Routing

Question 1

Fixed Routing

Answer 1

• single permanent route for each source to destination pair
• determines routes using a least cost algorithm
• route fixed, at least until a change in network topology

Question 2

Flooding Routing

Answer 2

• no network info required
• packet sent by node to every neighbor
• incoming packets retransmitted on every link except incoming link
• eventually a number of copies will arrive at destination
• each packet is uniquely numbered so duplicates can be discarded
• nodes can remember packets already forwarded to keep network load in bounds
• can include a hop
• properties:
• all possible routes are tried (robust)
• at least one packet will have taken min. hop count route
• all nodes are visited

Question 3

Random Routing

Answer 3

• node selects one outgoing path for retransmission of incoming packet
• selection can be random or round robin
• can select outgoing path based on probability calculation
• no network info needed
• route is typically not the least cost, nor the minimum hop

Question 4

Adaptive Routing

Answer 4

• used by almost all packets switching networks
• routing decisions change as conditions on the network change (failure, congestion)
• requires info about network
• decisions more complex
• tradeoff between quality of network info and overhead
• advantages:
• improved performance
• aid congestion control
• complex system - may not realize theoretical benefits

Question 5

ISP

Internet Service Provider

Answer 5

• ISP - company providing other companies or individuals access or presence on the Internet
• individual hosts and LANs are connected to an ISP through a point of presence (POP)

Question 6

Point of Presence

POP

Answer 6

• an internet access provider may operate several POPs distributed throughout its area of operation and represents a collection of telecommunications equipment

Question 7

CPE

Customer Premises Equipment

Answer 7

• communications equipment located onsite with the host (e.g. modem)

Question 8

Local loop or Last mile

Answer 8

• the infrastructure between a provider’s installation and the site where the host is located

Question 9

NAP

Network Access Point

Answer 9

• physical facility that provides the infrastructure to move data between connected networks
• serve to tie the ISPs together, ISP also connect using peering arrangements and interconnections within geographic regions

Question 10

CO

central office

Answer 10

• the place where telephone companies terminate customer lines and locate switching equipment to interconnect those lines with other networks

Question 11

Wireless Modems - ADSL

assymetric digital subscriber line

Answer 11

• assymetric because from the user point, there is greater capacity downstream (from service provider to customer) than upstream
• uses FDM for managing the 1 MHz bandwidth:
• 25 kHz for voice: 0 to 4 kHz for voice, rest for guard, avoiding interference with other channels
• use echo cancellation or FDM to allocate two bands: one for upstream, one for downstream
• use FDM within each of two bands
• supports loop length in the range of 5.5 km

Question 12

Echo cancellation

Answer 12

• signal processing technique allowing digital transmissions in both directions on a single line simultaneously
• transmitter must subtract the echo of its own transmission from the incoming signal to recover the signal sent by the other side
• advantages:
• more flexibility for upstream badnwidth changes, simply extending the area of overlap
• downstream bandwidth in the good part of the spectrum - lower attenuation

Question 13

Discrete Multitone

DMT

Answer 13

• allows multiple carrier signals at different frequencies
• upstream and downstream bandwidths are split in a number of 4kHz sub-channels, transmitting a number of bits on each channel
• initially, the modem sends test signal on each subchannel and then use those subchannels with better signal to noise ratio