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1. Identify and describe the functions of each of the seven layers of the OSI re
ference model.
Physical Layer
The physical layer defines the electrical, mechanical, procedural, and functiona
l specifications for activating, maintaining, and deactivating the physical link
between communicating network systems. Physical layer specifications define suc
h characteristics as voltage levels, timing of voltage changes, physical data ra
tes, maximum transmission distances, and the physical connectors to be used.
Data Link Layer
The data link layer provides reliable transit of data across a physical network
link. Different data link layer specifications define different network and prot
ocol characteristics, including the following:
Physical addressing -- Physical addressing (as opposed to network addressing) de
fines how devices are addressed at the data link layer.
Network topology -- Data link layer specifications often define how devices are
to be physically connected (such as in a bus or a ring topology).
Error notification -- Error notification involves alerting upper layer protocols
that a transmission error has occurred.
Sequencing of frames -- Sequencing of data frames involves the reordering of fra
mes that are transmitted out of sequence.
Flow control -- Flow control involves moderating the transmission of data so tha
t the receiving device is not overwhelmed with more traffic than it can handle a
t one time.
The Institute of Electrical and Electronics Engineers (IEEE) has subdivided the
data link layer into two sublayers: Logical Link Control (LLC) and Media Access
Control (MAC).
Network Layer
The network layer provides routing and related functions that allow multiple dat
a links to be combined into an internetwork. This is accomplished by the logical
addressing (as opposed to the physical addressing) of devices. The network laye
r supports both connection-oriented and connectionless service from higher-layer
protocols.
Transport Layer
The transport layer implements reliable internetwork data transport services tha
t are transparent to upper layers. Transport layer functions typically include t
he following:
Flow control -- Flow control manages data transmission between devices so that t
he transmitting device does not send more data than the receiving device can pro
cess.
Multiplexing -- Multiplexing allows data from several applications to be transmi
tted onto a single physical link.
Virtual circuit management -- Virtual circuits are established, maintained, and
terminated by the transport layer.
Error checking and recovery -- Error checking involves various mechanisms for de
tecting transmission errors. Error recovery involves taking an action (such as r
equesting that data be retransmitted) to resolve any errors that occur.
Some examples of transport layer implementations follow:
Transmission Control Protocol (TCP), Name Binding Protocol (NBP), OSI transport
protocols
Session Layer
The session layer establishes, manages, and terminates communication sessions be
tween presentation layer entities. Communication sessions consist of service req
uests and service responses that occur between applications located in different
network devices. These requests and responses are coordinated by protocols impl
emented at the session layer. Some examples of session layer implementations fol
low:
Apple ZIP, DEC SCP, NFS, SQL, RPC, X Windows, ASP
Presentation Layer
The presentation layer provides a variety of coding and conversion functions tha
t are applied to application layer data. These functions ensure that information
sent from the application layer of one system will be readable by the applicati
on layer of another system. Some examples of presentation layer coding and conve
rsion schemes follow:
Common data representation formats -- The use of standard image, sound, and vide
o formats allow the interchange of application data between different types of c
omputer systems.
Conversion of character representation formats -- Conversion schemes are used to
exchange information with systems using different text and data representations
(such as EBCDIC and ASCII).
Common data compression schemes -- The use of standard data compression schemes
allows data that is compressed at the source device to be properly decompressed
at the destination.
Common data encryption schemes -- The use of standard data encryption schemes al
lows data encrypted at the source device to be properly unencrypted at the desti
nation.
Presentation layer implementations are not typically associated with a particula
r protocol stack. Some well known standards follow:
Data: ASCII, EBCDIC, Encryption
Visual Imaging: PICT, TIFF, GIF, JPEG
Video: MIDI, MPEG, QuickTime
Application Layer
The application layer interacts with software applications that implement a comm
unicating component. Application layer functions typically include the following
:
Identifying communication partners -- The application layer identifies and deter
mines the availability of communication partners for an application with data to
transmit.
Determining resource availability -- The application layer must determine whethe
r sufficient network resources for the requested communication are available.
Synchronizing communication -- Communication between applications requires coope
ration that is managed by the application layer.
The application layer is the OSI layer closest to the end user. That is, both th
e OSI application layer and the user interact directly with the software applica
tion. Some examples of application layer implementations follow:
TCP/IP applications -- TCP/IP applications are protocols in the Internet Protoco
l suite, such as Telnet, File Transfer Protocol (FTP), and Simple Mail Transfer
Protocol (SMTP).
OSI applications -- OSI applications are protocols in the OSI suite such as File
Transfer, Access, and Management (FTAM), Virtual Terminal Protocol (VTP), and C
ommon Management Information Protocol (CMIP).
2. Describe connection-oriented network service and connectionless network servi
ce and identify the key differences between them.
Connection-Oriented Network Service
Connection-oriented service involves three phases:
Connection establishment -- During the connection establishment phase, a single
path between the source and destination systems is determined. Network resources
are typically reserved at this time to ensure a consistent grade of service (su
ch as a guaranteed throughput rate).
Data transfer -- During the data transfer phase, data is transmitted sequentiall
y over the path that has been established. Data always arrives at the destinatio
n system in the order in which it was sent.
Connection termination -- During the connection termination phase, an establishe
d connection that is no longer needed is terminated. Further communication betwe
en the source and destination systems requires that a new connection be establis
hed.
Connection-oriented service has two significant disadvantages as compared to con
nectionless network service:
Static path selection -- Because all traffic must travel along the same static p
ath, a failure anywhere along that path causes the connection to fail.
Static reservation of network resources -- A guaranteed rate of throughput requi
res the commitment of resources that cannot be shared by other network users. Un
less full, uninterrupted throughput is required for the communication, bandwidth
is not used efficiently.
Connection-oriented services are useful for transmitting data from applications
that are intolerant of delays and packet re-sequencing. Voice and video applicat
ions are typically based on connection-oriented services.
Connectionless Network Service
Connectionless network service does not predetermine the path from the source to
the destination system, nor are packet sequencing, data throughput, and other n
etwork resources guaranteed. Each packet must be completely addressed because di
fferent paths through the network might be selected for different packets, based
on a variety of influences. Each packet is transmitted independently by the sou
rce system and is handled independently by intermediate network devices. Connect
ionless service offers two important advantages over connection-oriented service
:
Dynamic path selection -- Because paths are selected on a packet-by-packet basis
, traffic can be routed around network failures.
Dynamic bandwidth allocation -- Bandwidth is used more efficiently because netwo
rk resources are not allocated bandwidth that they are not going to use.
Connectionless services are useful for transmitting data from applications that
can tolerate some delay and re-sequencing. Data-based applications are typically
based on connectionless service.
3. Describe data link addresses and network addresses and identify the key diffe
rences between them.
Data Link Layer Addresses
A data link layer address uniquely identifies each physical network connection o
f a network device. Data link addresses are sometimes referred to as physical or
hardware addresses. Data link addresses usually exist within a flat address spa
ce and have a pre-established and typically fixed relationship to a specific dev
ice. End systems typically have only one physical network connection, and thus h
ave only one data link address. Routers and other internetworking devices typica
lly have multiple physical network connections. They therefore have multiple dat
a link addresses.
Network Layer Addresses
A network layer address identifies an entity at the network layer of the OSI ref
erence model. Network addresses usually exist within a hierarchical address spac
e. They are sometimes called virtual or logical addresses. The relationship of a
network address with a device is logical and unfixed. It is typically based eit
her on physical network characteristics (the device is on a particular network s
egment) or on groupings that have no physical basis (the device is part of an Ap
pleTalk zone). End systems require one network layer address for each network la
yer protocol they support. (This assumes that the device has only one physical n
etwork connection.) Routers and other internetworking devices require one networ
k layer address per physical network connection for each network layer protocol
supported. For example, a router with three interfaces, each running AppleTalk,
TCP/IP, and OSI, must have three network layer addresses for each interface. The
router therefore has nine network layer addresses.
4. Define and describe the function of a MAC address.
Media Access Control (MAC) addresses are a subset of data link layer addresses.
MAC addresses identify network entities in LANs implementing the IEEE MAC sublay
er of the data link layer. Like most data link addresses, MAC addresses are uniq
ue for each LAN interface. MAC addresses are 48 bits in length and are expressed
as 12 hexadecimal digits: The first 6 hexadecimal digits are the manufacturer i
dentification (or vendor code), called the Organizational Unique Identifier (OUI
). These 6 digits are administered by the IEEE. The last 6 hexadecimal digits ar
e the interface serial number or another value administered by the specific vend
or. MAC addresses are sometimes called burned-in addresses (BIAs) because they a
re burned into read-only memory (ROM) and copied into random-access memory (RAM)
when the interface card initializes.
5. Define flow control and describe the three basic methods used in networking.
Flow control is a function that prevents network congestion by ensuring that tra
nsmitting devices do not overwhelm receiving devices with data. There are a numb
er of possible causes of network congestion. For example, a high-speed computer
might generate traffic faster than the network can transfer it, or faster than t
he destination device can receive and process it. There are three commonly used
methods for handling network congestion:
Buffering - Buffering is used by network devices to temporarily store bursts of
excess data in memory until they can be processed. Occasional data bursts are ea
sily handled by buffering. However, excess data bursts can exhaust memory, forci
ng the device to discard any additional datagrams that arrive.
Source quench messages - Source quench messages are used by receiving devices to
help prevent their buffers from overflowing. The receiving device sends source
quench messages to request that the source reduce its current rate of data trans
mission, as follows:
1. The receiving device begins discarding received data due to overflowing buffe
rs.
2. The receiving device begins sending source quench messages to the transmittin
g device, at the rate of one message for each packet dropped.
3. The source device receives the source quench messages and lowers the data rat
e until it stops receiving the messages.
4. The source device then gradually increases the data rate as long as no furthe
r source quench requests are received.
Windowing - Windowing is a flow-control scheme in which the source device requir
es an acknowledgement from the destination after a certain number of packets hav
e been transmitted. With a window size of three, the source requires an acknowle
dgment after sending three packets, as follows:
1. The source device sends three packets to the destination device.
2. After receiving the three packets, the destination device sends an acknowledg
ment to the source.
3. The source receives the acknowledgment and sends three more packets.
4. If the destination does not receive one or more of the packets for some reaso
n (such as overflowing buffers), it does not receive enough packets to send an a
cknowledgment. The source, not receiving an acknowledgment, retransmits the pack
ets at a reduced transmission rate.
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