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Chapter 12
Modern Computer Systems
12.1. (BL2-) The dedicated path used with circuit switching assures that the packets will arrive in the
correct sequence, with no delay except propagation delay, therefore it is easy to check for errors
and the receipt of complete messages. However, the switches along the path cannot accept packets
from other sources during the period when they are dedicated to the particular circuit, which limits
the capacity of the channel. Although the path is consistent in virtual channel switching, the
switches may also form connections of other virtual circuits, which means that packets may arrive
from a number of different sources. Bursts of traffic from other sources can delay the message, but
other advantages of circuit switching are preserved. Message switching is even less efficient than
circuit switching, because a message may be delayed until a channel is available. A channel is tied
up for the entire length of a message. Packet switching creates an independent path for each
packet. Packet switching allows maximum utilization of a network, since any available switch can
be used to move a packet. This allows the balancing of traffic across the network. However,
individual packets may be routed in different directions of substantially different time delay, which
requires that the receiving node be capable of reconstructing messages from packets received out
of sequence. The network must also be designed to prevent loops: situations in which a packet is
routed through the same switch more than once.
12.2. (BL1+) Circuit switches are dedicated to a single circuit for the length of transmission. Virtual
circuits create the appearance of a dedicated circuit, but in reality, the switches may form parts of
several different circuits, each with their own routes. The switches in a virtual circuit will direct
packets from each source to their appropriate destination consistent with the virtual circuit for those
packets.
12.3 (BL2) Both virtual circuits and packet switching pass a message in the form of packets through a
network. The primary difference is that all of the packets in a virtual circuit follow the same path;
with packet switching, the path of each packet is independent of the path of other packets in the
message.
12.4 (BL2) There are lots of possible answers for this exercise. Here are just a few:
Between its various locations:
Allocating orders for manufacture between different locations (improving assembly line efficiency)
Coordinating shipments from different nearby locations to particular locations or dealers (cut costs)
Coordinating ordering and inventory sharing needs (cut costs, minimize excess inventory)
Distributing personnel to locations where they are needed (effective use of personnel)
Video conferencing between managers at different locations (cut costs, improve communication)
Sharing and tallying marketing data and sales data (improve marketing)
Between iself and suppliers:
Automate parts ordering (minimize inventory, reduce costs)
Collaborate on new designs (improve products, reduce costs, improve communication)
Coordinate shipping (improve delivery control, minimize production delays, reduce errors)
Process billing (Cut costs, control cash flow)
Between itself and dealers:
Automate order processing (reduce delivery time, control costs, improve customer relations)
Process billing (Cut costs, minimize billing problems)
Special repair parts ordering by dealers (improve customer service)
Video conferencing with dealers for marketing
Dealer feedback on service problems (product imrpovment)
12.5. (BL2-) The "user-friendly" address is one that can be easily recognized and remembered by a user,
in contrast, for example, to an IP address, which is typically hard to remember. The "user-friendly"
address appears at the application level, and is used to specify a destination address for a message.
Examples include the URLs used with HTTP and the email @ format used by SMTP and other email protocols. The "user-friendly" address is typically translated by the DNS application into an IP
address.
The port number serves as an address to identify the applications sending and receiving messages.
At the sending end, the port number identifies the application originating the message to the
transport layer; it can be used by the transport layer to identify the corresponding application at the
destination address. (In some cases, a different destination port may be specified as part of the
URL.) It can also be used by the transport layer to send responses to the originating application.
IP addresses are unique throughout the Internet; their locations are catalogued and readily
identified. Therefore, IP addresses serve as the primary addresses that identify the source and
destination nodes, as well as for routing through the network from source to destination. IP
addressing is generated either directly, at the application level, or by use of the Domain Name
Service, directed from the transport level. IP addresses are used predominantly at the network level.
MAC or hardware addresses are provided with any NIC or other component that may be connected
to a network. They are used at the data link and physical layers. IP addresses are converted by
router tables to MAC addresses, using ARP, the address resolution protocol in the network layer.
12.6 (BL2) On the computer side, the NIC arranges and manages DMA transfers between the network
interface and memory; it also provides an interrupt capability to notify the CPU when the network
requires attention. Network attention would include network requests and network connection
issues, among other things. On the network side, the NIC manages the interface between the
network and the computer system. Management includes network connection, implementation of
the physical layer of the network protocols, and more.
12.7
(BL2) A short-circuit failure in an NIU is potentially serious, because it grounds out the entire
bus, making all communication over the network impossible. Worse, this type of failure is difficult
to find. If the bus is divided into different physical parts, it may be possible to isolate the part of
the bus that contains the defective junction. Otherwise, it is necessary to remove junctions from
the bus, one NIU at a time, until the defective station is identified. An open-circuit failure has no
effect on the bus or on any station except for the station with the defective NIU.
12.8
(BL2) This company would be well-served by a network that links the buildings with microwave
communication. Microwave communication is relatively inexpensive and would be capable of
high capacity communication, if necessary. Alternative methods would be to use leased telephone
communications between the buildings or to use local wireless communication technology.
12.9 (BL2+) Packages and mail represent "messages"; the various departments correspond to the
TCP/IP applications. These "messages" are passed to their building mailrooms (transport centers)
for delivery to the appropriate destinations.
At the "transport" center, the department and building addresses of the packages are identified,
perhaps on a label similar to the transport layer header. Packages remaining in the building are
simply passed on for delivery. This is similar to passing the packets to the IP layer and then to the
local area network for delivery. The department address will serve as the application that will
ultimately receive the package.
Other packages are passed to the OPS delivery service, corresponding to the network layer. These
packages will be routed through a district office, then rerouted through one of several air shipping
facilities. Each of these facilities "readdresses" the package to the next shipping point, until it
reaches the appropriate district office, and then the mailroom of the destination. The mailroom
identifies the department for which it is destined, and the package is delivered to the recipient.
12.10(BL2-) The datagram enters an intermediate node at the physical layer and is passed to the data link
layer. The data link layer strips the data link layer preamble and trailer and checks the frame for
errors. The frame is next passed to the network layer. The network layer determines the address of
the next node, and encapsulates the remainder of the frame with new addressing data for the next
node. It then passes the frame back to the data link layer, which reformats the frame suitably for the
next node, and creates new error checking data. The frame is then passed to the physical layer for
transmission to the next node.
12.11(BL2) In many cases it is possible. Each protocol to be recognized is identified by a unique
characteristic of the packet, for example, a preamble synchronizing sequence that forms a bit
pattern unique to one protocol or the other, or a field that contains a piece of data such as an address
or length which is positioned differently in each packet. Two protocols that are not separable in this
way can not be used in the same network.
12.12(BL2) TCP/IP applications are tailored for each system to include any conversions that are required
for presentation and control the dialog between systems that would normally be provided in the
session layer. Therefore, these functions are not required as separate layers when TCP/IP
applications are used. The establishment of a connection between systems, which is normally a
session layer function, is handled as part of TCP or UDP services.
12.13.(BL2-) The logical topology of a network is the representative of the connectivity of the network;
most network topology diagrams represent the logical topology. The physical topology of a
network is the actual physical layout of the wiring that connects the network components together.
As an example, the logical topology of a wired hub network or a WiFi network is a bus topology,
regardless of where the components are located, because the components are all effectively tied to
the same medium and share that medium. This, despite the fact that the physical appearance of the
hub is that of a star. Conversely, the logical topology of a network in which each node is connected
to a central switch is a star, even if the connections are all bundled together in a single cable wired
along the wall of a room.
12.15. (BL2) IP is described as a “best-effort delivery service” because IP attempts to deliver packets, but
does not guarantee that they will be delivered. IP datagrams may “time out” because of delays in
the network, because of poor routing, because a packet is defective or corrupted, because a
destination has logged off, or many other reasons. Because of different routings, IP packets may
also at the destination in the wrong order, Resolution of the problem occurs at the transport level.
If UDP was the transport mechanism, the communication is connectionless; therefore, the
destination UDP peer will correct errors in the packet, if possible. Otherwise, the packet is simply
discarded. Neither the sending node, nor the receiving node will be aware of UDP packets that do
not arrive at the destination node, since UDP does not provide communication between the nodes.
On the other hand, TCP sets up a connection between the source and destination, with a response
mechanism from the destination to the source that indicates successful receipt. The response
mechanism will request resending of missing or corrupted packets. The receiving node will also
resort packets that arrive in the wrong order.
12.16. (BL2) A connectionless communication does not establish communication between the source and
destination nodes. The source simply sends packets to the destination. No advance notice is given
to the destination node. The destination makes the decision as to how to use and manage the
packets that it receives. UDP is a connectionless protocol.
A connection-oriented communication establishes an initial communication between the source and
destination nodes that provides information to the destination that will be used in processing the
packets at the destination, as well as a response mechanism that assures reliable communication of
the message. TCP is a connection-oriented protocol. The details of the TCP setup mechanism that
establishes a connection are laid out in Chapter 13.
12.17 (BL2)
a. You will need to know how many computers your aunt will connect to the network, and where
they will be located within the home.. This will allow you to determine the capacity of the router
and the wiring needs of the system. It will also determine the need for WiFi support. You will need
to know her usage requirements; an estimate of the amount of data will be transported between
computers and between the network and the Internet service. This will help you to establish the
minimum capacity of the Internet "pipe". It will also determine what level of WiFi, if any, is
required. You will need to know what data services are available to provide Internet access to her
home. This will determine the type of router necessary to connect to the ISP and any outside vendor
connection that will need to be arranged.
b. The critical components that you must specify are the network switch and router (probably a
combined unit), the modem or codec that connects to the ISP, the WiFi cards, if any, that will be
needed for the networked computers, and any special items related to wiring and computer
connectivity, such as cable or WiFi extenders.
12.18 (BL2) Each input and output port on a router must have a separate MAC address because packets
are routed based on the particular MAC addresses that they pass through, and the relationship
between each port and its corresponding IP address.
12.19 (BL1+) Corresponding layers of the TCP/IP communication model are peers. Information
encapsulated at a particular layer of the source will be used by the corresponding layer at the
destination. For the application and transport layers, the corresponding nodes are the source host
and the final destination host; for the lower layers, the source and destination respresent the nodes
at each end of the current link.
12.20 (BL2) The birds represent the lower three layers of the TCP/IP model. A human application copies
the message into paper packets, and determines the final destination address of the packets
(transport layer) (S)he also numbers the packets, in case they should arrive out of order or be lost
along the way. (As the specification and corresponding test indicate, this is a fairly common
occurrence.) Packets are strapped to a bird's leg (network layer), and the bird heads for the next
node (data link layer and physical layer), where the packets are removed, from the bird's leg, read,
and, if necessary, readdressed (by strapping the packet to the leg of another bird) and forwarded to
another node. When the birds arrive at the final destination, the packets are removed, sorted in the
correct order, and delivered to the recipient. If necessary, a packet can be returned with the bird to
indicate the need for retransmission.
12.21 (BL3) Like TCP/IP, IBM System Network Architecture (SNA) is a layered protocol suite.
Functionally, the layers are relatively similar to those of TCP/IP or ISO/OSI. The following is a
diagram showing the rough equivalence of SNA and TCP/IP:
transaction
services
application
transport
network
presentation
services
data flow control
Transmission
control
path control
data link
data link control
physical
physical
Although the actual protocols are different, SNA transaction services are similar to TCP/IP
applications. Presentation services perform data conversions. Presentation services are similar to
the presentation layer of the OSI model, Transmission control provides reliable end-to-end control,
similar to that of the TCP/IP transport layer. SNA data flow control manages the request and
response aspect of the TCP/IP transport control mechanism. The SNA path control layer is
responsible for routing and other functions; it is similar to the TCP/IP network layer. The data link
control and physical layers are similar in both suites, although the SNA data link control is limited
primarily to a hierarchical routing method known as SDLC, for synchronous data link control, to
x.25, and to the various IBM mainframe I/O channels, described briefly in the text on pages 257358, and in a supplementary chapter describing the IBM zSeries. x.25 was originally designed for
connections to terminals and other equipment via telephone and ISDN lines.
(There are many references that explain SNA. One reasonable on-line reference can be found at
http://www.cisco.com/en/US/docs/internetworking/technology/handbook/IBM-SNAProtocols.html.)
12.22 (BL2) This is a laboratory exercise.
12.23 (BL1+)
The formula predicts that there should be (6 x 5) / 2 = 15 connections. A fifty-node full mesh network
would require (50 x 49) / 2 = 1225 connections.