Question

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SUBJECT: IT-210: Computer Networks

Q:Let us assume that some hubs, rather than switches, are connected in a way that they are forming a loop. Elaborate the effect of transmission when a host sends message over such a network.

Apparently, it would not be easy / possible to implement the spanning tree mechanism for hubs. Explain why? Suggest a technique using which the hubs can identify presence of loops and turn off some ports to eliminate the loop. Your solution must be logical and does not need to handle the situation every time.

Answer 1

1) The main difference between hub and switch is that the hub has single collision domain, while in switch, each port have their own collision domain.It means that if hub wants to send a packet to port it will simply forward to all other ports and a hub will blindly transmit everything it receives to all other connections. Also known as broadcasting device or broadcast transmission.

If we have hub based network, as soon as first piece of data is sent on network, a single ethernet frame will cycle through the loop repeatedly and consume 100% of the possible bandwidth.

2)Hub operates at layer 1- the physical layer and does not know anything about layer 2 or STP(spanning tree protocol).

Carrier Sense Multiple Access with Collison Detection technique can identify the presence of loop and eliminate loops. CSMA/CD is commonly used in network with half duplex device like hub.

Packet collisions occur when packets are transmitted from different host at the same time. To prevent this, CSMA/CD forces a transmitting station to check for the presence of a digital signal on the wire. If no other hosts are transmitting packets, the sender begins sending the frame. The sender also monitors the wire to make sure no other hosts begin transmitting. However, if another host begins transmitting at the same time and a collision occur, the transmitting host sends a jam signal that causes all hosts on the network segment to stop sending data. After a random period of time, hosts retransmit their packets

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