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Chapter: Computer Networks

Data Communication

1. Introduction to networks 2. Network architecture 3. OSI architecture 4. Network performance 5. Direct link networks 6. Encoding 7. Framing 8. Error detection 9. Transmission 10. Ethernet 11. Rings 12. Switched networks 13. Wireless networks 14. Bridges



1. Network Definition


     A network can be defined as two or more computers connected together in such a way that they can share resources.


     The purpose of a network is to share resources.


A resource may be:

     A file


     A folder


     A printer


     A disk drive

     Or just about anything else that exists on a computer.


     A network is simply a collection of computers or other hardware devices that are connected together, either physically or logically, using special hardware and software, to allow them to exchange information and cooperate. Networking is the term that describes the processes involved in designing, implementing, upgrading, managing and otherwise working with networks and network technologies


Advantages of networking.

     Connectivity and Communication


     Data Sharing


     Hardware Sharing


     Internet Access


     Internet Access Sharing


     Data Security and Management


     Performance Enhancement and Balancing




Layered system with alternative abstractions available at a given layer




o     Protocol defines the interfaces between the layers in the same system and with the layers of peer system

o     Building blocks of a network architecture


o     Each protocol object has two different interfaces


§    service interface: operations on this protocol


§    peer-to-peer interface: messages exchanged with peer

o     Term “protocol” is overloaded


§    specification of peer-to-peer interface


§    module that implements this interface




·         Protocol Specification: prose, pseudo-code, state transition diagram


·         Interoperable: when two or more protocols that implement the specification accurately


·         IETF: Internet Engineering Task Force



Description of Layers


Physical Layer


o     Handles the transmission of raw bits over a communication link


Data Link Layer


o     Collects a stream of bits into a larger aggregate called a frame


o     Network adaptor along with device driver in OS implement the protocol in this layer


o     Frames are actually delivered to hosts


Network Layer


o     Handles routing among nodes within a packet-switched network

o     Unit of data exchanged between nodes in this layer is called a packet


The lower three layers are implemented on all network nodes

Transport Layer


o     Implements a process-to-process channel


o     Unit of data exchanges in this layer is called a message


Session Layer


o     Provides a name space that is used to tie together the potentially different transport streams that are part of a single application


Presentation Layer

o     Concerned about the format of data exchanged between peers


Application Layer

o   Standardize common type of exchanges


The transport layer and the higher layers typically run only on end-hosts and not on the intermediate switches and routers



Internet Architecture

     Defined by IETF


     Three main features


o     Does not imply strict layering. The application is free to bypass the defined transport layers and to directly use IP or other underlying networks


o     An hour-glass shape – wide at the top, narrow in the middle and wide at the bottom. IP serves as the focal point for the architecture


o     In order for a new protocol to be officially included in the architecture, there needs to be both a protocol specification and at least one (and preferably two) representative implementations of the specification


                  Application Programming Interface


o     Interface exported by the network


o     Since most network protocols are implemented (those in the high protocol stack) in software and nearly all computer systems implement their network protocols as part of the operating system, when we refer to the interface “exported by the network”, we are generally referring to the interface that the OS provides to its networking subsystem


o     The interface is called the network Application Programming Interface (API)


o     Interface exported by the network


o     Since most network protocols are implemented (those in the high protocol stack) in software and nearly all computer systems implement their network protocols as part of the operating system, when we refer to the interface “exported by the network”, we are generally referring to the interface that the OS provides to its networking subsystem


o   The interface is called the network Application Programming Interface (API)

o     Socket Interface was originally provided by the Berkeley distribution of Unix

Now supported in virtually all operating systems


o     Each protocol provides a certain set of services, and the API provides a syntax by which those services can be invoked in this particular OS




Socket Family


o     PF_INET denotes the Internet family


o     PF_UNIX denotes the Unix pipe facility


o     PF_PACKET denotes direct access to the network interface (i.e., it bypasses the TCP/IP protocol stack)


o     Socket Type


o     SOCK_STREAM is used to denote a byte stream


o     SOCK_DGRAM is an alternative that denotes a message oriented service, such as that provided by UDP


Creating a Socket

int sockfd = socket(address_family, type, protocol);

·         The socket number returned is the socket descriptor for the newly created socket


·         int sockfd = socket (PF_INET, SOCK_STREAM, 0);


·         int sockfd = socket (PF_INET, SOCK_DGRAM, 0);


o   The combination of PF_INET and SOCK_STREAM implies TCP



·         Binds the newly created socket to the specified address i.e. the network address of the local participant (the server)

·         Address is a data structure which combines IP and port



o   Defines how many connections can be pending on the specified socket


o     Carries out the passive open


o     Blocking operation


o     Does not return until a remote participant has established a connection


o     When it does, it returns a new socket that corresponds to the new established connection and the address argument contains the remote participant’s address



o     Application performs active open

o     It says who it wants to communicate with


Client invokes


o   int connect (int socket, struct sockaddr *address, int addr_len)




o     Does not return until TCP has successfully established a connection at which application is free to begin sending data

o     Address contains remote machine’s address



·         Bandwidth


o     Width of the frequency band


o     Number of bits per second that can be transmitted over a communication link


·         1 Mbps: 1 x 106 bits/second = 1x220 bits/sec


·         1 x 10-6 seconds to transmit each bit or imagine that a timeline, now each bit occupies 1 micro second space.


·         On a 2 Mbps link the width is 0.5 micro second.


·         Smaller the width more will be transmission per unit time.

transmitted at a particular bandwidth can be regarded as having some width:

a.    bits transmitted at 1Mbps (each bit 1 μs wide);

b.    bits transmitted at 2Mbps (each bit 0.5 μs wide).

·         Latency = Propagation + transmit + queue


·         Propagation = distance/speed of light


·         Transmit = size/bandwidth


·         One bit transmission => propagation is important


·         Large bytes transmission => bandwidth is important Delay X Bandwidth


·         We think the channel between a pair of processes as a hollow pipe


·         Latency (delay) length of the pipe and bandwidth the width of the pipe


·         Delay of 50 ms and bandwidth of 45 Mbps

50 x 10-3 seconds x 45 x 106 bits/second

2.25 x 106 bits = 280 KB data.


Relative importance of bandwidth and latency depends on application For large

file transfer, bandwidth is critical

For small messages (HTTP, NFS, etc.), latency is critical

Variance in latency (jitter) can also affect some applications (e.g., audio/video conferencing)

How many bits the sender must transmit before the first bit arrives at the receiver if the sender keeps the pipe full

Takes another one-way latency to receive a response from the receiver

If the sender does not fill the pipe—send a whole delay × bandwidth product’s worth of data before it stops to wait for a signal—the sender will not fully utilize the network

Infinite bandwidth

RTT dominates

Throughput = TransferSize / TransferTime

TransferTime = RTT + 1/Bandwidth x TransferSize

Its all relative


1-MB file to 1-Gbps link looks like a 1-KB packet to 1-Mbps link







o     Gives the upper bound to the capacity of a link in terms of bits per second (bps) as a function of signal-to-noise ratio of the link measured in decibels (dB).

o     C = Blog2(1+S/N)


§    Where B = 3300 – 300 = 3000Hz, S is the signal power, N the average noise.


§    The signal to noise ratio (S/N) is measured in decibels is related to dB = 10 x log10(S/N). If there is 30dB of noise then S/N = 1000.


§    Now C = 3000 x log2(1001) = 30kbps.


o     All practical links rely on some sort of electromagnetic radiation propagating through a medium or, in some cases, through free space


o     One way to characterize links, then, is by the medium they use


§    Typically copper wire in some form (as in Digital Subscriber Line (DSL) and coaxial cable),


§    Another important link characteristic is the frequency


§    Measured in hertz, with which the electromagnetic waves oscillate


§    Distance between the adjacent pair of maxima or minima of a wave measured in meters is called wavelength


§    Speed of light divided by frequency gives the wavelength.


§    Frequency on a copper cable range from 300Hz to 3300Hz; Wavelength for 300Hz wave through copper is speed of light on a copper / frequency


§    2/3 x 3 x 108 /300 = 667 x 103 meters.

§    Placing binary data on a signal is called encoding.


§    Modulation involves modifying the signals in terms of their frequency, amplitude, and phase.


§    Optical fiber (as in both commercial fiber-to-the home services and many long-distance links in the Internet’s backbone), or Air/free space (for wireless links)




Problem with NRZ


o     Baseline wander


§    The receiver keeps an average of the signals it has seen so far


§    Uses the average to distinguish between low and high signal


§    When a signal is significantly low than the average, it is 0, else it is 1


§    Too many consecutive 0’s and 1’s cause this average to change, making it difficult to detect


Problem with NRZ


o     Clock recovery


§    Frequent transition from high to low or vice versa are necessary to enable clock recovery


§    Both the sending and decoding process is driven by a clock


§    Every clock cycle, the sender transmits a bit and the receiver recovers a bit


§    The sender and receiver have to be precisely synchronized




o     Non Return to Zero Inverted


o     Sender makes a transition from the current signal to encode 1 and stay at the current signal to encode 0


o     Solves for consecutive 1’s

Manchester encoding


o     Merging the clock with signal by transmitting Ex-OR of the NRZ encoded data and the clock


o     Clock is an internal signal that alternates from low to high, a low/high pair is considered as one clock cycle

o     In Manchester encoding


§    0: lowà high transition

§    1: highà low transition

Problem with Manchester encoding


o     Doubles the rate at which the signal transitions are made on the link


§    Which means the receiver has half of the time to detect each pulse of the signal


o     The rate at which the signal changes is called the link’s baud rate


o     In Manchester the bit rate is half the baud rate


4B/5B encoding


§    Insert extra bits into bit stream so as to break up the long sequence of 0’s and 1’s


§    Every 4-bits of actual data are encoded in a 5- bit code that is transmitted to the receiver


§    5-bit codes are selected in such a way that each one has no more than one leading


§  0(zero) and no more than two trailing 0’s.

§    No pair of 5-bit codes results in more than three consecutive 0’s





§    We are focusing on packet-switched networks, which means that blocks of data (called frames at this level), not bit streams, are exchanged between nodes.

§    It is the network adaptor that enables the nodes to exchange frames.


§    When node A wishes to transmit a frame to node B, it tells its adaptor to transmit a frame from the node’s memory. This results in a sequence of bits being sent over the link.


§    The adaptor on node B then collects together the sequence of bits arriving on the link and deposits the corresponding frame in B’s memory.


§    Recognizing exactly what set of bits constitute a frame—that is, determining where the frame begins and ends—is the central challenge faced by the adaptor

§    Byte-oriented Protocols


To view each frame as a collection of bytes (characters) rather than bits


BISYNC (Binary Synchronous Communication) Protocol


Developed by IBM (late 1960)


DDCMP (Digital Data Communication Protocol)


Used in DECNet


§    BISYNC – sentinel approach


     Frames transmitted beginning with leftmost field


     Beginning of a frame is denoted by sending a special SYN (synchronize) character


     Data portion of the frame is contained between special sentinel character STX (start of text) and ETX (end of text)

     SOH : Start of Header

     DLE : Data Link Escape

·         PPP Frame Format

     Recent PPP which is commonly run over Internet links uses sentinel approach


·         Special start of text character denoted as Flag


·         0 1 1 1 1 1 1 0


·         Address, control : default numbers


·         Protocol for demux : IP / IPX


·         Payload : negotiated (1500 bytes)


·         Checksum : for error detection


     Byte-counting approach


o     DDCMP


o     count : how many bytes are contained in the frame body


o     If count is corrupte


§    Framing error

·         Bit-oriented Protocol


o     HDLC : High Level Data Link Control


§    Beginning and Ending Sequences


0 1 1 1 1 1 1 0

HDLC Protocol


On the sending side, any time five consecutive 1’s have been transmitted from the body of the message (i.e. excluding when the sender is trying to send the distinguished 01111110 sequence)

The sender inserts 0 before transmitting the next bit


HDLC Protocol


On the receiving side


5 consecutive 1’s


Next bit 0 : Stuffed, so discard it


1 : Either End of the frame marker


Or Error has been introduced in the bitstream Look at the next bit

If 0 ( 01111110 ) à End of the frame marker

If 1 ( 01111111 ) à Error, discard the whole frame

The receiver needs to wait for next 01111110 before it can start receiving again





     Bit errors are introduced into frames


     Because of electrical interference and thermal noises


     Detecting Error


     Correction Error


     Two approaches when the recipient detects an error


     Notify the sender that the message was corrupted, so the sender can send again.


     If the error is rare, then the retransmitted message will  be error-free


     Using some error correct detection and correction algorithm, the receiver reconstructs the message


     Common technique for detecting transmission error


     CRC (Cyclic Redundancy Check)


     Used in HDLC, DDCMP, CSMA/CD, Token Ring


     Other approaches


     Two Dimensional Parity (BISYNC)


     Checksum (IP)


     Basic Idea of Error Detection


     To add redundant information to a frame that can be used to determine if errors have been introduced


     Imagine (Extreme Case)


     Transmitting two complete copies of data


n    Identical à No error

n    Differ à Error

n    Poor Scheme ???


n    n bit message, n bit redundant information


n    Error can go undetected


     In general, we can provide strong error detection technique


n    k redundant bits, n bits message, k << n


n    In Ethernet, a frame carrying up to 12,000 bits of data requires only 32-bit CRC


     Extra bits are redundant


     They add no new information to the message


     Derived from the original message using some algorithm


     Both the sender and receiver know the algorithm


Sender Receiver Receiver computes r using m If they match, no error


            9. TRANSMISSION


     CRC is used to detect errors.


     Some error codes are strong enough to correct errors.


     The overhead is typically too high.


     Corrupt frames must be discarded.


A link-level protocol that wants to deliver frames reliably must recover from these discarded frames.


     This is accomplished using a combination of two fundamental mechanisms


     Acknowledgements and Timeouts


     An acknowledgement (ACK for short) is a small control frame that a protocol sends back to its peer saying that it has received the earlier frame.


     A control frame is a frame with header only (no data).


     The receipt of an acknowledgement indicates to the sender of the original frame that its frame was successfully delivered.


     If the sender does not receive an acknowledgment after a reasonable amount of time, then it retransmits the original frame.

     The action of waiting a reasonable amount of time is called a timeout.


     The general strategy of using acknowledgements and timeouts to implement reliable delivery is sometimes called Automatic Repeat reQuest (ARQ).

 Stop and Wait Protocol

     Idea of stop-and-wait protocol is straightforward


     After transmitting one frame, the sender waits for an acknowledgement before transmitting the next frame.

     If the acknowledgement does not arrive after a certain period of time, the sender times out and retransmits the original frame

     If the acknowledgment is lost or delayed in arriving


The sender times out and retransmits the original frame, but the receiver will think that it is the next frame since it has correctly received and acknowledged the first frame

     As a result, duplicate copies of frames will be delivered


     How to solve this


     Use 1 bit sequence number (0 or 1)


     When the sender retransmits frame 0, the receiver can determine that it is seeing a second copy of frame 0 rather than the first copy of frame 1 and therefore can ignore it (the receiver still acknowledges it, in case the first acknowledgement was lost)


     The sender has only one outstanding frame on the link at a time

This may be far below the link’s capacity

Consider a 1.5 Mbps link with a 45 ms RTT

The link has a delay bandwidth product of 67.5 Kb or approximately 8 KB

Since the sender can send only one frame per RTT and assuming a frame size of 1


Maximum Sending rate

 Bits per frame Time per frame = 1024 8 0.045 = 182 Kbps Or about one-eighth of the link’s capacity

To use the link fully, then sender should transmit up to eight frames before having to wait for an acknowledgement

Sliding Window Protocol

Sender assigns a sequence number denoted as SeqNum to each frame.


     Assume it can grow infinitely large


     Sender maintains three variables


     Sending Window Size (SWS)


     Upper bound on the number of outstanding (unacknowledged) frames that the sender can transmit


     Last Acknowledgement Received (LAR)


     Sequence number of the last acknowledgement received


     Last Frame Sent (LFS)


     Sequence number of the last frame sent


     When an acknowledgement arrives


     the sender moves LAR to right, thereby allowing the sender to transmit another frame


     Also the sender associates a timer with each frame it transmits


     It retransmits the frame if the timer expires before the ACK is received


     Note that the sender has to be willing to buffer up to SWS frames


     Receiver maintains three variables


     Receiving Window Size (RWS)


     Upper bound on the number of out-of-order frames that the receiver is willing to accept


     Largest Acceptable Frame (LAF)


     Sequence number of the largest acceptable frame


     Last Frame Received (LFR)



     Sequence number of the last frame received

     Receiver also maintains the following invariant


If SeqNum ≤ LFR or SeqNum > LAF

     Discard it (the frame is outside the receiver window)


     If LFR < SeqNum ≤ LAF


     Accept it


     Now the receiver needs to decide whether or not to send an ACK


     Let SeqNumToAck


     Denote the largest sequence number not yet acknowledged, such that all frames with sequence number less than or equal to SeqNumToAck have been received


     The receiver acknowledges the receipt of SeqNumToAck even if high-numbered packets have been received

     This acknowledgement is said to be cumulative.


     The receiver then sets


     LFR = SeqNumToAck and adjusts


     LAF = LFR + RWS

For example, suppose LFR = 5 and RWS = 4

(i.e. the last ACK that the receiver sent was for seq. no. 5)

LAF = 9

If frames 7 and 8 arrive, they will be buffered because they are within the receiver window

But no ACK will be sent since frame 6 is yet to arrive Frames 7 and 8 are out of order

Frame 6 arrives (it is late because it was lost first time and had to be retransmitted)

Now Receiver Acknowledges Frame 8 and bumps LFR to 8

and LAF to 12





     Most successful local area networking technology of last 20 years.


     Developed in the mid-1970s by researchers at the Xerox Palo Alto Research Centers (PARC).


     Uses CSMA/CD technology


     Carrier Sense Multiple Access with Collision Detection.


     A set of nodes send and receive frames over a shared link.


     Carrier sense means that all nodes can distinguish between an idle and a busy link.


     Collision detection means that a node listens as it transmits and can therefore detect when a frame it is transmitting has collided with a frame transmitted by another node.


     Uses ALOHA (packet radio network) as the root protocol


     Developed at the University of Hawaii to support communication across the Hawaiian Islands.


     For ALOHA the medium was atmosphere, for Ethernet the medium is a coax cable.


     DEC and Intel joined Xerox to define a 10-Mbps Ethernet standard in 1978.

     This standard formed the basis for IEEE standard 802.3


     More recently 802.3 has been extended to include a 100-Mbps version called Fast Ethernet and a 1000-Mbps version called Gigabit Ethernet.

     An Ethernet segment is implemented on a coaxial cable of up to 500 m.


     This cable is similar to the type used for cable TV except that it typically has an impedance of 50 ohms instead of cable TV’s 75 ohms.


     Hosts connect to an Ethernet segment by tapping into it.


     A transceiver (a small device directly attached to the tap) detects when the line is idle and drives signal when the host is transmitting.


     The transceiver also receives incoming signal.


     The transceiver is connected to an Ethernet adaptor which is plugged into the host.


     The protocol is implemented on the adaptor.


     Multiple Ethernet segments can be joined together by repeaters.


     A repeater is a device that forwards digital signals.


     No more than four repeaters may be positioned between any pair of hosts.


     An Ethernet has a total reach of only 2500 m.


     Any signal placed on the Ethernet by a host is broadcast over the entire network


     Signal is propagated in both directions.


     Repeaters forward the signal on all outgoing segments.


     Terminators attached to the end of each segment absorb the signal.


     Ethernet uses Manchester encoding scheme.


     New Technologies in Ethernet


     Instead of using coax cable, an Ethernet can be constructed from a thinner cable known as 10Base2 (the original was 10Base5)


     10 means the network operates at 10 Mbps


     Base means the cable is used in a baseband system


     2 means that a given segment can be no longer than 200 m


     New Technologies in Ethernet


     Another cable technology is 10BaseT


     T stands for twisted pair


     Limited to 100 m in length


     With 10BaseT, the common configuration is to have several point to point segments coming out of a multiway repeater, called Hub


Access Protocol for Ethernet

     The algorithm is commonly called Ethernet’s Media Access Control (MAC).


     It is implemented in Hardware on the network adaptor.


     Frame format


     Preamble (64bit): allows the receiver to synchronize with the signal (sequence of alternating 0s and 1s).


     Host and Destination Address (48bit each).


     Packet type (16bit): acts as demux key to identify the higher level protocol.


     Data (up to 1500 bytes)


     Minimally a frame must contain at least 46 bytes of data.


     Frame must be long enough to detect collision.


     CRC (32bit)


     Ethernet Addresses


     Each host on an Ethernet (in fact, every Ethernet host in the world) has a unique Ethernet Address.

     The address belongs to the adaptor, not the host.


     It is usually burnt into ROM.


     Ethernet addresses are typically printed in a human readable format


     As a sequence of six numbers separated by colons.


     Each number corresponds to 1 byte of the 6 byte address and is given by a pair of hexadecimal digits, one for each of the 4-bit nibbles in the byte


     Leading 0s are dropped.


     For example, 8:0:2b:e4:b1:2 is


   00001000 00000000 00101011 11100100 10110001 00000010


     To ensure that every adaptor gets a unique address, each manufacturer of Ethernet devices is allocated a different prefix that must be prepended to the address on every adaptor they build


   AMD has been assigned the 24bit prefix 8:0:20







A ring toplogy network developed in the late 1960s. Supported mainly by IBM. Pushed into the background by Ethernet in the 1990s.


a LAN protocol which resides at the data link layer (DLL) of the OSI model


Shielded Twisted Pair with unique hermaphroditic connectors (IBM “Type 1”) or


Symmetric pair. Speed:


–   4 Mbps (1985)


16 Mpbs (1989, IBM Ring operation


When nobody is transmitting a token circles.

When a station needs to transmit data, it converts the token into a data frame.

When the sender receives its own data frame, it converts the frame back into a token.


If an error occurs and no token frame, or more than one, is present, a special station

(“Active Monitor”) detects the problem and removes and/or reinserts tokens as necessary.

The Abort frame: used to abort transmission by the sending station




            according to the osi-rm, fddi specifies layer 1 (physical layer) and part of layer 2 (data link control layer)


            the physical layer handles the transmission of raw bits over a communications link


            the data link control (dlc) layer is responsible for maintaining the integrity of information exchanged between two points


            high bandwidth (10 times more than ethernet)


            larger distances between fddi nodes because of very low attenuation (0.3 db/km) in fibers

improved signal-to-noise ratio because of no interference from external radio frequencies and electromagnetic noise


            ber typical of fiber-optic systems (10^-11) is substantially better than that in copper (10^-5) and microwave systems (10^-7)


            very difficult to tap signals form a fiber cable


            high cost of optical components required for transmission/reception of signals (especially for single mode fiber networks)


            more complex to implement than existing low speed lan technologies such as ieee 802.3 and ieee 802.5


            office automation at the desktop

            backbones for factory automation

            backend data center applications

            campus lan interconnection

            intercampus backbones or metropolitan area networks (mans)

            interconnection of private branch exchanges (pbxs)

            workgroup and departmental lans

            integrated transport for multimedia applications




            Datagram network is not either connection-oriented

or connectionless.

              Internet provides both connection-oriented (TCP) and

connectionless services (UDP) to apps.

mesh of interconnected routers


the fundamental question: how is data transferred through net?


     circuit switching: dedicated circuit per call: telephone net


     packet-switching: data sent thru net in discrete “chunks”


End-end resources reserved for “call”

link bandwidth, switch capacity


dedicated resources: no sharing


circuit-like (guaranteed) performance


call setup required


network resources (e.g., bandwidth) divided into “pieces”

pieces allocated to calls


resource piece idle if not used by owning call (no sharing)


each end-end data stream divided into packets

     user A, B packets share network resources


     each packet uses full link bandwidth


     resources used as needed




Wireless links transmit electromagnetic signals


     Radio, microwave, infrared


Wireless links all share the same “wire” (so to speak)


     The challenge is to share it efficiently without unduly interfering with each other


     Most of this sharing is accomplished by dividing the “wire” along the dimensions of frequency and space


Exclusive use of a particular frequency in a particular geographic area may be allocated to an individual entity such as a corporation


These allocations are determined by government agencies such as FCC (Federal Communications Commission) in USA

Specific bands (frequency) ranges are allocated to certain uses.


     Some bands are reserved for government use


     Other bands are reserved for uses such as AM radio, FM radio, televisions, satellite communications, and cell phones


     Specific frequencies within these bands are then allocated to individual organizations for use within certain geographical areas.

     Finally, there are several frequency bands set aside for “license exempt” usage


     Bands in which a license is not needed


Devices that use license-exempt frequencies are still subject to certain restrictions


     The first is a limit on transmission power


     This limits the range of signal, making it less likely to interfere with another signal


     For example, a cordless phone might have a range of about 100 feet.


he second restriction requires the use of Spread Spectrum technique


     Idea is to spread the signal over a wider frequency band

     So as to minimize the impact of interference from other devices

     Originally designed for military use


     Frequency hopping


     Transmitting signal over a random sequence of frequencies


     First transmitting at one frequency, then a second, then a third…


     The sequence of frequencies is not truly random, instead computed algorithmically by a pseudorandom number generator


     The receiver uses the same algorithm as the sender, initializes it with the same seed, and is


     Able to hop frequencies in sync with the transmitter to correctly receive the frame

     A second spread spectrum technique called Direct sequence


     Represents each bit in the frame by multiple bits in the transmitted signal.


     For each bit the sender wants to transmit


     It actually sends the exclusive OR of that bit and n random bits


     The sequence of random bits is generated by a pseudorandom number generator known to both the sender and the receiver.


     The transmitted values, known as an n-bit chipping code, spread the signal across a frequency band that is n times wider

     Wireless technologies differ in a variety of dimensions


     How much bandwidth they provide


     How far apart the communication nodes can be


     Four prominent wireless technologies




     Wi-Fi (more formally known as 802.11)


     WiMAX (802.16)


     3G cellular wireless




     Bridges and LAN Switches


     Class of switches that is used to forward packets between shared-media LANs such as Ethernets


     Known as LAN switches


     Referred to as Bridges


     Suppose you have a pair of Ethernets that you want to interconnect


     One approach is put a repeater in between them


     It might exceed the physical limitation of the Ethernet


     No more than four repeaters between any pair of hosts


     No more than a total of 2500 m in length is allowed


     An alternative would be to put a node between the two Ethernets and have the node forward frames from one Ethernet to the other


     This node is called a Bridge


     A collection of LANs connected by one or more bridges is usually said to form an Extended LAN


     Simplest Strategy for Bridges


     Accept LAN frames on their inputs and forward them out to all other outputs

     Used by early bridges


Learning Bridges


     Observe that there is no need to forward all the frames that a bridge receives


Broadcast and Multicast


     Forward all broadcast/multicast frames


     Current practice


     Learn when no group members downstream


     Accomplished by having each member of group G send a frame to bridge multicast address with G in source field


Limitation of Bridges


     Do not scale


     Spanning tree algorithm does not scale


     Broadcast does not scale,Do not accommodate heterogeneity


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