Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Recursive InterNetwork Architecture (RINA) wikipedia , lookup
Point-to-Point Protocol over Ethernet wikipedia , lookup
IEEE 802.1aq wikipedia , lookup
Piggybacking (Internet access) wikipedia , lookup
Computer network wikipedia , lookup
Zero-configuration networking wikipedia , lookup
Network tap wikipedia , lookup
Airborne Networking wikipedia , lookup
An Introduction to Computer Networks Lecture 5: Direct Link Networks University of Tehran Dept. of EE and Computer Engineering By: Dr. Nasser Yazdani 1 Outline Issues ALOHA Network Ethernet Token Ring Wireless Univ. of Tehran Introduction to computer Network 2 Main Issues Local Area Network (LAN) : Three or more machines are physically connected and communicating. Problems: How to connect them? Topology How to address each machine? Addressing How to regulate accessing to the media? Sharing links MAC (Media Access method or protocol) Different technology address each problem in different way. Problems are not independent Univ. of Tehran Introduction to computer Network 3 LAN Technologies. Session Transport Network Link Physical The 7-layer OSI Model TCP TFTP HTTP NNTP Presentation Telnet FTP SMTP Application UDP IP LAN-LINK The 4-layer Internet Model Link layer can have two types of technologies; • Point to point link like PPP where there are only 2 nodes. • Broadcast link like Ethernet when there are more than 2 nodes. Univ. of Tehran Introduction to computer Network 4 Data link sublayers Our focus will be on MAC sublayer. MAC = “Medium Access Control” Multiplexing Media Access (MAC) Error Detection Framing • The link is shared among different sender and receivers. • Since every frame is simultaneously accessed by different nodes; •They are called multiaccess links. •They are called broadcast links. (important) •LAN because of limited area. • We need some type of medium access rules to avoid collision. • Multicast capability of LANs. Univ. of Tehran Introduction to computer Network 5 Simple Random Examples of MAC Protocols Packet-Switched Radio Network Aloha Carrier Sense Multiple Access/Collision Detection Complex Deterministic Ethernet (IEEE 802.3) Token Passing Token Ring (IEEE 802.5) Fiber Distributed Data Interface (FDDI) Wireless Univ. of Tehran Introduction to computer Network 6 Goals of MAC Protocols MAC Protocols arbitrate access to a common shared channel among a population of nodes Goals: 1. Fair among users 2. High efficiency 3. Low delay 4. Fault tolerant Univ. of Tehran Introduction to computer Network 7 Aloha Protocol All nodes transmit on one freq. Central Node Central node relays packets on the other frequency f0 f1 If more than one node transmit at the same time Collision! If there is a collision, both nodes re-transmit packets Univ. of Tehran Introduction to computer Network 8 CSMA/CD Protocol All nodes transmit & receive on one channel Packets are of variable size. 1. Carrier Sense: Check if the line is idle before transmitting. 2. Collision Detection: If more than one node transmit. Collision! All nodes detect collision, wait for random delay. Goto 1. binary exponential backoff Univ. of Tehran Introduction to computer Network 9 CSMA/CD Network Size Restriction Node must be able to hear that there is a collision before its packet is transmitted completely. i.e. Packet Transmission Time > Round trip propagation time i.e. TRANSP > 2.PROP Univ. of Tehran Introduction to computer Network 10 Performance of CSMA/CD Assume time-slotted channel 1. Find ( p): Probability that exactly one node transmits in a given slot, where: p = Prob{a node tries to transmit a packet in a time slot}, N = number of nodes N ( p) p(1 p) N 1 1 d N (1 p ) N 1 pN ( N 1)(1 p ) N 2 dp max 36% 40% when : p 1 / N Univ. of Tehran Introduction to computer Network 11 Ethernet Overview History developed by Xerox PARC in mid-1970s roots in Aloha packet-radio network standardized by Xerox, DEC, and Intel in 1978 similar to IEEE 802.3 standard Uses CSMA/CD technique for Media access. Uses 10Mbps physical link originally and now extended to 100Mbps, Fast Ethernet, and recently to 1000Mbps, Gigabit Ethernet. Uses variable frame length, 64-1500 bytes. Univ. of Tehran Introduction to computer Network 12 The Original Ethernet Repeaters every 500m 10Mb/s l 1500m PROPmax l / c 1500 / 2.5 108 6s Thick copper coaxial cable TRANSP 2 PROP TRANSP 12s Packetsize 12s 10Mb / s 120bits In practice, minimum packet size = 512 bits. • allows for extra time to detect collisions. • allows for “repeaters” that can boost signal. Univ. of Tehran Introduction to computer Network 13 The Original Ethernet Original picture drawn by Bob Metcalfe, inventor of Ethernet (1972 – Xerox PARC) The Ethernet protocol is implemented in Contoroler (Adaptor) Univ. of Tehran Introduction to computer Network 14 Ethernet Frame Format Bytes: 7 1 6 6 Preamble SFD DA SA 2 Type 0-1500 Data 0-46 4 Pad CRC 1. Preamble: trains clock-recovery circuits 2. Start of Frame Delimiter: indicates start of frame 3. Destination Address: 48-bit globally unique address assigned by manufacturer. 1b: unicast/multicast 1b: local/global address 4. Type: Indicates protocol of encapsulated data (e.g. IP = 0x0800) 5. Pad: Zeroes used to ensure minimum frame length 6. Cyclic Redundancy Check: check sequence to detect of Tehran Introduction to computer Network 15 bitUniv. errors. Ethernet Addresses Unique, 6 bytes or 48-bit address assigned to each adapter by manufacturer. It is read in : notation, for example: 8:0:e4:b1:2 An address with all 1s is a broadcast address. multicast: first bit is 1 In order to make the address unique, first 24 bits are assigned to manufacturers and the last 24 bits are assigned locally. Each adaptor accept the packet if the destination address is its own address, broadcast address or multicast to which this adaptor belongs. Univ. of Tehran Introduction to computer Network 16 The 10Mb/s Ethernet Standard IEEE 802.3 Ethernet MAC Protocol 10Base-5 Different physical layer options 10Base-2 10: 10Mbs 10Base-T 10Base-F Base: baseband 5: 500 Meter 10Base-5: Original Ethernet: large thick coaxial cable. 10Base-2: Thin coaxial cable version. 10Base-T: Voice-grade unshielded twisted-pair Category-3 telephone cable. 10Base-F: Two optical fibers in a single cable. Univ. of Tehran Introduction to computer Network 17 10Base-T “Twisted pair Ethernet” Repeater “Hub” 100m max cable length Router Designed to run over existing voice-grade “Category3” twisted pair telephone wire. Centralized management (“managed hubs”) lead to more reliability. Created a huge increase in Ethernet usage. Univ. of Tehran Introduction to computer Network 18 Transmit Algorithm If line is idle… send immediately upper bound message size of 1500 bytes must wait 9.6us between back-to-back frames If line is busy… wait until idle and transmit immediately called 1-persistent (special case of p-persistent) (sending with probability of p) Univ. of Tehran Introduction to computer Network 19 Algorithm (cont) If collision… jam for 32 bits, then stop transmitting frame (minimum frame is 64 bytes (header + 46 bytes of data)) delay and try again 1st time: 0 or 51.2us 2nd time: 0, 51.2, or 102.4us 3rd time51.2, 102.4, or 153.6us nth time: k x 51.2us, for randomly selected k=0..2n - 1 give up after several tries (usually 16) exponential backoff Univ. of Tehran Introduction to computer Network 20 Increasing the data rate Increasing the data rate create the following Problem: E.g. CSMA/CD at 100Mb/s over 1500m of TRANSP 2PROP cable: 8 PROP 1500 / 2.5 10 6s TRANSP 12s Packetsize 1200bits To overcome this two techniques used: Cable length limited to 100m: PROP 200 / 2.5 108 Packetsize 160bits Use “Ethernet Switching” to prevent collisions. Univ. of Tehran Introduction to computer Network 21 Ethernet Switching An Ethernet switch is the same as an Ethernet Bridge. A Bridge: Examines the header of each arriving frame. If the DA is in its table, it forwards the frame to the correct output port. If the DA is not in its table, it broadcasts the frame to all ports (except the one through which it arrived). The table is learned by examining the SA of arriving packets. Univ. of Tehran Introduction to computer Network 22 Ethernet Switch Ethernet Switch/Bridge Router • If only one computer per port, no collisions can take place (each cable is now a self-contained point-to-point Ethernet link). • Capacity is increased: the switch can forward multiple frames to different computers at the same time. • An Ethernet switch must contain buffers to hold Univ. of Tehran Introduction to computer Network 23 frames during times of congestion. Ethernet Switch Ethernet Switch/Bridge Router Ethernet Hub • If only one computer per port, no collisions can take place (each cable is now a self-contained point-to-point Ethernet link). • Capacity is increased: the switch can forward multiple frames to different computers at the same time. • An Ethernet switch must contain buffers to hold frames during times of congestion. Univ. of Tehran Introduction to computer Network 24 Token Ring Listen: Talk: Data l4 l1 l3 l2 Token/Data PROP i li / c TRTmin TRT=Token Rotation Time Univ. of Tehran Introduction to computer Network 25 Token Ring (cont) It is like people talking in a ring in the round robin manner. Common features. Frames flow in one direction: upstream to downstream special bit pattern (token) rotates around ring must capture token before transmitting release token after done transmitting immediate release delayed release remove your frame when it comes back around stations get round-robin service Univ. of Tehran Introduction to computer Network 26 Timed Token Algorithm Token Holding Time (THT) Token Rotation Time (TRT) how long it takes the token to traverse the ring. ActiveNodes x THT + RingLatency TRT Target Token Rotation Time (TTRT) (FDDI) upper limit on how long a station can hold the token agreed-upon upper bound on TRT Each node measures TRT between successive tokens if measured-TRT > TTRT: token is late so don’t send if measured-TRT < TTRT: token is early so OK to send Univ. of Tehran Introduction to computer Network 27 Token Maintenance Lost Token no token when initializing ring bit error corrupts token pattern node holding token crashes Solution- have a monitor Monitor role in the link Generating Tokens Announces its presence periodically Check for the corrupt or orphaned frames and remove them from the ring. Orphaned frame are those whose sending station have died. Sets the monitor bit to 0 in sending and to 1 when pass the monitor. Univ. of Tehran Introduction to computer Network 28 Token Maintenance (cont) How about when the monitor dies? Or the network just powered up? Any station tries to become monitor send a claim frame that includes the node’s TTRT bid if your claim frame makes it all the way around the ring: Everyone has accepted you as the monitor everyone knows TTRT you insert new token How about receiving another monitor claim at the same time? Break the tie with The lowest TTRT bid wins. The highest address wins. Etc. Univ. of Tehran Introduction to computer Network 29 Maintenance (cont) Monitoring for a Valid Token should periodically see valid transmission (frame or token) maximum gap = ring latency + max frame < = 2.5ms set timer at 2.5ms and send claim frame if it fires Univ. of Tehran Introduction to computer Network 30 Release After Reception (RAR) Computer captures token, transmits data, waits for data to successfully travel around ring, then releases token again. Allows computer to detect errored frames and retransmit them. Example time evolution in which host 1 and host 3 have packets to transmit: TRANSP PROP TRANST Data Token l1/c l2/c Token arrives at host 1 Univ. of Tehran TRANST lN/c TRANSP Data Token l1/c l2/c Token departs Token arrives at host 3 from host 1 Token arrives at host 2 Introduction to computer Network l3/c time 31 Release After Transmission (RAT) Computer captures token, transmits data, then releases token again. FDDI uses this technique. Example time evolution in which host 1 and host 3 have packets to transmit: TRANSP Data TRANST TRANST Token l1/c Token arrives at host 1 Univ. of Tehran Token departs from host 1 Token TRANSP Data l2/c Token arrives at host 3 Token time Token arrives at host 2 Introduction to computer Network 32 Frame format Bytes 1 Start Delimt. 1 Access control 1 Frame control 6 6 Dest. Addr Src. Addr Variable Body 4 Checks um 1 1 End Delimt. Frame status Access control is for priority. Frame control is a Demux key for the higher layer protocol. Addresses are like Ethernet. They can also be 16 bit. Frame Status include two A and C bits. A, Active bit, is set by the receiver indicating the station is alive and has seen the frame. C, Copy bit, is also set by the receiver indicating the frame has been copied. Univ. of Tehran Introduction to computer Network 33 FDDI: Fiber Distributed It is a Dual counter-rotating ring for fault tolerance. It can be also Single Attachment (SAS- Single Attachment Station). 100 Mbps on optical fibers Up to 500 nodes Total length less than or equal to 200 km Uses 4B/5B encoding. Modulation: non-return to zero with inversion (NRZI) Univ. of Tehran Introduction to computer Network 34 FDDI Timed Token Rotation Protocol 1. All hosts agree on a common Target Token Rotation Time (TTRT). They will aim to make the token rotate around the network at least once per TTRT. Hence, they can each expect to see the token once TTRT. 2. Each host on the network maintains a timed token Rotation (TRT) timer, that indicates when the token is next expected to arrive. 3. If the token arrives before TRT expires, we say it is “Early”. If the token arrives after TRT expires, we say it is “Late”. 4. A host can only transmit if it receives the token, AND the token is Early. Univ. of Tehran Introduction to computer Network 35 Wireless LANs It is shared media like Ethernet The standard is IEEE 802.11 Bandwidth: 1 or 2 Mbps Physical Media spread spectrum radio (Up to 2.4GHz)spread signal over a wider frequency band diffused infrared (10m), sender and receiver do not have to aimed at each (up to 10m, bluetooth tech). Univ. of Tehran Introduction to computer Network 36 Spread Spectrum Idea is to spread signal over wider frequency band than required Frequency Hopping transmit over random sequence of frequencies sender and receiver share… pseudorandom number generator seed 802.11 uses 79 x 1MHz-wide frequency bands Univ. of Tehran Introduction to computer Network 37 Spread Spectrum (cont) Direct Sequence for each bit, send XOR of that bit and n random bits random sequence known to both sender and receiver The sent code is called n-bit chipping code 802.11 defines an 11-bit chipping code 1 0 Data stream: 1010 1 0 Random sequence: 0100101101011001 1 0 XOR of the two: 1011101110101001 Univ. of Tehran Introduction to computer Network 38 Collisions Avoidance It is similar to Ethernet, but there is mobility problem here and also Problem: hidden and exposed nodes B can exchange data with A and C, but not D., How A and C send to B, they are not aware, this is the hidden node problem. If B send to A, C can send to D, exposed problem. A Univ. of Tehran B C Introduction to computer Network D 39 MACA MACA- Multiple Access with Collision Avoidance The idea is the sender and receiver to exchange control information. Univ. of Tehran Introduction to computer Network 40 MACA (cont) Sender transmits RequestToSend (RTS) frame Receiver replies with ClearToSend (CTS) frame Neighbors… Receiver sends ACK when has the frame see CTS: keep quiet see RTS but not CTS: OK to transmit neighbors silent until see this ACK Collisions if two sender transmit RTS at the same time. no collisions detection known when don’t receive CTS exponential backoff Univ. of Tehran Introduction to computer Network 41 Supporting Mobility Case 1: ad hoc networking when node may or may not be able to communicate. Case 2: access points (AP) connected node to wire each mobile node associates with an AP Distribution system AP-1 AP-3 F AP-2 A B G H C Univ. of Tehran E D Introduction to computer Network 42 Mobility (cont) Scanning (selecting an AP) node sends Probe frame all AP’s w/in reach reply with ProbeResponse frame node selects one AP; sends it AssociateRequest frame AP replies with AssociationResponse frame new AP informs old AP via network When active: when join or move passive: AP periodically sends Beacon frame Univ. of Tehran Introduction to computer Network 43 Network Adaptors All functionalities are implemented in Adaptors or network cards. Each vender has it own adaptor. Host I/o bus Univ. of Tehran Bus interface Link Interface Network Adaptor Introduction to computer Network 44 Network Adaptors (cont) Adaptor, like other devices, are programmed by CPU. Adaptor has a Control Status Register (CSR), usaully located in the memory. CPU communicate with Adaptor through CSR. Two methods for Communication, polling and interrupt. Univ. of Tehran Introduction to computer Network 45 Network Adaptors (cont) How to transfer data? Direct memory access (DMA) and programmed I/O (PIO) Device drivers are routines to connect OS with the network hardware. Memory is bottleneck. Each frame might be written/read several times from the memory. It has limited Bandwidth, usually 32 bit x 100 MHz, (around 3.2 Gbps), however, each packet goes at least two time and there are overheads. Univ. of Tehran Introduction to computer Network 46