lab7 - Cristinel Ababei
Contents
1. Structure Information is sent around an Ethernet network in discreet messages known as frames The frame structure consists of the following fields Preamble This consists of seven bytes all of the form 10101010 This allows the receiver s clock to be synchronized with the sender s Start Frame Delimiter This is a single byte 10101011 which is used to indicate the start of a frame Destination Address This is the address of the intended recipient of the frame The addresses in 802 3 use globally unique hardwired 48 bit addresses Source Address This is the address of the source in the same form as above Length This is the length of the data in the Ethernet frame which can be anything from 0 to 1500 bytes Data This is the information being sent by the frame Pad 802 3 frame must be at least 64 bytes long so if the data is shorter than 46 bytes the pad field must compensate The reason for the minimum length lies with the collision detection mechanism In CSMA CD the sender must wait at least two times the maximum propagation delay before it knows that no collision has occurred If a station sends a very short message then it might release the ether without knowing that the frame has been corrupted 802 3 sets an upper limit on the propagation delay and the minimum frame size is set at the amount of data which can be sent in twice this figure CRC Cyclic Redundancy Check to detect errors that occur during tr2. address to a physical address found on your NIC card Using this command can tell us what physical address belongs to which IP address 2 Netstat Netstat is a tool that displays local and remote connections to the computer It displays IP addresses ports protocol being used and the status of the connection 3 Ping Ping is a simple diagnostic tool that can check for connectivity between two points on a network It is one of the most used TCP IP utilities when setting up a network or changing network settings 4 TraceRT Tracert or traceroute is a command that shows the path that packets of data take while being sent It s handy for checking to see where a possible network failure lies or even for ensuring that data packets are taking the fastest route possible on a network 5 FTP TFTP FTP and TFTP are both used for transferring files It is important to note that FTP is a TCP utility while TFTP is a UDP utility TFTP tends to be less secure than FTP and is generally only used for transferring non confidential files over a network when speed is concerned 6 Hostname Hostname is a simple command that displays the hostname of the current computer simple yet effective 7 Whois Whois information is just like an online phonebook It shows the contact information for owners of a particular domain By using a Whois search you will find that Google is based in California APPENDIX C Key Features of TCP IP Five of the most im
3. duplex or simplex operation and establishes checkpointing adjournment termination and restart procedures This layer also separates the data of different applications from each other 4 Transport layer The transport layer provides transparent transfer of data between end users providing reliable data transfer services to the upper layers The transport layer controls the reliability of a given link through flow control segmentation desegmentation and error control The transport layer is responsible basically for segmentation and reassembly S amp R The data from upper layer is combined together and sent as a single data stream The Transmission Control Protocol TCP and the User Datagram Protocol UDP of the Internet Protocol Suite are commonly categorized as layer 4 protocols within OSI 3 Network layer The network layer provides the functional and procedural means of transferring variable length data sequences from a source host on one network to a destination host on a different network in contrast to the data link layer which connects hosts within the same network while maintaining the quality of service requested by the transport layer This layer basically is used for routing It tracks location of devices Data travels organized as packets data packets and route update packets 2 Data Link Layer DLL The data link layer provides the functional and procedural means to transfer data between network entities and to detect and
4. effect of collisions 1 Collision Avoidance and 2 Collision Resolution Collision Avoidance involves systems which prevent any collisions occurring 1 in the first place such as polling or token passing Collision Resolution or Contention MAC Strategies rely on the fact that collisions will occur and try to cope with them as well as possible Ethernet uses Collision Resolution Below we discuss some collision resolution techniques ALOHA The most basic form of Collision Resolution is to simply allow any station to send a message or packet whenever it is ready to send one This form of transmission was first used in a prototype packet radio network ALOHANET commissioned in Hawaii in 1970 and has been known ever since as unslotted ALOHA In Pure ALOHA packets contain some form of error detection which is verified by the receiver If the packet is received correctly the destination returns an acknowledgment If a collision occurs and the message is destroyed or corrupted then no acknowledgment will be sent If the sender does not receive an acknowledgment after a certain delay it will re send the message Carrier Sense Multiple Access CSMA The next stage in Collision Resolution after ALOHA was to add the ability for devices to detect whether the shared medium is idle or not This is called Carrier Sense Multiple Access or CSMA This however does not completely eliminate collisions since two devices could detect the medium as idl
5. internetarchitecture html Internet The Big Picture http navigators com internet_architecture html Internet Technical Resources http www cs columbia edu hgs internet Internet Wikipedia entry http en wikipedia org wiki Internet 2 LPC17xx user manual 2010 http www nxp com documents user_manual UM10360 pdf APPENDIX A Manchester Phase Encoding MPE 802 3 Ethernet uses Manchester Phase Encoding MPE A data bit 1 from the level encoded signal 1 e that from the digital circuitry in the host machine sending data 1s represented by a full cycle of the inverted signal from the master clock which matches with the 0 to 1 rise of the phase encoded signal linked to the phase of the carrier signal which goes out on the wire 1 e V in the first half of the signal and V in the second half The data bit 0 from the level encoded signal is represented by a full normal cycle of the master clock which gives the 1 to 0 fall of the phase encoded signal 1 e V in the first half of the signal and V in the second half The following diagram shows graphically how MPE operates The example at the bottom of the diagram indicates how the digital bit stream 10110 is encoded 1 bit D bit Figure 13 Illustration of MPE operation 13 A transition in the middle of each bit makes it possible to synchronize the sender and receiver At any instant the ether can be in one of three state
6. layers A layer serves the layer above it and is served by the layer below it Example Application Web Application Presentation HTTP Session 80 Transmision Control mzzzzzzaznnn im Transport Protocol TCP _ 3 Network Layer 94 Bf Daca fT i 3 Network Layer 2 Datalink Layer I Daca i 2 Datalink Layer Network Internet Protocol IP ee A OE Data Link ii Data Transmission E J System Logical Connection in each Layer system 2 FH Physical CAT5 PZ Realisation of the communication Figure 4 Illustration of the 7 layers of the OSI model An example A brief explanation of each of these seven layers follows 7 Application layer The top most layer of the OSI model The primary role of the application layer is that it checks resource usability and synchronization with the remote partner The application layer is the closest to the end user which means that both the OSI application layer and the user interact directly with the software application 6 Presentation layer The function of this layer is very critical as it provides encryption services Other services apart from encryption include decryption data compression and decompression 5 Session layer The session layer controls the dialogues connections between computers It establishes manages and terminates the connections between the local and remote application It provides for full duplex half
7. of the network stack is unaware of the layers above and below The information coming from the layers above are simply treated as data to be encapsulated Many application protocols can be packed into TCP When the packet is received at its final destination the same process is repeated in reverse The packet is de encapsulated and the headers stripped off when it is received by the intended target 8 user data 7 application application iii a BEH t n TOP header application data newark j It segrnnent TEP IP header TCP header application data Mak i Ethemet e F tert o oo driver eader TCP header application data a eueeesbanenes Lamehester pp os 14 A A 4 Fhe ane lt 45 io 1800 bytes ______ Figure 7 Illustration of encapsulation 5 The Internet The Internet is a global system of interconnected computer networks that use the standard Internet Protocol suite TCP IP to serve billions of users worldwide It is a network of networks that consists of millions of private public academic business and government networks of local to global scope that are linked by a broad array of electronic wireless and optical networking technologies The Internet carries an extensive range of information resources and services such as the inter linked hypertext documents of the World Wide Web WWW and the infrastructure to support email A simplified archite
8. possibly correct errors that may occur in the physical layer This layer is usually divided into 2 sublayers 1 the upper one is LLC Logical Link Control and the lower one is MAC Medium Access Control The DLL deals with the movement of data on Local Area Networks LANs The data movement is in the form of frames and forwarded on the basis of hardware address called the MAC address 1 Physical layer The physical layer defines electrical and physical specifications for devices It defines the relationship between a device and a transmission medium such as a copper or fiber optical cable This includes the layout of pins voltages line impedance cable specifications signal timing hubs repeaters network adapters etc Data travels in form of digital signals e g 01110001000 The real transmission takes place here 1 e traveling through a medium like cable fiber optic or air 4 The Internet Protocol a k a TCP IP Protocol The Internet Protocol suite is the set of communications protocols used for the Internet and similar networks It is generally the most popular protocol stack for wide area networks It is commonly known as TCP IP because of its most important protocols Transmission Control Protocol TCP and Internet Protocol IP which were the first networking protocols defined in this standard It is also referred to as the DoD model due to the foundational influence of the ARPANET in the 1970s operated
9. 7xx keil emac pdf file included in the downloadable archive for this lab to see how to set up and run this example use the CAT 5E cable from your TA Compile and download to the board Observe operation and comment When up and running you should see the webpage shown in the figure below Take some time and read the source code in order to get a good understanding of what s happening gt easy WEB dynamic webside gt C ff 192 168 0100 Hello World Figure 12 Webpage that shows the value of the ADC value of the MCB1700 board 8 Lab assignment This is optional If done correctly you may get up to 2 of the final grade Create a new uVision project and write a program that uses Ethernet to connect two boards and transmit the value of the ADC from one board to the other where it is displayed on the LCD screen 12 9 Credits and references 1 Ethernet Introduction Ross MCIlroy 2004 http www dcs gla ac uk ross Ethernet index htm Data Network Resource Rhys Haden 2013 http www rhyshaden com eth_intr htm OSI Model Wikipedia entry http en wikipedia org wiki OSI_model Internet Protocol Wikipedia entry http en wikipedia org wiki TCP IP_model How the Application Layer Works http learn networking com tcp ip how the application layer works Introduction to Internet Architecture and Institutions Ethan Zuckerman and Andrew McLaughlin 2003 http cyber law harvard edu digitaldemocracy
10. Lab 7 Introduction to Ethernet EE 379 Embedded Systems and Applications Electrical Engineering Department University at Buffalo Last update Cristinel Ababei April 2013 1 Objective The objective of this lab is to introduce you to Ethernet We also study the EMAC EasyWEB example from Keil which illustrates a simple web server hosted on the MCB1700 board Note This presentation has been adapted from various references listed at the end of this lab 2 Ethernet Ethernet is now the world s most pervasive networking technology History In 1973 Xerox Corporation s Palo Alto Research Center began the development of a bus topology LAN local area network In 1976 Xerox built a 2 94 Mbps network to connect over 100 personal workstations ona 1 km cable This network was called the Ethernet named after the ether the single coaxial cable used to connect the machines Xerox Ethernet was so successful that in 1980 Digital Equipment Corporation Intel Corporation and Xerox had released a de facto standard for a 10 Mbps Ethernet informally called DIX Ethernet from the initials of the 3 companies This Ethernet Specification defined Ethernet II and was used as a basis for the IEEE 802 3 specification in 1985 Strictly Ethernet refers to a product which predates the IEEE 802 3 Standard However nowadays any 802 3 compliant network is referred to as an Ethernet Ethernet has largely replaced competing wired LAN technologies Over the
11. a might be represented as a silent spot c Logical Addressing Most computers today come standard with Network Interface Cards NICs These cards are the actual hardware used to communicate to other computers Each card has a unique physical address that is set at the factory and can t be changed Essentially this is an identifier for the computer it is installed on Networks rely on the physical address of a computer for data delivery but we have a problem The NIC card is constantly looking for transmissions that are addressed to it what if the network was very large in size To put it into perspective imagine your computer looking at every single bit of data on the internet to see if any of the millions of data packets are addressed to it This is where logical addressing comes in You are probably more familiar with the term IP address however These IP addresses can be subnetted on a network to divide a large network into tiny pieces Instead of looking at every bit of data on the internet logical addressing allows for computers to just look at data on a home network or subnet d Routing A router is a device used to read logical addressing information and to direct the data to the appropriate destination Routers are commonly used to separate networks into portions this greatly reduces network traffic if done correctly 16 TCP IP includes protocols that tell routers how to find a path through the network This is a vital
12. ansmission DIX version of FCS or FCS Frame Check Sequence to detect errors that occur during transmission 802 3 version of CRC This 32 bit code has an algorithm applied to it which will give the same result as the other end of the link provided that the frame was transmitted successfully bytes 5 5 1500 G z J6 4 Ols Ethernet Packet bytes 46 1500 i b b z 4 IEEE 602 35 Frame Figure 1 Structure of an Ethernet frame Ethernet vs 802 3 Although the Ethernet and 802 3 standards are effectively the same thing there are some subtle differences between Ethernet II and 802 3 The IEEE 802 3 standard was part of a bigger standard 802 This contains a number of different network technologies such as token ring and token bus as well as Ethernet These technologies are brought together by a layer on top of these MAC Layers called Logical Link Control LLC as shown in the figure below Ethernet II however does not use this LLC layer LLC Logical Link Control MAC Media Access Control Physical Layer Figure 2 Illustration of LLC layer b Physical Layer The Physical Layer is concerned with the low level electronic way in which the signals are transmitted In Ethernet signals are transmitted using Manchester Phase Encoding MPE see Appendix A This encoding is used to ensure that clocking data 1s sent along with the data so that the sending and receiving device clocks are in sync The logic levels are tran
13. by DARPA an agency of the United States Department of Defense TCP IP provides end to end connectivity specifying how data should be formatted addressed transmitted routed and received at the destination It has four abstraction layers see figure below which are used to sort all Internet protocols according to the scope of networking involved From lowest to highest the layers are 1 Link layer contains communication technologies for a local network 2 Internet layer IP connects local networks thus establishing internetworking 3 Transport layer handles host to host communication 4 Application layer contains all protocols for specific data communications services on a process to process level It focuses more on network services APIs utilities and operating system environments For example HTTP specifies the web browser communication with a web server See Appendix B for more info Application Presentation Session Process Application m TCP UDP etc Host to Host Data Link Ethernet ATM TCP IP DoD model OS Model Figure 5 Comparison of TCP IP and OSI layer stacks Note 1 Sometimes people mix or are not aware of the meaning and differences between Ethernet and TCP IP Generally speaking they are different levels or layers of a network Ethernet covers the physical medium plus some low level things like message collision detection TCP IP worries about getting a message to where it is
14. cture of the Internet network is shown in the figure below y Internet Providers Corporate Network Zz T L a Teephore Campus Network Line Ethernet LAN a Figure 8 Simplified Internet architecture Another view of the Internet that illustrates various components at different hierarchy levels is shown in figure below Conventional Phone Digital Subscriber or Cable Modem Line Home Business Figure 9 Illustration of the hierarchy of the Internet 6 Ethernet Block of LPC1768 The Ethernet block contains a full featured 10 Mbps or 100 Mbps Ethernet MAC Media Access Controller designed to provide optimized performance through the use of DMA hardware acceleration Features include a generous suite of control registers half or full duplex operation flow control control frames hardware acceleration for transmit retry receive packet filtering and wake up on LAN activity Automatic frame transmission and reception with Scatter Gather DMA off loads many operations from the CPU The Ethernet block is an AHB master that drives the AHB bus matrix Through the matrix it has access to all on chip RAM memories A recommended use of RAM by the Ethernet is to use one of the RAM blocks exclusively for Ethernet traffic That RAM would then be accessed only by the Ethernet and the CPU and possibly the GPDMA giving maximum bandwidth to the Ethernet function The Ethernet block interfaces between an off chip Etherne
15. e then attempt to send at approximately the same time CSMA is actually a family of protocols which vary by the method which they wait for the medium to become idle known as the persistence strategy Here is a list of two major strategies e 1 Persistent CSMA In this strategy when a device wants to send a message it first listens to the medium If it is idle the message is sent immediately however if it is busy the device continues to listen to the medium until it becomes idle and then sends the message immediately The problem is that if a number of devices attempt to send during a busy period then they shall all send as soon as the medium becomes idle leading to a collision e nonpersistent CSMA This strategy attempts to reduce the greediness of 1 Persistent CSMA It again first listens to the medium to see if it is idle if so it sends immediately If the medium is busy instead of continuing to listen for the medium to become idle and transmitting immediately it waits a random period then it tries again This means that in high load situations there is less chance of collisions occurring Collision Window A collision occurs when two devices send at approximately the same time But how long does a device have to wait until it knows that its message has not been corrupted by a collision Messages take a certain amount of time to travel from the device to the end of the signaling medium which is known as the propagation delay I
16. feature of the TCP IP suite that enables massive LAN connections to be created e Name Resolution Finally we have name resolution If you wanted to conduct a search on the internet you would probably just type Google s URL into your address bar What you probably didn t know is that you aren t necessarily connecting to google com but rather an IP address Instead of having to remember an IP address name resolution allows you to remember Google s name It might not be so bad if IP addresses were not so lengthy in size Which is easier to remember http 74 125 224 72 or google This handy service is accomplished on name servers which are just computers that store tables that translate domain names to and from IP addresses 17
17. going TCP IP is usually found on Ethernet based networks but it can be used on other networks as well Also you can have Ethernet without TCP IP and in fact a lot of proprietary industrial networks do exactly that In addition you can also run TCP IP in parallel with other things like UDP on the same Ethernet connection 7 Note 2 In the seven layer OSI model of computer networking packet strictly refers to a data unit at layer 3 the Network Layer At this layer a packet is also commonly called a datagram The correct term for a data unit at the Data Link Layer Layer 2 of the seven layer OSI model is a frame and at Layer 4 the Transport Layer the correct term is a segment Hence e g a TCP segment is carried in one or more IP Layer datagrams or packets which are each carried in one or more Ethernet frames though the mapping of TCP IP and Ethernet to the layers of the OSI model is not exact as we ll discuss in the next section Some prefer to refer to all these simply as network packets Network packets are described like Russian dolls a k a Matroishka An IP packet resides within an Ethernet packet A TCP packet resides within an IP packet A HTTP packet resides within a TCP packet See figure below for an illustration of this point Ethernet Packet Receiver Sender Mumber Data M C address MAC address of bytes IP Packet vITHL tos 10 EL fol ttil Prot chs 28092 Recelver Da
18. mes to allow the filter to filter out certain frames before storing them to memory HOST REGISTERS register interface AHB slave TRANSMIT ETHERNET MAC RMI ADAPTER DMA interface AHB master RECEIVE BUFFER ETHERNET PHY RECEIVE FILTER ETHERNET BLOCK Figure 10 Block diagram of the Ethernet block of LPC17xx microcontrollers The fields of the Ethernet packet are as shown in the figure below _ _ _ lt _ _ _ _ so sthemet packet PREAMBLE ETHERNET FRAME T bytes start of frame delimiter 1 byte a I w DESTINATION ADDRESS l Mii I time Figure 11 Fields of the Ethernet packet 11 7 Example 1 EasyWEB This is the EasyWEB example that comes as part of the code bundle from NXP You can also find it inside the downloadable archive with the files of this lab The EMAC project is a simple embedded web server for the NXP LPC17xx microcontrollers also known as EasyWEB This tiny web server was taken from the Design amp Elektronik magazine extra issue Embedded Internet This software was adapted to work with a Keil MCB1700 board and the ARM RealView C Compiler with as few modifications as possible The served web page shows the value of one analog input AN2 which can be controlled via the blue potentiometer on the MCB1700 board Please read the 1_ code bundle Ipc1
19. n network To send a frame a station on an 802 3 network first listens to check if the medium is busy If it is then the station uses the 1 persistent strategy and transmits after only a short fixed delay the inter frame gap after the medium becomes idle If there is no collision then this message will be sent normally If the device detects a collision however the frame transmission stops and the station sends a jamming signal to alert other stations of the situation The station then decides how long to wait before re sending using a truncated binary exponential backoff algorithm The station waits for some multiple of 51 2us slots The station first waits for either 0 or 1 slots then transmits If there is another collision then the station waits for 0 1 2 or 3 slots before transmitting This continues with the station choosing to wait a random number of slots from 0 to 2k 1 if there have been k collisions in the current transmission until k 10 where the number of slots chosen from stops growing After 16 continuous collisions the MAC layer gives up and reports a failure to the layer above Many companies offer Ethernet MAC EMAC Controllers as SW or HW cores 3 Open Systems Interconnection OSI Model The Open Systems Interconnection OSI model is a prescription of characterizing and standardizing the functions of a communications system in terms of abstraction layers Similar communication functions are grouped into logical
20. portant features of TCP IP are a Application support 15 b Error and flow control c Logical Addressing d Routing e Name resolution a Application Support Assume you are multitasking you are uploading files to your website sending an email streaming music and watching video all at the same time How does the computer know where to send each packet of data if multiple applications are running We sure wouldn t want to use our email program to watch video and vice versa This problem is addressed by using channels called ports These numbered ports each correspond to a certain action For example the email is likely using port 25 for operation Files you upload to your website use the FTP port which is usually port 20 and 21 Browsing a webpage uses a specific port the HTTP port 80 In total there are 65 535 ports for controlling the flow of information b Error and Flow Control TCP IP is considered a connection oriented protocol suite This means that if data isn t received correctly a request to resend the data is made This is compared to User Datagram Protocol UDP which is connectionless UDP is a suite of protocols just like TCP IP with a few notable differences UDP is great for broadcasting data such as streaming radio music If part of the data was lost we wouldn t want to go back and retrieve it it would waste bandwidth and would create collisions or noise in our signal With UDP the lost dat
21. rotocols floating around this 1s a good feature to have It also has support for security encryption and data compression 3 Session Layer The fifth layer of the OSI model is the Session Layer It manages communication between applications on a network and is usually used particularly for streaming media or using web conferencing To better see the concepts of the Application Layer let s take a look at a few examples of the Application Layer in action Application Layer APIs A good example of an API is DirectX If you ve ever run a multimedia application and used Windows at the same time odds are you have come into contact with DirectX DirectX is made up of many different components that allow programmers to create multimedia applications such as video games There are many types of APIs You may have heard of NetBIOS Winsock or WinAPI among others The world of APIs has also extended to web services You may have heard of a Google API for instance In this case Google allows developers to use its internal functions yet also keeps Google s internal code safe Network Services The Application Layer handles network services most notably file and printing name resolution and redirector services 14 Name resolution is the process of mapping an IP address to a human readable name You may be familiar with the name Google more so than the IP address of Google Without name resolution we would have to remember four octe
22. s transmitting a 0 bit 0 85v transmitting a 1 bit 0 85v or idle 0 volts Having a normal clock signal as well as an inverted clock signal leads to regular transitions which means that synchronization of clocks is easily achieved even if there are a series of 0 s or l s This results in highly reliable data transmission The master clock speed for Manchester encoding always matches the data speed and this determines the carrier signal frequency so for 10Mbps Ethernet the carrier is LOMHz APPENDIX B How the Application Layer of TCP IP layer stack model works One may ask why an Application Layer is needed for TCP IP since the Transport Layer handles a lot of interfacing between network and applications While this is true the Application Layer focuses more on network services APIs utilities and operating system environments By breaking the TCP IP Application Layer into three separate layers we can better understand what responsibilities the Application Layer actually has The OSI Equivalent of the TCP IP Application Layer 1 Application Layer The seventh OSI model layer which shouldn t be confused with the TCP IP stack s Application Layer It supports network access as well as provides services for user applications 2 Presentation Layer The Sixth OSI model layer is the Presentation Layer It translates data into a format that can be read by many platforms With all the different operating systems programs and p
23. smitted along the medium using voltage levels of 0 85V The table below lists some of the cable types utilized by Ethernet networks The structure of a typical Ethernet cable is shown in the figure below Pins 4 5 7 and amp are not used Fins 4 5 7 and amp are not used Pin number Wire Color Pin number Wire Color Pin 1 gt Orange White wire Becomes Pin 1 gt Orange White Pin 2 gt Orange 1 gt 4 Pin 2 gt Orange Pin 3 gt Green White Pin 3 gt Green White Pin 4 gt Blue 2 2 Pin 4 gt Blue Pin gt Blue White Pin 5 gt Blue White Pin 6 gt Green 3 gt 3 Pin 6 gt Green Pin 7 gt Brown White Pin 7 gt Brown White Pin 6 gt Brown 6 ma 6 Pin 8 gt Brown Pins 4 5 7 and amp are not used a E m Pins 4 5 7 and8 are not used Pin number Wire Color E Crossed Over Pin number Wire Color Pin 1 gt Orange White Pin 1 gt Green White Pin 2 gt Orange 1 3 Pin 2 gt Green Pin 3 gt Green White Pin 3 gt Orange White Pin 4 gt Blue 2 mnb 6 Pin 4 gt Blue Pin gt Blue White Pin gt Blue White Pin 6 gt Green 3 aa Pin 6 gt Orange Pin 7 gt Brown White Pin 7 gt Brown White Pin amp gt Brown 6 gt 2 Pin 8 gt Brown Figure 3 Ethernet cable structure c Media Access Control MAC Operation Ethernet is a CSMA CD Carrier Sense Multiple Access Collision Detectio
24. t PHY using the RMII Reduced Media Independent Interface protocol and the on chip MIIM Media Independent Interface Management serial bus also referred to as MDIO Management Data Input Output The block diagram of the Ethernet block shown in the figure below consists of e The host registers module containing the registers in the software view and handling AHB accesses to the Ethernet block The host registers connect to the transmit and receive data path as well as the MAC e The DMA to AHB interface This provides an AHB master connection that allows the Ethernet block to access on chip SRAM for reading of descriptors writing of status and reading and writing data buffers e The Ethernet MAC which interfaces to the off chip PHY via an RMII interface e The transmit data path including The transmit DMA manager which reads descriptors and data from memory and writes status to memory The transmit retry module handling Ethernet retry and abort situations 10 The transmit flow control module which can insert Ethernet pause frames e The receive data path including The receive DMA manager which reads descriptors from memory and writes data and status to memory The Ethernet MAC which detects frame types by parsing part of the frame header The receive filter which can filter out certain Ethernet frames by applying different filtering schemes The receive buffer implementing a delay for receive fra
25. t would seem that a device only needs to wait for one propagation delay until the message reaches the last receiver to know if a collision has occurred This however is not the case Take for example the following situation A device sends a message which takes propagation delay to reach the last device on the medium This last device on the medium could then send a message just before the original message reaches it 1 e just before 1 propagation delay This new message would take an additional propagation delay to reach the original device which means that this device would not know that a collision had occurred until after 2 propagation delays Collision Detection Knowing how long is needed to wait to discover if a collision has occurred we can use this to increase the effectiveness of CSMA CSMA behaves inefficiently when a collision occurs since both stations continue to send their full packet even though it will be corrupted A simple enhancement to CSMA is the addition of Collision Detection CSMA CD A simple check is made to make sure that the signal present on the medium is the same as the outgoing message If it isn t then a collision is occurring and the message can be aborted This means that the time spent sending the doomed messages can utilized for something else Ethernet Protocol The Ethernet protocol is made up of a number of components a Ethernet frames b Physical Layer c MAC operation a Frame
26. ta IP address IP address TCP Packet Sender Receiver e e Figure 6 TCP packet or segment vs IP datagram or packet vs Ethernet packet or frame A network packet is nothing more than a chunk of data that an application wants to deliver to another system on the network This chunk of data has information added to the front and back that contains instructions for where the data needs to go and what the destination system should do with it once it arrives The addition of this routing and usage information is called encapsulation The figure below illustrates the process We start with a chunk of application data to which we add a header We take that data application data plus application header and package it up as a series of TCP segments by adding TCP headers We then add an IP header to each TCP segment making IP datagram Finally we add Ethernet headers and trailers to the IP datagrams making an Ethernet frame that we can send over the wire Each layer has its own function TCP the transport layer makes sure data gets from point A to point B reliably and in order IP the network layer handles routing based on IP addresses and should be familiar to you and Ethernet the link layer adds low level MAC media access control addresses that specify actual physical devices It s also important to note that there are several choices at each layer of the model at the transport layer you can see either TCP UDP or ICMP Each layer
27. ts of numbers for each website we wanted to visit A redirector otherwise known as a requester is a service that is largely taken for granted It is a handy service that looks at requests a user may make if it can be fulfilled locally it is done so If the request requires a redirection to another computer then the request is forwarded onto another machine This enables users to access network resources just like they were an integral part of the local system A user could browse files on another computer just like they were located on the local computer Lastly we have file and print services If a computer needs to access a file server or a printer these services will allow the computer to do so While fairly self explanatory it s worth reviewing Network Utilities This is where most people have experience within the network utilities section of the Application Layer Every time you use a Ping Arp or Traceroute command you are taking full advantage of the Application Layer It s quite convenient that the Application Layer is located on the top of the TCP IP stack We can send a Ping and if successful can verify that the TCP IP stack is successfully functioning It s a good idea to commit each utility to memory as they are very useful for maintaining configuring and troubleshooting networks Listed below are seven of the most used utilities 1 ARP Arp stands for Address Resolution Protocol It is used to map an IP
28. years Ethernet has continued to evolve with 10Base5 using thick coaxial cable approved in 1986 10Base2 using cheaper thin coaxial cable approved in 1986 Twisted pair wiring was used in 10BaseT approved in 1991 and fiber optic in 1OBaseF approved in 1994 95 In 1995 100Mbps Ethernet was released increasing the speed of Ethernet which has since been further increased with the release of Gigabit Ethernet in 1998 99 In 2002 100 Gigabit was published and recently 100 Gigabit Ethernet or 100GbE and 40 Gigabit Ethernet or 40GbE emerged and were first defined by the IEEE 802 3ba 2010 standard In the future Ethernet will continue to increase in speed Broadcast Network Operation Ethernet is a Broadcast Network hosts are connected to a network through a single shared medium This has the advantage that messages don t have to be routed to their destination as all hosts are present on the shared medium but it does incur another set of problems The main problem which needs to be addressed is that of Media Access Control MAC or giving fair access to multiple nodes on a shared medium Collisions When a number of nodes are connected to a single shared medium one of the issues is the possibility of two or more nodes trying to broadcast at the same time This is called a collision and prevents any information passing along the network because the multiple messages would corrupt each other destroying both There are two main methods for reducing the
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