Computer Network Unit 1 q what are the topologies in computer n/w ?

MSC/VLR Service Area: The area covered by one MSC is called the MSC/VLR service area.

PLMN: The area covered by one network operator is called PLMN. A PLMN can contain one or more MSCs.


Q 9. What is the purpse of Jam signal in CSMA/CD ?
Ans: Collision
A condition where two devices detect that the network is idle and end up trying to send packets at exactly the same time. (within 1 round-trip delay) Since only one device can transmit at a time, both devices must back off and attempt to retransmit again.
CSMA/CD is designed to handle collisions with a re-transmit. The retransmission algorithm requires each device to wait a random amount of time, so the two are very likely to retry at different times, and thus the second one will sense that the network is busy and wait until the packet is finished. If the two devices retry at the same time (or almost the same time) they will collide again, and the process repeats until either the packet finally makes it onto the network without collisions, or 16 consecutive collision occur and the packet is aborted.
Jam
This is part of the CSMA/CD algorithm, that tells all stations that a collision has occurred, and to hold off transmitting for a short time, called the back off time, which is a random number. When a workstation detects a collision during transmission of a frame - none of the other stations are aware that the collision has occurred. So the station transmits a 32 to 48-bit jam signal so all other stations will see the collision also. When a repeater detects a collision on one port, it puts out a jam on all other ports, causing a collision to occur on those lines that are transmitting, and causing any non-transmitting stations to wait to transmit.
Interestingly enough, the actual format of jam is unspecified in the 802.3 specifications. Most manufacturers have used alternating 1s and 0s as jam, which is displayed as 0x5 (0101) or 0xA (1010) depending on when the jam is captured in the data stream.
Retransmission
When a collision is detected, the station sends a jam signal and then waits for a random backoff time, and then retransmits the frame. It will retry n attempts, where n is a user-defined number. If all attempts fail, it will report this to the LLC layer, which will then decide whether to retry another n times, or report that the link is down.