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COMPUTER NETWORK |
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UNIT-III |
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Lecture-1 |
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MAC Sublayer
[RGPV JUNE 2011] In the seven-layer OSI model of computer networking, media access control (MAC) data communication protocol is a sublayer of the data link layer (layer 2). The MAC sublayer provides
addressing and channel access control mechanisms that make it
possible for several terminals or network nodes to communicate
within a multiple access network that incorporates a shared
medium, e.g.Ethernet.
The hardware that implements the MAC is referred to as a media access controller. The MAC sublayer acts as an
interface between the logical link control (LLC) sublayer and the network's physical layer.
The MAC layer emulates a full-duplex logical communication channel in a
multi-point network. This channel may provide unicast, multicast orbroadcast communication service.
MAC Addressing (Media
Access Control address)
In a local area network (LAN) or other network, the MAC (Media Access
Control) address is your computer's unique hardware number.
In a local area network (LAN) or
other network, the MAC (Media Access Control) address is your computer's
unique hardware number. (On an Ethernet LAN, it's the same as your Ethernet address.) When
you're connected to the Internet from your computer (or host as the Internet protocol thinks of it), a
correspondence table relates your IP
address to your computer's
physical (MAC) address on the LAN. What Is a MAC Address?
The
MAC address is a unique value associated with a network adapter. MAC addresses are also known as hardware addresses or physical addresses. They
uniquely identify an adapter on a LAN. MAC addresses are 12-digit
hexadecimal numbers (48 bits in length). By convention, MAC addresses are
usually written in one of the following two formats: MM:MM:MM:SS:SS:SS MM-MM-MM-SS-SS-SS The
first half of a MAC address contains the ID number of the adapter
manufacturer. These IDs are regulated by an Internet standards body (see
sidebar). The second half of a MAC address represents the serial number assigned
to the adapter by the manufacturer. In the example, 00:A0:C9:14:C8:29 The prefix 00A0C9 indicates the
manufacturer is Intel Corporation. Why MAC Addresses?
Recall
that TCP/IP and other mainstream networking architectures generally adopt the OSI
model. In this model, network functionality
is subdivided into layers. MAC addresses function at the data link layer
(layer 2 in the OSI model). They allow computers to uniquely identify
themselves on a network at this relatively low level. MAC vs. IP Addressing
Whereas MAC addressing works at the data link layer, IP
addressing functions at the network layer (layer 3). It's a slight
oversimplification, but one can think of IP addressing as supporting the
software implementation and MAC addresses as supporting the hardware
implementation of the network stack. The MAC address generally remains fixed
and follows the network device, but the IP address changes as the network
device moves from one network to another. IP
networks maintain a mapping between the IP address of a device and its MAC
address. This mapping is known as the ARP
cache or ARP table. ARP, the Address
Resolution Protocol, supports the logic for obtaining this mapping
and keeping the cache up to date. DHCP also usually relies on MAC addresses
to manage the unique assignment of IP addresses to devices. Binary Exponential Back-off (BEB) Algorithm In a variety of computer networks, binary exponential backoff or truncated
binary exponential backoff refers
to an algorithm used to space out repeated retransmissions of the same block of data, often as part of network congestion avoidance. Examples are the
retransmission of frames in carrier sense
multiple access with collision avoidance (CSMA/CA) and carrier sense multiple
access with collision detection(CSMA/CD) networks, where this
algorithm is part of the channel access method used to send data on these
networks. In Ethernet networks, the algorithm is commonly
used to schedule retransmissions after collisions. The retransmission is
delayed by an amount of time derived from the slot time and the number of attempts to
retransmit. After c collisions, a random number of slot
times between 0 and 2c -
1 is chosen. For the first collision, each sender will wait 0 or 1 slot
times. After the second collision, the senders will wait anywhere from 0 to 3
slot times (inclusive). After the third collision, the
senders will wait anywhere from 0 to 7 slot times (inclusive), and so forth.
As the number of retransmission attempts increases, the number of
possibilities for delay increases exponentially. The 'truncated'
simply means that after a certain number of increases, the exponentiation
stops; i.e. the retransmission timeout reaches a ceiling, and thereafter does
not increase any further. For example, if the ceiling is set at i = 10 (as it is in the IEEE 802.3 CSMA/CD standard),
then the maximum delay is 1023 slot times. Because these delays
cause other stations that are sending to collide as well, there is a
possibility that, on a busy network, hundreds of people may be caught in a
single collision set. Because of this possibility, the process is aborted after
16 attempts at transmission. |
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Lecture-2 |
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Distributed Random
Access Schemes/Contention Schemes: for Data Services (ALOHA and Slotted
ALOHA) ALOHA: ALOHA is a system for coordinating and arbitrating access
to a shared communication Networks channel. It was developed in the 1970s by
Norman Abramson and his colleagues at the University of Hawaii. The original
system used for ground based radio broadcasting, but
the system has been implemented in satellite communication systems. A shared communication
system like ALOHA requires a method of handling collisions that occur when
two or more systems attempt to transmit on the channel at the same time. In
the ALOHA system, a node transmits whenever data is available to send. If
another node transmits at the same time, a collision occurs, and the frames
that were transmitted are lost. However, a node can listen to broadcasts on
the medium, even its own, and determine whether the frames were transmitted. Aloha means "Hello". Aloha is a multiple access protocol at
the datalink layer and proposes how multiple terminals access the medium without
interference or collision. In 1972 Roberts developed a protocol that would
increase the capacity of aloha two fold. The Slotted
Aloha protocol involves dividing the time interval into discrete slots and
each slot interval corresponds to the time period of one frame. This method
requires synchronization between the sending nodes to prevent collisions. There are two different versior.s/types of ALOHA: (i) Pure ALOHA (i) Pure ALOHA • In pure ALOHA, the stations
transmit frames whenever they have data to send. • When two or more stations transmit
simultaneously, there is collision and the frames are destroyed. • In pure ALOHA, whenever any station
transmits a frame, it expects the acknowledgement from the receiver. • If acknowledgement is not received within
specified time, the station assumes that the frame (or acknowledgement) has
been destroyed. • If the frame is destroyed because of
collision the station waits for a random amount of time and sends it again.
This waiting time must be random otherwise same frames will collide again and
again. • Therefore pure
ALOHA dictates that when time-out period passes, each station must wait for a
random amount of time before resending its frame. This randomness will help
avoid more collisions. • Figure shows an example of frame
collisions in pure ALOHA.
In fig there are four stations that .contended with one another for access to shared
channel. All these stations are transmitting frames. Some of these frames
collide because multiple frames are in contention for the shared channel.
Only two frames, frame 1.1 and frame 2.2 survive. All other frames are
destroyed. • Whenever two frames try to occupy the
channel at the same time, there will be a collision and both will be damaged.
If first bit of a new frame overlaps with just the last bit of a frame almost
finished, both frames will be totally destroyed and both will have to be
retransmitted. (ii) Slotted ALOHA • Slotted ALOHA was invented to
improve the efficiency of pure ALOHA as chances of collision in pure ALOHA
are very high. • In slotted ALOHA, the time of the
shared channel is divided into discrete intervals called slots. • The stations can send a frame only at
the beginning of the slot and only one frame is sent in each slot.
In slotted ALOHA, if any station is not
able to place the frame onto the channel at the beginning of the slot i.e. it misses the time slot then the
station has to wait until the beginning of the next time slot. • In slotted ALOHA, there is still a
possibility of collision if two stations try to send at the beginning of the
same time slot as shown in fig. • Slotted ALOHA still has an edge over
pure ALOHA as chances of collision are reduced to one-half. |
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Lecture-3 |
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For Local-Area Networks (CSMA,
CSMA/CD, CSMA/CA) [RGPV DEC 2012] Carrier sense multiple access (CSMA) is a probabilistic media access control (MAC) protocol in which a node
verifies the absence of other traffic before transmitting on a shared transmission medium, such as an electrical
bus, or a band of the electromagnetic spectrum. Carrier sense means that a transmitter uses feedback from a receiver to determine whether
another transmission is in progress before initiating a transmission. That
is, it tries to detect the presence of a carrier wave from another station before
attempting to transmit. If a carrier is sensed, the station waits for the
transmission in progress to finish before initiating its own transmission. In
other words, CSMA is based on the principle "sense before transmit"
or "listen before talk". Multiple access means that multiple stations send
and receive on the medium. Transmissions by one node are generally received
by all other stations connected to the medium. CSMA/CA
(Carrier Sense Multiple Access/Collision Avoidance) is a protocolfor carrier transmission in 802.11 networks. Unlike CSMA/CD (Carrier Sense Multiple
Access/Collision Detect) which deals with transmissions after a collision has
occurred, CSMA/CA acts to prevent collisions before they happen. CSMA/CD - Carrier Sense Multiple Access / Collision Detection Short for Carrier Sense Multiple Access / Collision Detection, a set of rules determining how network devices respond when two devices attempt to
use a data channel simultaneously (called acollision).
Standard Ethernet networks use CSMA/CD to
physically monitor the traffic on the line at participating stations. If no
transmission is taking place at the time, the particular station can
transmit. If two stations attempt to transmit simultaneously, this causes a
collision, which is detected by all participating stations. After a random time interval, the stations that collided attempt to
transmit again. If another collision occurs, the time intervals from which
the random waiting time is selected are increased step by step. This is known
as exponential back off. CSMA/CD is a type of contention protocol. Networks using the
CSMA/CD procedure are simple to implement but do not have deterministic
transmission characteristics. The CSMA/CD method is internationally
standardized inIEEE 802.3 and ISO 8802.3. CSMA/CA - Carrier Sense Multiple Access/Collision Avoidance Short for Carrier Sense Multiple Access/Collision Avoidance,
network contention protocol that listens to a network in order to avoid
collisions, unlike CSMA/CD that deals with network transmissions once
collisions have been detected. CSMA/CA contributes to network traffic
because, before any real data is transmitted, it has to broadcast a signal
onto the network in order to listen for collision scenarios and to tell other
devices not to broadcast. What is CSMA/CA? The
Carrier-Sense Multiple Access/Collision Avoidance (CSMA/CA)
access method, as the name indicates, has several characteristics in common
with CSMA/CD. The difference is in the last of the three components: Instead
of detecting data collisions, the CSMA/CA
method attempts to avoid them altogether. Although it sounds good in
theory, the method it uses to do this causes some problems of its own, which
is one reason CSMA/CA is a far less popular access method than CSMA/CD. How CSMA/CA works? On a network that uses the CSMA/CA
access method, when a computer has data to transmit, its NIC first checks the
cable to determine if there is already data on the wire. So far, the process
is identical to CSMA/CD. However, if the NIC senses that the cable is not in
use, it still does not send its data packet. Instead, it sends a signal of
intent--indicating that it is about to transmit data--out onto the cable. |
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Lecture-4 |
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Collision Free Protocols: Basic Bit
Map, BRAP, Binary Count Down Collision Free Protocols A collision-free protocol for transmitting frames between stations
connected over a shared transmission medium such as an IEEE 802.3 Ethernet
LAN. A logical ring is formed and a token is circulated among the connected
stations part of the logical ring (not all connected stations are required to
be part of the logical ring). Transmitting from any one station, part of the
logical ring, is permitted only while holding the token, therefore preventing
collisions. A collision-free protocol, over a standard Ethernet
infrastructure, becomes feasible, yet remains compatible with the standard
collision protocol, thus improving performances. Basic Bit Map This is
how the Basic Bit-Map Protocol works.
BRAP Backup
Route Aware Routing Program (BRAP) is a protocol that provides interdomain
routing. BRAP uses reverse paths and backup paths to ensure fast failure
recovery in networking systems. Binary Count Down One
problem with Basic Bit-Map Protocol is that the overhead is 1 bit per frame
per station. We can do better by using binary station addresses.
For
exapmle, if stations 0010, 0100, 1001, and 1010 are all trying to get the
channel, in the first bit time the four stations transmit 0, 0, 1, and 1,
respectively. These are ORed together resulting in a 1. Stations 0010 and
0100 see the 1 and know that a higher-numbered station is competing for the
channel, so they give up for the current round. Stations 1001 and 1010
continue. The next bit sent from both stations is 0, both continues. The next
bit is 1, so station 1001 gives up. The winner is 1010. This station
transmits its frame. Then a new bidding process begins. The channel
efficiency is now d/(d + ln N) |
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Lecture-5 |
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MLMA Limited Contention
Protocols: Adaptive Tree Walk Under
conditions of light load, contention is preferable due to its low delay. As
the load increases, contention becomes increasingly less attractive, because
the overload associated with channel arbitration becomes greater. Just the
reverse is true for contention - free protocols. At low load, they have high
delay, but as the load increases , the channel
efficiency improves rather than getting worse as it does for contention
protocols. Obviously it would
be better if one could combine the best properties of the contention and
contention - free protocols, that is, protocol which used contention at low
loads to provide low delay, but used a cotention-free technique at high load
to provide good channel efficiency. Such protocols do exist and are called
Limited contention protocols. It is obvious that the probablity of some
station aquiring the channel could only be increased by decreasing the amount
of competition. The limited contention protocols do exactly that. They first
divide the stations up into ( not necessarily
disjoint ) groups. Only the members of group 0 are permitted to compete for
slot 0. The competition for aquiring the slot within a group is contention
based. If one of the members of that group succeeds, it aquires the channel
and transmits a frame. If there is collision or no node of a particular group
wants to send then the members of the next group compete for the next slot.
The probablity of a particular node is set to a particular value ( optimum ). Adaptive Tree Walk
Protocol
The
following is the method of adaptive tree protocol. Initially all the nodes
are allowed to try to aquire the channel. If it is able to aquire the
channel, it sends its frame. If there is collision then the nodes are divided
into two equal groups and only one of these groups compete for slot 1. If one
of its member aquires the channel then the next slot is reserved for the
other group. On the other hand, if there is a collision then that group is
again subdivided and the same process is followed. This can be better
understood if the nodes are thought of as being organised in a binary tree . What is MLMA protocol? Multi-Level
Multi-Access (MLMA): The problem with BRAP is the delay when the channel is
lightly loaded. When there is no frame to be transmitted, the N-bit headers
just go on and on until a station inserts a 1 into its mini slot. On average,
the waiting time would be N=2. MLAM scheme [41] is nearly as efficient under
high channel load, but has shorter delay under low channel load. In MLAM, a
station wants to transmit a frame sends its identification in a particular
format. A group of 10 bits (called decade) is used to represent a digit of
the station number [48]. |
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Lecture-6 |
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URN Protocol Uniform resource name In computing, a uniform resource name (URN) is the historical name for a uniform resource identifier (URI) that uses the urn scheme.
A URI is a string of characters used
to identify a name of a web resource.
Such identification enables interaction with representations of the web
resource over a network, typically the World Wide Web,
using specific protocols. URNs were intended to serve as persistent, location-independent identifiers,
allowing the simple mapping ofnamespaces into a single URN namespace.[1] The existence of such a URI does not
imply availability of the identified resource, but such URIs are required to
remain globally unique and persistent, even when the resource ceases to exist
or becomes unavailable. Since RFC 3986[2] in 2005, the use of the term has
been deprecated in favor of the less-restrictive "URI", a view
proposed by a joint working group between the World Wide Web Consortium (W3C) and Internet Engineering Task Force (IETF). Both URNs and uniform resource locators (URLs) are URIs, and a particular
URI may be a name and a locator at the same time.URNs were originally
intended in the 1990s to be part of a three-part information architecture for the Internet, along with URLs
anduniform resource characteristics (URCs), a metadata framework. However, URCs never
progressed past the conceptual stage, and
other technologies such as the Resource Description Framework later took their place.
(Uniform Resource Name) A name that identifies a resource on the Internet.
Unlike URLs, which use network addresses (domain, directory path, file name),
URNs use regular words that are protocol and location independent. Providing
a higher level of abstraction, URNs are persistent (never change) and require
a resolution service similar to the DNS system in order to convert names into
real addresses. For the most part, URNs have evolved into XRI identifiers
(see XDI). See URI and URL. |
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Lecture-7 |
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High Speed LAN: Fast Ethernet, Gigabit Ethernet What is high speed LAN? Most
modern local networks use cables, adaptors and connecting devices that can
communicate at a maximum speed of 100Mbits/sec (quick enough - in theory - to
move an 8Mb file from one computer to another in 1 second). HCL VM -10 -
Very High Speed LAN Extender
HCL
VM -10 LAN Extender is a Long Reach Ethernet media converter with one
Ethernet port (RJ-45 connector) and one VDSL port (RJ-11 connector) This model
is a bridge mode modem, well accommodating VDSL2 (Very-high-data-rate Digital
Subscribe Loop) technologies to extend Ethernet service over single-pair
phone line. Supporting both symmetric and asymmetric transmission, it can
reach up to 100/75 Mbps bandwidth (line rate) within 300M or 10/10 Mbps (line
rate) for 1 Km long range connections. By providing ultra-high speed, HCL VM
-10 LAN Extender makes your telephone line achieve its best performance than
before. It has the advantage of minimum installation time (simply as
plug-n-play) and minimum expense by allowing video streaming and data to
share the same telephone pair without interference.
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Lecture-8 |
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FDDI [RGPV JUNE 2011] FDDI (Fiber
Distributed Data Interface) is a set of ANSI and ISO standards for data transmission on fiber
optic lines in a local
area network (LAN) that can extend in range up to 200 km (124 miles). The
FDDI protocol is based on the token
ring protocol. In
addition to being large geographically, an FDDI local area network can
support thousands of users. FDDI is frequently used on the backbone for a wide area network (WAN). Fiber
Distributed Data Interface (FDDI)
is a standard for data transmission in a local area network. It uses optical
fiber as its standard
underlying physical medium, although it was also later specified to use copper cable, in which case it may be called CDDI (Copper Distributed Data Interface),
standardized as TP-PMD (Twisted-Pair Physical
Medium-Dependent), also referred to as TP-DDI (Twisted-Pair Distributed Data
Interface). Performance Measuring Metrics Network Performance Evaluation
For a given network, one might be
interested to know how well it is performing. One might also wish to know
what could be done to further improve the performance, or if the network is
giving the peak performance. Thus, one needs to do a comparative study of the
network by considering different options. This performance evaluation helps the
user to determine the suitable network configuration that serves him best. For example
consider a new startup organization which has setup its own web portal. As
the portal gradually becomes popular then network traffic increases which
would degrade its performance. Therefore, one should have a well configured
network with proper load balancing capabilities. Performance Evaluation Metrics
Before we can proceed with performance
evaluation, we must choose the different metrics that would help us in making
comparisons. There could be different metrics to determine the performance
like throughput, delay, jitter, packet loss. The choice of metric would
depend upon the purpose the network has been setup for. The metrics could be
related to the different layers of the network stack. For example, TCP
throughput is based on the application layer, whereas IP round trip time is
based on the network layer. For example, a network supporting multimedia
applications should have minimum delay and jitter.Packet
loss might not be a critical issue for such network. However, packet loss
might be a considerable factor for networks supporting textual data oriented applications, say someone downloading by
FTP. Once the metrics have been chosen, one goes
for their quantitative evaluation by subjecting the network under diverse
conditions. For example, one could make step by step increments in bandwidth
of the links, which in turn improve the throughput. However, the throughput
might get saturated beyond the certain point. That is, further increase in
bandwidth would not improve throughput. Thus, the optimum value of bandwidth
has been determined. The table below shows different metrics of evaluation,
and categories
·
It might not be always possible or feasible to
obtain best performance from a network due to various factors like high cost,complexity, compatibility. In such cases one would
like to obtain optimum performance by balancing different factors. Following are some of the performance measurement metrics:
Parameters Affecting the Performance of a Network
·
Different parameters can together or
independently determine how well a network would perform. A few such are
mentioned below:
Performance Evaluation Techniques
·
Before starting with tuning the performance of
a network one must remember that the performance, to some extent, depends on
the workload as well as the topology. A given topology might give different
throughputs under CBR and exponential traffic. Keeping this in mind, one can
go for studying an actual network. Otherwise one can simulate its performance
using suitable parameters. these simulations would largely depend on queuing
theory. Network Performance Evaluation using NS2
·
In this section we discuss how to evaluate
performance of a network by simulating it with ns2.
·
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Lecture-9 |
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IEEE
Standards 802 series & their variant
IEEE 802 refers to a family of IEEE standards dealing with local area networks and metropolitan area networks. More specifically, the IEEE
802 standards are restricted to networks carrying variable-size packets. (By
contrast, in cell relay networks data is transmitted in
short, uniformly sized units called cells. Isochronous networks,
where data is transmitted as a steady stream of octets, or groups of octets,
at regular time intervals, are also out of the scope of this standard.) The
number 802 was simply the next free number IEEE could assign,[1] though “802” is sometimes associated
with the date the first meeting was held — February 1980. The services and protocols
specified in IEEE 802 map to the lower two layers (Data Link and Physical) of
the seven-layer OSI networking reference model. In fact,
IEEE 802 splits the OSI Data Link Layer into two sub-layers named Logical Link Control (LLC) and Media Access Control (MAC), so that the layers can be
listed like this: The IEEE 802 family of
standards is maintained by the IEEE 802 LAN/MAN Standards Committee (LMSC).
The most widely used standards are for the Ethernet family, Token Ring,
Wireless LAN, Bridging and Virtual Bridged LANs. An individual Working Group provides the focus for each area. The IEEE
802.x Standard
The bottom two
layers of the OSI reference model pertain to hardware: the NIC and the
network cabling. To further refine the requirements for hardware that operate
within these layers, the Institute of Electrical and Electronics Engineers
(IEEE) has developed enhancements specific to different NICs and cabling.
Collectively, these refinements are known as the 802 project.
This lesson describes these enhancements and how they relate to OSI. The
802 Project Model
When
local area networks (LANs) first began to emerge as potential business tools
in the late 1970s, the IEEE realized that there was a need to define certain
LAN standards. To accomplish this task, the IEEE launched what became known
as Project 802, named for the year and month it began (1980, February). Although the published
IEEE 802 standards actually predated the ISO standards, both were in
development at roughly the same time, and both shared information that
resulted in the creation of two compatible models. Project 802 defined
network standards for the physical components of a network (the interface
card and the cabling) that are accounted for in the physical and data-link
layers of the OSI reference model. The 802
specifications set
standards for:
The 802 specifications define the ways NICs access and
transfer data over physical media. These include connecting, maintaining, and
disconnecting network devices. |
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|
RGPV PAPER QUESTIONS |
|
S.NO |
Questions |
Year |
Marks |
|
1 |
Define FDDI? |
June 2011 |
10 |
|
2 |
Explain MAC Sublayer? |
June 2013 |
10 |
|
3 |
Explain ALOHA Protocol? |
June 2014 |
7 |
|
4 |
Explain Contention Protocol? |
June 2012 |
10 |
|
5 |
How does CSMA/CD different from CSMA/CA? |
June 2013 |
10 |