodically(special packet contains the modem configuration and sender of this is the headend),After getting the packet, the new modem sends a packet on one of the upstream channel.

After getting a packet from a modem, headend sends a packet to the new modem assigning the modem with downstream and upstream channels. The modem then determines its distance from the headend by sending it a special packet and seeing how long it takes to get the response. This process is called ranging. It is important for the modem to know its distance to accommodate the way the upstream channels and to get the timing right.




They are divided in time in minislots Each upstream packet must fit in one or more consecutive minislots. The headend announces the start of a new round of minislots periodically, but the starting gun is not heard at all modems simultaneously due to the propagation time down the cable. By knowing  how  far it is from the headend, each modem can compute how long ago the first minislot really started. Minislot length is network dependent. A typical payload  is  8 bytes

During  initialization the headend also assigns each modem to a minislot to use for requesting upstream bandwidth. As rule, multiple modems will be assigned the same minislot, which leads to contention. When a computer wants to send a packet, it transfers the packet to the modem, which then requests the necessary number of minislots for it.

If the request is accepted, the headend puts an acknowledgement on the downstream channel telling the modem which minislots have been reserved for its packet. The packet is then sent ., starting  in the minislot allocated  to it .Additional packets can be requested using a field in the header.

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