7.In some LANs, most of the computers talk with the server, but others use no server. What are these two approaches called?
A dedicated server LAN has one or more computers that are permanently assigned to being the network server(s). These
servers enable users to share files and often are also used to share printers.
Peer-to-peer networks do not require a dedicated server. All computers run special network software that enables them to
function both as a client and as a server. Authorized users can connect to any computer in the LAN that permits access
and use their hard drive(s) and printer as though they were physically attached to their own computers. Peer-to-peer
networks often are slower than dedicated server networks because if you access a computer that is also being used by its
owner, it slows down both the owner and the network.
22.How does switched Ethernet differ from traditional Ethernet?
Switched Ethernet provides individual dedicated ports to each node connected to it physically and processes packets
based on this unique port. Traditional Ethernet whether 10Base2, 10Base5 or a 10BaseT HUB processes packets
based on the entire LAN segment to which a node is physically connected.
23.How do layer-2 Ethernet switches know where to send the packets they receive? Describe how
switches gather and use this knowledge.
Layer-2 switches operate on the destination MAC address of each packet processed to determine which port to pass
on each packet presented for transmission.
Level-2 switches learn and store in memory in the form of a forwarding table, the specific port location of each
MAC address for every device connected to any of its ports.
25.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.
29.Explain how Bluetooth works?
Unlike 802.11 WLANs, Bluetooth is designed to operate within a maximum distance of 30 feet. The new standard IEEE
802.15 which is being written at the present time is evaluating Bluetooth to operate at a maximum distance of 300 feet.
Using Bluetooth connecting devices, Bluetooth technology provides a 1Mbps shared circuit with approximately 780Kbps
actual throughput. It uses FHSS.
35.Some people believe Bluetooth as a revolution, whereas others see it as being similar to our current short-range
infrared communication between computer devices.
What do you think? Is Bluetooth a revolution?
Bluetooth is yet another evolution of network technology that currently is focused on higher speed connectivity for
short-range devices. Perhaps it is best characterized as the beginning of a revolution in which much higher speed
connectivity will be provided to personal devices and in which the range of such connectivity will exceed the
threshold of what normally would be considered the LAN environment.
connectivity as well.
Perhaps even including satellite
20.How do datagram services differ from virtual circuit services?
@Two communicating computers do not need to know through which intermediate nodes their data are routed because the
packet network takes care of it by either of two methods. The first method is called datagram, is a connectionless service. It
adds a destination address and sequence number to each packet, in addition to information about the data stream to which the
packet belongs. In this case, a route is chosen for each packet as it is accepted into the packet network. Each packet may
follow a different route through the network. At the destination address the sequence number tells the network how to
reassemble the packets into a continuous message. The sequence number is necessary because different routes may deliver
packets at different speeds, so data packets often arrive out of sequence.The second routing method is a connection-oriented
approach called a virtual circuit. In this case, the packet switched network establishes what appears to be one end-to-end circuit
between the sender and receiver. All packets for that transmission take the same route over the virtual circuit that has been set
up for that particular transmission. The two computers believe they have a dedicated point-to-point circuit (but in fact they do
not).
24.Explain the differences between CIR and MAR.@The committed information rate (CIR) is the data rate the circuit must
guarantee to transmit. If the network accepts the connection, it guarantees to provide that level of service. Most connections
also specify a maximum allowable rate (MAR), which is the maximum rate that the frame relay network will attempt to
provide, over and above the CIR. The circuit will attempt to transmit all packets up to the MAR, but all packets that exceed
the CIR are marked as discard eligible (DE). If the network becomes overloaded, DE packets are discarded. So while can
transmit faster than the CIR, they do so at a risk of lost packets.
25.How do VPN services differ from common carrier services?@A new type of VAN, called a virtual private network (VPN)
(or sometimes software defined network) has recently emerged. VPNs provide circuits that run over the Internet but appear to
the user to be private networks. Internet access is inexpensive compared to the cost of leasing dedicated circuits, circuit
switched services, or packet switched services from a common carrier.Different VPNs provide different services, but most
offer packet switching hardware that will communicate via the Internet, or VPN services which you lease from the VPN in
much the same way as leasing a service from a common carrier. These VPN hardware (or services) take your data, encrypt it,
and send it through the Internet through a series of “tunnels” -- a virtual circuit through the Internet which constrains the
source and destination to only those within the VPN.
17.What is social engineering?@Social engineering refers to breaking security simply by asking. For example, hackers
routinely phone unsuspecting users and imitate someone else (e.g., a technician, a boss, a network expert) and ask for a
password. Most security experts no longer test for social engineering attacks; they know from past experience that social
engineering will eventually succeed in any organization and therefore assume that hackers can gain access at will to "normal"
user accounts. Training end users not to divulge passwords may not eliminate social engineering attacks, but it may reduce its
effectiveness so that hackers give up and move on to easier targets.
22.Describe the purpose of a call-back modem.
In the case of a call-back modem, the user dials the organization’s
modem’s telephone number, and logs into to his or her account. Once the user enters the correct password, the modem
automatically hangs ups and dials the user’s modem’s telephone number. In this way, unauthorized intruders cannot access
others’ accounts because the host computer or communications server will only permit access via modems calling from
prespecified numbers.
15. How does TCP/IP perform address resolution for network layer addresses？
Server name resolution is the translation of application layer addresses into network layer addresses (e.g., translating
an Internet address such as www.cba.uga.edu into an IP address such as 128.192.98.3). This is done using the Domain
Name Service (DNS). Throughout the Internet there are a series of computers called name servers that provide DNS
services. These name servers run special address databases that store thousands of Internet addresses and their
corresponding IP addresses. These name servers are in effect the "directory assistance" computers for the Internet.
Any time a computer does not know the IP number for a computer, it sends a message to the name server requesting
the IP number.
When TCP/IP needs to translate an application layer address into an IP address, it sends a special TCP-level packet
to the nearest DNS server. This packet asks the DNS server to send the requesting computer the IP address that matches
the Internet address provided. If the DNS server has a matching name in its database, it sends back a special TCP packet
with the correct IP address. If that DNS server does not have that Internet address in its database, it will issue the same
request to another DNS server elsewhere on the Internet.
Once your computer receives an IP address it is stored in a server address table. This way, if you ever need to
access the same computer again, your computer does not need to contact a DNS server. Most server address tables are
routinely deleted whenever you turn off your computer.
19. What are the differences between connectionless and connection-oriented routing？
Connectionless routing means each packet is treated separately and makes its own way through the network. It is
possible that different packets will take different routes through the network depending upon the type of routing used and
the amount of traffic. Because packets following different routes may travel at different speeds, they may arrive out of
sequence at their destination. The sender’s network layer therefore puts a sequence number on each packet, in addition
to information about the message stream to which the packet belongs. The network layer must reassemble them in the
correct order before passing the message to the application layer.
Connection-oriented routing sets up a virtual circuit between the sender and receiver. In this case, a temporary
virtual circuit is defined between the sender and receiver. The network layer makes one routing decision when the
connection is established, and all packets follow the same route. All packets in the same message arrive at the destination
in the same order in which they were sent. In this case, packets only need to contain information about the stream to
which it belongs; sequence numbers are not needed, although many connection-oriented protocols include a sequence
number to ensure that all packets are actually received.
Connection-oriented routing has greater overhead than connectionless routing, because the sender must first “open
” the circuit by sending a control packet that instructs all the intervening devices to establish the circuit routing.
Likewise, when the transmission is complete, the sender must “close” the circuit. Connection-oriented protocols also
tend to have more overhead bits in each packet.
２. What is media access control and why is it important？
Media access control becomes important when several computers share the same communication circuit.
Controlling when and what computer transmit
—Important when more than one computer wants to send data (at the same time over the same circuit); e.g.:
．Point-to-point half duplex links
-- computers to take turns
．Multipoint configuration
-- Ensure that no two computers attempt to transmit data at the same time
３. Under what conditions is media access control unimportant？
With point-to-point full duplex configurations, media access control is unnecessary because there are only two
computers on the circuit and full duplex permits either computer to transmit at anytime. There is no media access
control.
４. Compare and contrast roll call polling, hub polling（or token passing）, and contention.
Roll Call Polling － 被動式
Hub polling (Token Passing) － 主動式
Contention
．當 circuit 為 free 時，則 transmit
．Collisions 衝突
－ 在超過一台 computer 同時傳送時則會發生
－ 需要確定哪一台 computer 是在發生 collision 是被擁許 transmit
．Used commonly in Ethernet LANs
５. Which is better, hub polling or contention？Explain.
10. Describe five types of noise. Which is likely to pose the greatest problem to network managers？
13. Describe three approaches to detecting errors, including how they work, the probability of detecting an
error, and any other benefits or limitations.
Three common error detection methods are ．Parity checks．Longitudinal Redundancy Checking (LRC)
．Polynomial checking －Cyclic Redundancy Check(CRC)
．Polynomial checking[投 4-23]
．Adds 1 or more characters to the end of message (based on a mathematical algorithm)
．Cyclic Redundancy Check (CRC)
－ Computed by calculating the remainder to a division problem
15. Briefly describe how polynomial checking works.
．Cyclic Redundancy Check (CRC)
－ Computed by calculating the remainder to a division problem
16. How does cyclical redundancy checking work？
CRC － 要注意最高位數是多少，就要補多少。 用 XOR
17. Which is the simplest (least sophisticated) protocol described in this chapter?
XMODEM is the simplest (least sophisticated) protocol described in this chapter.
23. What is transparency, and why is this a problem with SDLC？ （課 p132、講 4-39）
ａ. The protocol is not “transparent” because it cannot automatically send all types of data with any bit
patterns.
ｂ. It is possible that the user’s data to be transmitted contains the same bit pattern as the flag
(e.g., 01111110). If this is not prevented, the receiver will mistakenly believe that this data marks
the end of the frame and ignore all the data that follows it.
24. How does SDLC overcome its transparency problems？ （課 p132、講 4-39）
Solution：Bit stuffing（aka、zero insertion）
－ Sender inserts 0 anytime it detects 11111 ( five 1’s )
－ If receiver sees five 1’s, check next bit(s)
* if 0, remove it（stuffed bit）
* if 10, end of frame marker（0111110）
* if 11, error（7 1’s cannot be in data）
－ Works but increases complexity（增加複雜度）
25. Explain why Ethernet does not suffer from transparency problems. （課 p132）
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.
27. What is transmission efficiency？ （課 p135、講 4-45）
Transmission efficiency is defined as the total number of information bits (i.e., bits in the message sent by the user)
divided by the total bits in transmission (i.e., information bits plus overhead bits).
Transmissi on efficiency 
Total number of info bits to be transmitt ed
Total number of bits transmitt ed