Question

1. Consider a PON network unications a What is the main problem in the b. How the problem of collision is Consider the enclosed graph of BERFesolved in What is the dispersion power penalty for b. How is the main in the PONUD graph of BER VS. received a. What is the di BER = 10" b. Explain how you compute the dis its value on the graph. upstream transmission? ved in Ethernet-based PON (E-PON)? ed power: OF SMF fiber as compared to LEAF fiber at dispersion powerp Dispersion Penalty for SME TWK "power penalty from this graph and show Y for SME, TWRS and + Back-to-back + SMF TWRS LEAF Log(BER) -22 21.5 -21 20.5 17.5 -17 -16.5 -20 -19.5 19 185 -18 Received Power (dBm)

Answer 1

1.(a)

PON stands for Passive Optical Networks, it is based in optical fibres (which are transparent fibres meant for transmitting data/information using the phenomenon of Total Internal Reflection) and are intended to provide broadband network services to the consumers. PONs are based in point-to-multipoint topology wherein single service provider (or ISP) provides internet access to multiple nodes by utilizing unpowered optical fibres. It is due to this topology that PON has to use different transmission modes for upstream and downstream transmission.

The downstream transmission works well with this topology since single source can transmit upstream data to the nodes without much hassle but the problem lies with the upstream transmission since a single source/ISP has to receive upstream request/upload data from multiple nodes/users in the network which causes collision of data packets at the receiver/ISP’s end (even with the use of TDM or time division multiplexing) this is because, the distance of each node in the network is different from the ISP, and even if each node transmits the data in its time slot allotted, then even because of the greatly varying distances, data may reach at the receiver/ISP at random times owing to different propagation delays (which depends on distance of two nodes).This causes increased number of collisions at the ISP’s end during upstream transmission.

1.(b)

E-PON or Ethernet based PONs solve the problem through a mechanism called DBA (or Dynamic Bandwidth Allocation) which uses the technique of polling for granting bandwidth to the nodes/users of the network and use message packets for communication thereby overcoming the problem of packet collision/loss at service provider side in TDM.