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[[Category:Michigan Technological University]]
[[Category: MTU Classes]]
CS4461 Computer Networks Final Exam Study Guide
'''Chapter 4 - The Network Layer'''
Virtual Circuit Networks:
-Options: allow for the IP header to be extended
-Data: the payload of the packet contains the transport-layer segment to be delivered
Link State Routing Algorithm:
The network topology and all link costs are known and available as input to the LS algorithm. Dijkstra's algorithm is popular.
Problem - oscillations in updates. A solution would be to mandate that no 2 routers can run the LS algorithm at the same time.
Popular LS Algorithm in use: OSPF
Distance Vector Routing Algorithm:
The DV algorithm is iterative and ''distributed'' meaning that each node receives information from one or more of its directly connected neighbors. It is iterative in that it continues until no more information is exchanged between neighbors.
Link-Cost change and Link Failure:
-At time t0, y detects the link-cost change
-updates its distance vector
-informs its neighbors of the change
-At time t1, z receives the update from y and updates its table
-it compares a new least cost to x
-sends its new distance vector to its neighbors
-At time t2, y receives z's update and updates its table
-y's least costs do not change
-y sends no messages to z
-Algorithm stabilizes
Popular DV Algorithm In Use: RIP
Border Gateway Protocol:
BGP is an inter-AS routing protocol. BGP provides each AS a means to
-Obtain subnet reachability information from neighboring ASs
-Propagate the reachability information to all routers internal to the AS
-Determine "good" routes to subnets based on the reachability information and on AS policy
Essentially BGP allow for a subnet to shout out "OMG I'M OVER HERE!!!!111!one"
'''Chapter 5 - The Link Layer'''
Purpose: To move a datagram over an individual link. The link layer has the end to end job of moving network layer datagrams over a ''single link'' in the path.
Possible services offered by a link-layer protocol include:
-Framing: Encapsulate the network-layer datagram within a link-layer frame
-Link Access: A medium access control (MAC) protocol specifies the rules by which a frame is transmitted onto the link.
-Reliable Delivery: When a protocol provides a reliable delivery service it guarentees to move each network-layer datagram across the link without error
-Flow Control: The nodes on each side of a link have a limited amount of frame buffering capacity.
-Error Detection: A node's receiver can incorrectly decide that a bit in a frame is zero when it was transmitted as a one and vice versa.
-Error Correction: Receiver not only detects whether errors have been intorduced in the frame but also determines exactly where in the frame the errors have occured.
-Half-duplex and full-duplex: With full-duplex the nodes at both ends of a link may transmit packets at the same time.
Channel Partitioning Protocols
Time division multiplexing
-The Good
-Eliminates Collisions
-Perfectly fair
-The Bad
-Each node is limited to an average of the bandwidth even if its the only one talking
-A particular node is the only one with something to say, it can't say it all at once it must take breaks
Code division multiplexing
-The Good
-Different nodes can transmit simultaneously
Random Access Protocols
When there is a collision, each node involved in the collision repeatedly retransmits its frame until the frame gets through without a collision. It doesn't necessarily retransmit right away however. It waits a random ammount of time to retransmit thus decreasing the probability of future collisions on the same frame.
Carrier Sense Multiple Access With Collision Detection (CSMA/CD)
-An adapter may begin to transmit at any time
-An adapter never transmits a frame when it senses that some other adapter is transmitting
-A transmitting adapter aborts its transmission as soon as it detects that another adapter is also transmitting
-Before attempting a retransmission, an adapter waits a random time that is typically small compared with the time to transmit a frame
How CSMA/CD works in a specific adapter
1. The adapter obtains a network-layer datagram from its parent node
-prepares an ethernet frame
-puts the frame in an adapter buffer
2. If the adapter senses that the channel is idle it starts to transmit the frame
-If it senses the channel is busy then it waits until it senses no signal energy and then starts to transmit
3. While transmitting the adapter monitors for the presence of signal energy coming from other adapters
-If the adapter transmits the entire frame without detecting signal energy from other adapters, the adapter is finished with the frame
4. If the adapter detects signal energy from other adapters while transmitting it stops transmitting its frame and instead transmits a 48-bit jam signal
5. After aborting the adapter enters an exponential backoff phase.
-After experiencing the nth collision in a row for this frame the adapter chooses a value for K at random from {0,1,2,...,2^m-1}
-where m=min(n,10)
-The adapter then waits K*512 bit times then returns to step 2.
Purpose of the jam signal is to ensure that all the other adapters are aware of the collision.
At 10Mbps a bit time is 0.1 microseconds.
The Ethernet standard imposes limits on the distance between any two nodes. These limits ensure that if adapter A chooses a lower value of K than all the other adapters involved in a collision, then adapter A will be able to transmit its frames without experiencing a new collision. It is possible that an adapter can sneak in a new frame while other adapters are in the exponential backoff state.
Taking Turns Protocols: Most popular is Token Ring.
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