#### A – Point-to-point connections

**Exercise 3.1.**

Two stations A and B are connected by a bidirectional data transmission system whose ARQ
line protocol is characterized by the following parameters:

• fixed size of IUs, whose header is considered negligible, ,

• fixed size of acknowledgments, ,

• negligible processing time of IUs and acknowledgments.

The data transmission between the two stations is carried out by means of a radio system which makes a communication channel available having the following characteristics:

• distance between the two stations, d = 35 km,

• link capacity, C = 34.368 Mbit/s.

The transmission from A to B of a data segment of length is considered. Evaluate the total transfer time of the data segment and the throughput achieved with the stop and wait protocol in the absence of transmission errors.

**Exercise 3.4.**

Two stations A and B are connected by a bidirectional data transmission system whose ARQ
line protocol is characterized by the following parameters:

• fixed size of IUs, whose header is considered negligible, ,

• fixed size of acknowledgments, ,

• negligible processing time of IUs and acknowledgments.

The data transmission between the two stations is carried out by means of a radio system which makes a communication channel available having the following characteristics:

• distance between the two stations, d = 15 km,

• link capacity, C = 155 Mbit/s.
A window flow control mechanism is also operating with parameters:

• opening of the transmitter window, ,

• arbitrarily large opening of the receiver window,

• time-out for start of retransmission, .

If the go-back-n protocol is used, evaluate in the absence of transmission errors the total transfer time from A to B of a data segment of length , the throughput and the link usage efficiency achieved.

**Exercise 3.6.**

Two terrestrial stations A and B are connected via two simplex radio links characterized as follows:
(i) direct connection A-B of length and capacity Mbit/s; (ii) connection B-A consisting of two serial sections connected via a radio reflector R with length of each section and link capacity .

The protocol that controls the transmission of the IUs on this link is of type ARQ, characterized as follows:

• IUs with variable size, which depends on the size of the packet transported, up to a maximum length , of which constitute the header,

• fixed size of acknowledgments, ,

• negligible processing time in stations A and B for IUs and acknowledgments.

The selective repeat protocol is used with a time-out and equal opening of the sender and receiver windows . A data segment of length D = 3355 byte is to be transferred from A to B, requiring that the IUs have maximum length except possibly the last one. Calculate, in the absence of errors on the link, the transfer time of the data segment, the actual data throughput of the link, measured in bit/s, and how much this is worth in percentage with respect to the maximum throughput theoretically achievable on channel A-B ( efficiency).). Also determine the optimal size of the transmitter window which maximizes the throughput of this link.

**Exercise 3.11.**

Two stations A and B are in communication through a data link on cable with length d = 8 km
and capacity C = 40 Mbit/s. The protocol that controls the data transfer on the link is ARQ go-back-n with a
fixed length IU byte and acknowledgments of length byte. It is assumed that the IU information flow is unidirectional from station A to station B, with positive or negative acknowledgments sent
in the opposite direction. At time t = 0, station A begins the transmission without interruption of 11 IUs, while station B sends 7 acknowledgments, which can be either positive or negative, with transmissions beginning
at times t = 100, 160, 280, 340, 400, 460, 520, 580, 640 . IUs are numbered with the smallest possible numbering module . Knowing that the link is subject to errors that can affect only the IUs but not the acknowledgments, identify the IUs and the acknowledgments having numbered the first transmitted IU with number 2 and determine the minimum opening of the sender window compatible with this exchange
of IUs.

#### B – Multipoint networks

**Exercise 3.17.**

Determine the expression of the maximum throughput of a reservation protocol with all active
stations, fixed-length frames with one transmission per cycle and a reservation phase in which the N stations
access a single minislot using the same contention mechanism as the slotted ALOHA protocol; It is assumed
that a centralized station will acknowledge the reservations successfully received (without collisions) in negligible time on a dedicated return channel.

**Exercise 3.20.**

Determine the minimum extension of a ring network with token transfer for a copper transmission
medium with the following characteristics:

• ring capacity, C = 4 Mbit/s,

• number of stations evenly distributed along the ring, N = 4,

• minimum length of the information unit, ,

• station latency,

**Exercise 3.23.**

Generate a graph similar to that of Figure 3.50 for the non-persistent CSMA protocol for the
three a values in the figure (a = 1, 0.1, 0.01).

**Figure 3.50** Throughput with slotted non-persistent CSMA protocol (Exercise 3.23).

**Exercise 3.24.**

Determine analytically the value of the maximum throughput and the corresponding value of
the offered traffic for the non-persistent CSMA protocol with a = (1, 0.1, 0.01).

**Exercise 3.31.**

Express the value of the minimum delay required to transfer a single-fragment IU in the
CSMA/CA protocol.

#### C – Routing protocols

**Exercise 3.35.**

Determine the routing table of nodes B and F of the six-node and nine-edge network of Figure
3.60 for the first four update steps of the distance vector algorithm, assuming that the distance vectors are
transmitted between nodes at the same times and that updating in each node occurs only once per step considering the vectors of all adjacent nodes.

**Figure 3.60** Network topology (Exercise 3.35).

**Exercise 3.38.**

Consider a six-node network whose LSPs issued, shown in Figure 3.68, are received in node
B in sequence from nodes E, D, A, F, C. Obtain the minimum spanning tree of node B.

**Figure 3.68** Link state packet issued by the six network nodes (Exercise 3.38).

**Exercise 3.39.**

For the network topology obtained in Exercise 3.38, apply the Dijkstra’s algorithm and obtain
the routing table of node B.

**Exercise 3.41.**

Consider a network with 9 nodes (A, B, C, D, E, F, G, H, I) and 17 edges whose costs are:

costo | rami

1 | F-G, F-I, G-H

2 | A-B, B-D

3 | B-C, C-D, E-F

4 | A-H, D-E, E-I

5 | D-I

6 | A-G, H-I

7 | D-H, G-I

8 | B-H

Apply the Dijkstra’s algorithm and obtain the minimum spanning tree for node H.

#### D – Flow and congestion control

**Exercise 3.43.**

Consider a network with 6 nodes, labelled A, B, C, D, E, F, and 8 edges with lengths 20 km
for A-B, 12 km for B-C, 10 km for C-D, 40 km for D-E, 10 km for E-F, 5 km for F-A, 35 km for A-C, 40 km
for B-F. All links are made of optical fibre and have a capacity C = 20 Mbit/s. The network nodes behave like
ideal store and forward switches: any processing and queuing delay is negligible, and a packet in transit is
retransmitted only after having received it entirely. An isarithmic flow control mechanism operates on the
network between the access nodes. The entrance into the network of a packet “consumes” a permit in the entry
node. The exit from the network of a packet generates in the exit node a new permit which is retransmitted
back to the entry node along the same path used by the packet. In the absence of permits, the input node keeps
the packets received from the source in a buffer waiting for new permits to be available. It is assumed that:

• the packets entering the network are all of equal length (byte) and have payloads of
= 100 byte, and an overhead ;

• the permits are carried by control packets that have length with .

Two virtual circuits are active, the first from node C to node B and the second from node C to node A which are routed along the shortest route. Consider the case in which C receives from the sources of the two virtual circuits the following contiguous sequence of information packets: two for B, three for A, three for B. At time t = 0 node C has received completely from the sources all the packets that must be sent; moreover, N = 4 permits are available in node C at time t = 0. Determine the overall transfer time of the packets from node C to the respective destinations.

**Exercise 3.45.**

Consider a periodic VBR source, whose activity for a period of 3 s is characterized as follows:

• source peak rate, ,

• activity and inactivity periods with constant duration, and , with the start of activity at time ,

• fixed size packets issued by the source, .

Obtain the graph of the outgoing flow from a leaky bucket and a token bucket in store and forward (S&F)
mode with the following features:

• output rate from the bucket, ,

• permit arrival rate,,

• time of first arrival of the permits, ,

• bucket capacity in the leaky bucket, W = 10 pacchetti,

• bucket capacity in the token bucket, W = 10 permessi.

**Exercise 3.50.**

Consider a non-periodic source, whose (constant) packet generation rate is characterized
as follows: for , for , for , for ,
linearly decreasing rate from to for . The source feeds a leaky bucket flow control device with permit interarrival time from t = 0 and memory size (buffer) of the information units W = 300 IUs. Obtain the time diagram representing the output rate from the device fout, the filling level of the memory , as well as the number of lost IUs, assuming that the device does not add overhead.