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计算机网络自顶向下方法与Internet特色第三版答案英文
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更新于2023-03-16
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计算机网络 自顶向下方法与internet特色,经典教材的配套答案,对学习很有帮助
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Chapter 1
Review Questions
3.A networking program usually has two programs, each running on a
different host, communicating with each other. The program that
initiates the communication is the client. Typically, the client program
requests and receives services from the server program.
4.The Internet provides its applications a connection-oriented service
(TCP) and a connectionless service (UDP). Each Internet application
makes use of one these two services. The two services will be discussed
in detail in Chapter 3.
Some of the principle characteristics of the connection-oriented service
are:
a) Two end-systems first "handshake" before either starts to send
application data to the other.
b) Provides reliable data transfer, i.e., all application data sent by one
side of the connection arrives at the other side of the connection in order
and without any gaps.
c) Provides flow control, i.e., it makes sure that neither end of a
connection overwhelms the buffers in the other end of the connection by
sending to many packets to fast.
d) Provides congestion control, i.e., regulates the amount of data that
an application can send into the network, helping to prevent the
Internet from entering a state of grid lock.
Some of the principle characteristics of connectionless service are:
a) No handshaking
b) No guarantees of reliable data transfer
c) No flow control or congestion control
5.Flow control and congestion control are two distinct control
mechanisms with distinct objectives. Flow control makes sure that
neither end of a connection overwhelms the buffers in the other end of
the connection by sending to many packets to fast. Congestion control
regulates the amount of data that an application can send into the
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network, helping to prevent congestion in the network core (i.e., in the
buffers in the network routers).
6.The Internet's connection-oriented service provides reliable data
transfer by using acknowledgements and retransmissions. When one
side of the connection doesn't receive an acknowledgement (from the
other side of the connection) for a packet it transmitted, it retransmits
the packet.
8.In a packet switched network, the packets from different sources
flowing on a link do not follow any fixed, pre-defined pattern. In TDM
circuit switching, each host gets the same slot in a revolving TDM frame.
9.At time t0 the sending host begins to transmit. At time t1 = L/R1, the
sending host completes transmission and the entire packet is received at
the router (no propagation delay). Because the router has the entire
packet at time t1, it can begin to transmit the packet to the receiving
host at time t1. At time t2 = t1 + L/R2, the router completes
transmission and the entire packet is received at the receiving host
(again, no propagation delay).
Thus, the end-to-end delay is L/R1 + L/R2.
10.In a VC network, each packet switch in the network core maintains
connection state information for each VC passing through it. Some of
this connection state information is maintained to a VC-number
translation table.The forwarding table in the switch need to be modified
at the same rate of one per millisecond.
11.The cons of VCs include (i) the need to have a signaling protocol to
set-up and tear-down the VCs; (ii) the need to maintain connection state
in the packet switches. For the pros, some researchers and engineers
argue that it is easier to provide QoS services - such as services that
guarantee a minimum transmission rate or services that guarantee
maximum end-to-end packet delay – when VCs are used.
14.A POP is a group of one or more routers in an ISPs network at which
routers in other ISPs can connect. NAPs are localized networks at which
many ISPs (tier-1, tier-2 and lower-tier ISPs) can interconnect.
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16.Ethernet LANs have transmission rates of 10 Mbps, 100 Mbps, 1
Gbps and 10 Gbps. For an X Mbps Ethernet (where X = 10, 100, 1,000
or 10,000), a user can continuously transmit at the rate X Mbps if that
user is the only person sending data. If there are more than one active
user, then each user cannot continuously transmit at X Mbps.
19.The delay components are processing delays, transmission delays,
propagation delays, and queuing delays. All of these delays are fixed,
except for the queuing delays, which are variable.
20.Five generic tasks are error control, flow control, segmentation and
reassembly, multiplexing, and connection setup. Yes, these tasks can be
duplicated at different layers. For example, error control is often
provided at more than one layer.
21. The five layers in the Internet protocol stack are – from top to
bottom – the application layer, the transport layer, the network layer,
the link layer, and the physical layer. The principal responsibilities are
outlined in Section 1.7.1.
22.
a) application-layer message: data which an application wants to send
and passed onto the transport layer;
b) transport-layer segment: generated by the transport layer and
encapsulates application-layer message with transport layer header;
c) network-layer datagram: encapsulates transport-layer segment
with a network-layer header;
d) link-layer frame: encapsulates network-layer datagram with a link-
layer header.
23.Routers process layers 1 through 3. (This is a little bit of a white lie,
as modern routers sometimes act as firewalls or caching components,
and process layer four as well.) Link layer switches process layers 1
through 2. Hosts process all five layers.
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Problems
1.There is no single right answer to this question. Many protocols would
do the trick. Here's a simple answer below:
a) Messages from ATM machine to Server
Msg name purpose
-------- -------
HELO <userid> Let server know that there is a card in the ATM
machine
ATM card transmits user ID to Server
PASSWD <passwd> User enters PIN, which is sent to server
BALANCE User requests balance
WITHDRAWL <amount> User asks to withdraw money
BYE user all done
b) Messages from Server to ATM machine (display)
Msg name purpose
-------- -------
PASSWD Ask user for PIN (password)
OK last requested operation (PASSWD, WITHDRAWL) OK
ERR last requested operation (PASSWD, WITHDRAWL) in
ERROR
AMOUNT <amt> sent in response to BALANCE request
BYE user done, display welcome screen at ATM
c) Correct operation flow
client server
HELO (userid) --------------> (check if valid userid)
<------------- PASSWD
PASSWD <passwd> --------------> (check password)
<------------- OK (password is OK)
BALANCE -------------->
<------------- AMOUNT <amt>
WITHDRAWL <amt> --------------> check if enough $ to cover
withdrawl
<------------- OK
ATM dispenses $
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BYE -------------->
<------------- BYE
d) In situation when there's not enough money
client server
HELO (userid) --------------> (check if valid userid)
<------------- PASSWD
PASSWD <passwd> --------------> (check password)
<------------- OK (password is OK)
BALANCE -------------->
<------------- AMOUNT <amt>
WITHDRAWL <amt> --------------> check if enough $ to cover
withdrawl
<------------- ERR (not enough funds)
error msg displayed
no $ given out
BYE -------------->
<------------- BYE
2.
a) A circuit-switched network would be well suited to the application
described, because the application involves long sessions with
predictable smooth bandwidth requirements. Since the transmission
rate is known and not bursty, bandwidth can be reserved for each
application session circuit with no significant waste. In addition, we
need not worry greatly about the overhead costs of setting up and
tearing down a circuit connection, which are amortized over the lengthy
duration of a typical application session.
b) Given such generous link capacities, the network needs no congestion
control mechanism. In the worst (most potentially congested) case, all
the applications simultaneously transmit over one or more particular
network links. However, since each link offers sufficient bandwidth to
handle the sum of all of the applications' data rates, no congestion (very
little queueing) will occur.
5.
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