ISQS 3349 Final review questions (Spring 2002)
Chapter 3
1.
Describe the three types of data flows?
The three types of data flows are simplex, half-duplex and full duplex. Simplex is one-way transmission,
such as that in radio or TV transmission. Half duplex is two-way transmission, but you can transmit in
only one direction at a time. A half duplex communication link is similar to a walkie-talkie link; only one
computer can transmit at a time. With full duplex transmission, you can transmit in both directions
simultaneously, with no turnaround time.
2.
Clearly explain the differences among analog data, analog transmission, digital data, and digital
transmission.
Data can be transmitted through a circuit in the same form they are produced. Most computers, for example,
transmit their data through digital circuits to printers and other attached devices. Likewise, analog voice data
can be transmitted through telephone networks in analog form. In general, networks designed primarily to
transmit digital computer data tend to use digital transmission, and networks designed primarily to transmit
analog voice data tend to use analog transmission (at least for some parts of the transmission).
3.
Describe how data could be transmitted using phase modulation.
Phase modulation (PM) (also called phase shift keying (PSK)), is the most difficult to understand. Phase
refers to the direction in which the wave begins. Until now, the waves we have shown start by moving up and
to the right (this is called a 0º phase wave). Waves can also start down and to the right. This is called a phase
of 180º. With phase modulation, one phase is defined to be a zero and the other phase is defined to be a one.
4.
Why is data compression so useful?
Data compression can increase throughput of data over a communication link literally by compressing the
data. A 2:1 compression ratio means that for every two characters in the original signal, only one is needed in
the compressed signal (e.g., if the original signal contained 1000 bytes, only 500 would needed in the
compressed signal).
In 1996, ITU-T revised the V.34 standard to include a higher data rate 33.6 Kbps. This revision is popularly
known as V.34+. The faster data rate is accomplished by using a new form of TCM that averages 9.8 bits per
symbol (symbol rate remains at 3429).
Chapter 4
1. What does the data link layer do?
The data link layer controls the way messages are sent on the physical media. The data link layer handles
three functions: media access control, message delineation, and error control. The data link layer accepts
messages from the network layer and controls the hardware that actually transmits them. The data link layer is
responsible for getting a message from one computer to another without errors. The data link layer also
accepts streams of bits from the physical layer and organizes them into coherent messages that it passes to the
network layer.
2.
Compare and contrast stop-and-wait ARQ and continuous ARQ.
With stop-and-wait ARQ, the sender stops and waits for a response from the receiver after each message or
data packet. After receiving a packet, the receiver sends either an acknowledgment (ACK) if the message was
received without error, or a negative acknowledgment (NAK) if the message contained an error. If it is an
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NAK, the sender resends the previous message. If it is an ACK, the sender continues with the next message.
Stop-and-wait ARQ is by definition, a half duplex transmission technique.
With continuous ARQ, the sender does not wait for an acknowledgment after sending a message; it
immediately sends the next one. While the messages are being transmitted, the sender examines the stream of
returning acknowledgments. If it receives an NAK, the sender retransmits the needed messages. Continuous
ARQ is by definition a full duplex transmission technique, because both the sender and the receiver are
transmitting simultaneously (the sender is sending messages, and the receiver is sending ACKs and NAKs).
3. Explain why Ethernet does not suffer from transparency problems.
Ethernet is a byte-count protocol because instead of using special characters or bit patterns to mark the end of
a packet, it includes a field that specifies the length of the message portion of the packet. Unlike SDLC and
HDLC, Ethernet has no transparency problems. Any bit pattern can be transmitted because Ethernet uses the
number of bytes, not control characters, to delineate the message
Chapter 5
1.
What does the network layer do?
The network layer performs three important functions: addressing, routing, and breaking long messages
into smaller packets for transmission by the data link layer. The network layer sits between the
application layer and the data link layer. The network layer accepts messages from the application layer
and formats and addresses them for transmission by the data link layer. The network layer also accepts
individual messages from the data link layer and organizes them into coherent messages that it passes to
the application layer.
2.
What is a subnet and why do networks need them?
Each organization must assign the IP addresses it has received to specific computers on its networks. In
general, IP addresses are assigned so that all computers on the same local area network have a similar
addresses. For example, suppose a university has just received a set of Class B addresses starting with
128.184.x.x. It is customary to assign all the computers in the same LAN numbers that start with the
same first three digits, so the Business School LAN might be assigned 128.184.56.x while the Computer
Science LAN might be assigned 128.184.55.x (see Figure 6-8). Likewise, all the other LANs at the
university and the backbone network that connects them, would have a different set of numbers. Each of
these LANs are called a TCP/IP subnet because they are logically grouped together by IP number.
Knowing whether a computer is on your subnet or not it very important for message routing.
3.
How does dynamic addressing work?
With dynamic addressing, a server is designated to supply a network layer address to a computer each
time the computer connects to the network. This is commonly done for client computers, but usually not
done for servers.
Instead of providing a network layer address in a configuration file, a special software package is
installed on the client that instructs it to contact bootp or DHCP servers using data link layer addresses.
This message asks the servers to assign the requesting computer a unique network layer address. The
server runs a corresponding bootp or DHCP software package that responds to these requests and sends a
message back to the client giving it its network layer address (and its subnet mask).
4.
What is address resolution?
In order to send a message, the sender must be able to translate the application layer address (or server
name) of the destination into a network layer address and in turn translate that into a data link layer
address. This process is called address resolution. There are many different approaches to address
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resolution that range from completely decentralized (each computer is responsible for knowing all
addresses) to completely centralized (there is one computer that knows all addresses).
Chapter 6
1. Briefly describe CSMA, CD, and CA.
CSMA/CD, like all contention-based techniques, is very simple in concept: wait until the bus is free
(sense for carrier) and then transmit. Computers wait until no other devices are transmitting, and then
transmit their data. As long as no other computer attempts to transmit at the same time, everything is fine.
However, it is possible that two computers located some distance from one another can both listen to the
circuit, find it empty, and begin to simultaneously. This simultaneous transmission is called a collision.
The two messages collide and destroy each other.
The solution to this is to listen while transmitting, better known as collision detection (CD). If the NIC
detects any signal other than its own, it presumes that a collision has occurred, and sends a jamming
signal. All computers stop transmitting and wait for the circuit to become free before trying to retransmit.
The problem is that the computers which caused the collision could attempt to retransmit at the same
time. To prevent this, each computer waits a random amount of time after the colliding message
disappears before attempting to retransmit.
2.
What is the topology of wireless Ethernet and how does it work?
The logical and physical topologies of wireless Ethernet are the same as those of traditional Ethernet. It
is both a physical start and a logical bus. A central wireless access point (AP) is a radio transmitter that
plays the same role as a hub in traditional Ethernet.
3.
What is a bottleneck and how can you locate one?
In order to improve performance, you must locate the bottleneck, the part of the network that is
restricting the data flow. Generally speaking, the bottleneck will lie in one of two places. The first is the
network server. In this case, the client computers have no difficulty sending requests to the network
server, but the server lacks sufficient capacity to process all the requests it receives in a timely manner.
The second location is the network circuit. The network server can easily process all the client requests it
receives, but the network circuit lacks enough capacity to transmit all the requests to server. It is also
possible that the bottleneck could also lie in the client computers themselves (e.g., they are receiving
data to fast for them to process it), but this is extremely unlikely.
Chapter 7
1.
How does a bridge differ from a layer 2 switch?
Both bridges and switches operate at the data link layer, may connect different types of cable, and use
the same data link and network protocol to connect computers or network segments. With the exception
of new address encounters in an address-learning phase, layer-2 switches replace the Ethernet broadcast
paradigm with a capability for simultaneous processing of multiple messages.
Bridges are commonly used to segment local area networks to improve performance. Bridges “learn”
whether to forward packets from one network segment to another. When a bridge receives a packet, it
reads the packet's data link layer source address and compares this address to its own internal address
table. If the destination address is on the same network segment from which the packet arrived, the
bridge discards the packet, which is a process known as filtering.
Layer-2 switches (or workgroup switches) typically provide ports for a small set of 16 to 24 computers.
Layer-2 witches operate at the same layers as bridges but differ from them in two ways. First, most
switches enable all ports to be in use simultaneously by managing paired combinations of ports as
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separate point-to-point circuits. Since all ports can be active at once, switches usually are faster than
bridges. Like bridges, layer-2 switches "learn" addresses; a layer-2 switch builds a forwarding table after
it is first turned on. To learn addresses, a layer-2 switch retransmits to all ports (except to the one from
which it was received) only for a packet with a destination address not already in the forwarding table.
The resulting ACK from the destination computer (that recognized its address) is then used by the layer2 switch to add the new port number and address to the forwarding table. As layer-2 switches become
more powerful, they render bridges obsolete.
2.
How does a bridge differ from a layer 3 switch?
Layer-3 switches switch messages on the basis of their network layer address (usually TCP/IP address,
but some also support IPX/SPX) while bridges (and also layer-2 switches) switch on the basis of data
link layer addresses (usually Ethernet addresses). These switches provide the best of both switches and
routers (or brouters). Since they utilize network layer addressing, layer-3 switches can replace routers
(rather than hubs or bridges as layer-2 switches) and deliver significant benefits at the network layer:
more simultaneously active ports and thus much faster transmission.
3.
Explain how routed backbones work.
Routed backbones move packets along the backbone based on their network layer address (i.e., layer 3
address). The most common form of routed backbone uses a bus topology (e.g., using Ethernet 100BaseT). Routed backbones can be used at the core or distribution layers.
At the core layer routed backbones are sometimes called subnetted backbones or hierarchical backbones
and are most commonly used to connect different buildings within the same campus network.
At the distribution layer a routed backbone uses routers or layer 3 switches to connect a series of LANs
(access layer) to a single shared media backbone network. Each of the LANs are a separate subnet.
Message traffic stays within each subnet unless it specifically needs to leave the subnet to travel
elsewhere on the network, in which case the network layer address (e.g., TCP/IP) is used to move the
packet.
Chapter 10
1.
What is a sniffer?
A sniffer program records all messages received for later (unauthorized) analysis. A computer with a
sniffer program could then be plugged into an unattended hub or bridge to eavesdrop on all message
traffic.
2.
What is a firewall?
A firewall is a router, gateway, or special purpose computer that examines packets flowing into and
out of a network and restricts access to the organization's network. The network is designed so that a
firewall is placed on every network connection between the organization and the Internet. No access
is permitted except through the firewall. Some firewalls have the ability to detect and prevent denialof-service attacks, as well as unauthorized access attempts. Two commonly used types of firewalls
are packet level, and application level.
3.
What is a NAT proxy server and how does it work?
The NAT proxy server (NAT means network address translation) uses an address table to translate
the private IP addresses used inside the organization into proxy IP addresses used on the Internet.
When a computer inside the organization accesses a computer on the Internet, the proxy server
changes the source IP address in the outgoing IP packet to its own address.
When the external computer responds to the request, it addresses the message to the proxy server's IP
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address. The proxy server receives the incoming message, and after ensuring the packet should be
permitted inside, changes the destination IP address to the private IP address of the internal computer
and changes the TCP port id to the correct port id before transmitting it on the internal network. This
way systems outside the organization never see the actual internal IP addresses, and thus they think
there is only one computer on the internal network.
4.
What is key management?
Key management is concerned with dispersing and storing keys carefully. Because the DES algorithm is
known publicly, the disclosure of a secret key can mean total compromise of encrypted messages.
Managing this system of keys can be challenging, especially with symmetric algorithms.
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