recitation midterm

Recitation: Midterm Review

Hung-Bin (Bing) Chang and Yu-Yu Lin

Electrical Engineering Department University of California (UCLA), USA,[email protected] and [email protected]

Prof. Izhak Rubin (UCLA) EE 132B 2014 Fall 1 / 14

Outline

1 Material SummaryProbabilityMultiplexing schemes and Multiples Access SchemesSwitching TechniquesARQ: Automatic repeat requestRouting

2 Midterm Sample


Material Summary Probability

Probability

Random variables (uniform, binomial, exponential, geometric,Poisson)CDF, PDF, expectation, moment generating functionGeometric series, Memoryless property


Material Summary Multiplexing schemes and Multiples Access Schemes

Multiplexing schemes and Multiples Access Schemes

Multiplexing schemesFixed assigned (FDM, TDM)Demand assigned (ATDM - statistical multiplexing)

Multiples Access SchemesFixed assigned (FDMA, TDMA)Demand assigned (reservation-based DA/TDMA vs. polling(centralized, token passing/token ring))Random access (ALOHA, slotted ALOHA, CSMA, CSMA/CA)Throughput calculation of a system (bps, packets/sec)


Material Summary Switching Techniques

Switching Techniques

Circuit switching (resource guarantee/dedicated, QoS) - phases ofoperation

Constant rate trafficResource guarantee, QoS (throughput and delay) assuranceCircuit setup cost and delay, low capacity utilization

Packet/Message switching (store and forward, best effort)Bursty trafficLow cost, rapid reaction to link/network failures/congestionNo QoS guarantees


Material Summary ARQ: Automatic repeat request

ARQ: Automatic repeat request

Positive acknowledgement, timeout timer, number packet andACK.Three ARQ schemes

Stop and WaitGo back NSelective repeated


Material Summary Routing

Routing

Distance Vector Routing (Bellman-Ford Algorithm)Each router estimates the length of its shortest path to each nodein the domain using estimates that it receives from its neighbors,and informs its estimates to its neighbors.Routers iteratively update their estimates based on the estimatesthat they receive from their neighbors, when a shorter route lengthis detected.

Link State Routing (Dijkstras Algorithm)Each router has complete topology info (map of routers and thenetworks they are connected to).Routers actively test the status of neighboring routers andpropagate link status to all neighbors.Each router does the routing locally by finding the shortest pathtree.



Dijkstras Algorithm

Step 1:Set P = {1} (i.e., Node 1 is the source node)Set T = {2, 3, . . . }j = 2, 3, . . . , set d(1, j) = h(1, j)

Step 2: Find i T where d(1, i) = minjT{d(1, j)}.Set T = T {i}Set P = P + {i}If T = , we are done!

Step 3: j T :d(1, j) = min{d(1, j),d(1, i) + h(i , j)}Go to step 2



Example: Dijkstras Algorithm

Step 1:

P = {1}T = {2, 3, 4}d(1, 2) = 3,d(1, 4) =0.5,d(1, 3) =

Step 2:

i = 4P = {1, 4},T = {2, 3}d(1, 2) = min{3, 0.5 +} =3d(1, 3) = min{, 0.5 + 8} =8.5

3 4

80.5

2

1

4

3

3 4

80.5

2

1

4

3

(1, 3)

(1, 0.5)

(1, )

3 4

80.5

2

1

4

3

(1, 3)

(1, 0.5)

(4, 8.5)

Step 1

Step 2

Figure : Dijkstras algorithm



Example - Contd

Step 3:

i = 2P = {1, 2, 4}T = {3}d(1, 2) = min{3, 0.5 +} =3d(1, 3) = min{8.5, 3+4} = 7

Step 4:

i = 3P = {1, 2, 3, 4}

3 4

80.5

2

1

4

3

(1, 3)

(1, 0.5)

(2, 7)

Step 3

3 4

80.5

2

1

4

3

(1, 3)

(1, 0.5)

(2, 7)

Step 4

Figure : Dijkstras algorithm


Midterm Sample

Problem 1

(a.) State the protocols for the following random access schemes:unslotted ALOHA, slotted ALOHA, CSMA/CA.

(b.) Consider a communications channel that is shared among 10stations. The transmission data rate across the channel is 10Mbps. Each station is noted to transmit (or retransmit) a packet ineach slot with probability 0.03. The packet size is equal to 4000bits, including overhead. The slot duration is equal to the time ittakes to transmit a packet.(i.) Calculate the probability that a given stations packet transmission

is successful.(ii.) Calculate the channels normalized throughput rate (which is equal

to the average number of successful packet transmissions in a slot).(iii.) Compute the average number of times that a packet is transmitted

(and retransmitted) until it is successfully received.


Midterm Sample

Problem 2

(a.) Define and state key differences between multiplexing andmultiple access schemes.

(b.) Define and state key differences between TDMA anddemand-assigned TDMA.


Midterm Sample

Problem 2 - Contd

(c.) Consider a communications channel that is shared among 10stations using a TDMA protocol. Each station is allocated a singleslot during each TDMA time frame. The transmission data rateacross the channel is equal to 2 Mbps. Each time slot issufficiently wide to allow the transmission of a single packet,including the propagation delay which equals to 1 msec. Eachpacket contains 5000 information bits and 240 overhead bits.(i.) Calculate the network throughput achieved by a single station,

assuming a noiseless channel such that all message transmissionsare received correctly.

(ii.) Assume that, for the above described system, the channel bit errorrate is equal to 104. Assume that a packet that is receivedincorrectly will be retransmitted by the station in its slot in the nextframe. Calculate the net throughput achieved by a single station.


Midterm Sample

Problem 3

Consider a Half-Duplex communications link which employs aStop-and-Wait ARQ error-control scheme. The transceiversequipment has a turn-around time of 3 msec. The link is 2000 Kmlong, and the propagation rate is 5 microsec/Km. The ACK packetcontains 360 bits. Assume ACK messages to be sent as separateframes. The information frame (on which the error control schemeoperates) contains a 760 bits header. The link is operated at adata rate of 240 Kbps. The channels bit error rate is equal to0.0001.(a.) Obtain the maximum length of the frame which must be selected to

ensure that the frame is retransmitted (at least once) for no morethan 30% of the time. Show whether such a maximum lengthcondition can be imposed.

(b.) Under the selected value for the frame, calculate the linksthroughput and its normalized throughput efficiency.


Material SummaryProbabilityMultiplexing schemes and Multiples Access SchemesSwitching TechniquesARQ: Automatic repeat requestRouting

Midterm Sample

recitation midterm

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