Data Link Layer. Today s News. Data Link Layer. Node Configuration

Transcription

1 Today s News Topics Comm., Networks, Architecture, Protocol(Sept. 4) PD: 1.3 Multiplexing, Queueing (Sept. 6, Sept. 11) PD: 1.2, 1.5 Layer 2 Data Link (Sept. 11 supp.) PD: 2.1 Socket programming HW description (TA) Error control (Sept. 13, Sept. 18) PD: 2.5 First Quiz is on Sept. 18 May bring one A4 sized reference note Should be original & handwritten Post prior year s Quiz, exams on the homepage Data Link Layer Chong-kwon Kim SNU SCONE lab. 1 Data Link Layer Data link layer is concerned with direct link networks Two nodes directly connected by a single point-topoint link Node Configuration Network adapter connects to I/O bus & network link Device driver manages a network adapter Manage message passes between main memory and network adapter buffer CPU A broadcast link segment with multiple attachments Cache Network Adapter (To network) Different names at different layers Frame: L2 Packet, Datagram: L3 Segment: L4 Message: L7 SNU SCONE lab. Memory I/O bus Example OpenWRT: An embedded kernel for WLAN AP ath9k: Device driver for Atheros WiFi chipsets SNU SCONE lab.

2 Data Link Layer Functions Framing Error control Flow control Medium access control (MAC) LAN Standard protocols HDLC (High-level Data Link Control) - ISO LAPB (Link Access Protocol Balanced) - ITU LLC (Logical Link Control) - IEEE Type 1, 2, 3 Error Control Chong-kwon Kim SNU SCONE lab. Error Control First, detect error occurrences and then ensure correct transmission Error types & detection methods Bit error Error detection code such as CRC Lost or duplicated frames Use sequence numbers (SN) that uniquely identify frames Principle A sender retransmits a frame repeatedly until it assures that correct one is transferred Two methods Positive Acknowledgement (ACK) + Automatic retransmission based on Timeout (TO) Retransmission based on Negative Acknowledgement (NAK) SNU SCONE lab. 7 ACK + Automatic Retransmission Basic action Sender Receiver ACK Automatic Retransmission based on TO Sender Receiver x TO 1 What happens if packet / ACK is erred / lost? SNU SCONE lab. 8

3 ACK + Automatic Retransmission Sender Receiver 1 TO x 1 The ACK is lost and the receiver receives the same frame What would you do? 1. Do nothing The sender will retransmit the same frame infinitely 2. Resend the ACK ACK + Automatic Retransmission Sender Receiver TO The frame #2 is lost, but two ACKs are received Should distinguish ACKs Use Request Number (RN) RN specifies the next packet expecting instead of the last correctly received packet x Sender TO SNU SCONE lab. 9 Receiver 2 2 x 10 NAK Based Method Sender Receiver x 1 ARQ (Automatic Repeat request) Stop-and-wait ARQ Continuous ARQ Go-back-N ARQ Selective-repeat ARQ When to send NAKs? Receive corrupted frames How do you know that a corrupted frame is the frame? Receive out of order frames Error control is triggered by following frames What if there is a long gap between frames? What if there is no next frame? Lost NAK NAK is used only as a supplementary mechanism SNU SCONE lab. 11 SNU SCONE lab. 12

4 Today s News Topics Layer 2 Data Link (Sept. 11 supp.) PD: 2.1 HW description (TA) Error control (Sept. 13) PD: 2.5 Socket programming (TA) MAC (Sept. 18, Sept. 18 Supp.) PD: 2.6 WLAN (Sept. 29, Oct. 6) PD: 2.7 First Quiz is on Sept. 18 May bring one A4 sized reference note Should be original & handwritten Post prior year s Quiz, exams on the homepage (Slightly) revised class note will be post by tomorrow Stop & Wait ARQ Chong-Kwon Kim SNU SCONE lab. 13 Stop & Wait ARQ - 1 Stop and Wait Send a single packet(frame) and stop & wait for an Ack. Stop & Wait ARQ - 2 Sender Algorithm Step 0: Set SN = 0 Step 1: Transmit a frame, Set the timer, Wait for ACK with right RN Step 2: One of two events occurs A: If receive right ACK before TO, Set SN (SN+1) mod 2, Goto Step 1 B: If TO, Goto Step 1 to retransmit Receiver Algorithm 15 Step 0: Set RN = 0 Step 1: Wait for a frame Step 2: Packet arrives A: If errorless & SN = RN, Accept the frame, RN (RN+1) mod 2, Send ACK B: Discard the frame, Retransmit ACK

5 Stop & Wait ARQ - Example Three Army Problem SN TO Two Generals Problem (Byzantine Generals Problem ) RN State SN represents the sender s state RN represents the receiver s state Combined state = (SN, RN) State transitions (0, 0) -> (0, 1) -> (1, 1) -> (1, 0) -> (0, 0) (0, 0) -> (0, 1), (1, 1) -> (1, 0) : Receiver knows the combined state (0, 1) -> (1, 1), (1, 0) -> (0, 0): Sender knows the combined state Sender & Receiver do not knows the combined state simultaneously SNU SCONE lab. Utilization (Efficiency) of Stop & Wait Propagation time : time taken for signal to travel from S to R. Thus first bit transmitted at t=0 arrives at R at t=t p =d/v Transmission time : time taken to emit all bits of frame at sender=t t =L/B 19 SNU SCONE lab. 20

6 Derive Utilization using the Little s Theorem With stop & wait scheme, for high channel utilization, we need a low α (since U=1/(1+2 α )) Go-Back-N ARQ In practice, it is not desirable to increase length L indefinitely Error probability increases with L Starvation/high avg. delay with multipoint lines Realtime applications short packet assembly time A more efficient scheme is called for, especially with HSN/WAN/satellite communication Chong-Kwon Kim 21 Go-Back-N Send multiple packets without receiving ACKs N: Window size Maximum number of unacked packets that can be sent Stop and Wait is a special case of Go-back-N Window: Packets that have been sent (but unacked) + that can be sent Window size = N MinSN: First packet that are not ACKed yet MaxSN: Highest packet that can be sent MaxSN = MinSN+N-1 SN: Next packet to be sent Example: N=8, MinSN = Go-Back-N Algorithm Sender algorithm Step 0: Set MinSN = SN = 0, MaxSN = MinSN+N-1, Send a frame, Set the timer Step 1: Do one of three events forever A: If SN < MaxSN, Set SN SN+1, Transmit the next frame, (Set the timer) B: If ACK with RN > MinSN arrives, Set MinSN RN, MaxSN MinSN+N-1 C: If TO, Retransmit (Which frames?) Receiver algorithm Step 0: Set RN = 0 Step 1: Receive frames forever A: If the frame is error-free and RN = SN, Accept the frame, RN RN+1, Transmit ACK B: O.W., Retransmit ACK Sent but not Acked yet SNU SCONE lab.

7 Quiz Solutions Quiz Solutions Suppose users share a 2 Mbps link. Also suppose each user transmits continuously at 1 Mbps when active, but each user is active only 20% of the time. a. When FDM (or Synchronous TDM) is used, how many users can be supported? (Ignore framing or guard band overheads) b. Suppose we use synchronous TDM. A framing bit alternates 0 and 1. The receiver picks one position and regards the position is the framing bit if 0/1 bit alternates N times continuously. Given that 0 and 1 bits in user data appear randomly and in equal probability, compute the probability that the receive makes a wrong decision. c. Assume that user data is packetized into 11Kbits long packets. Each packet consists of 10 Kbit paylord and 1 Kbit header. How many packets does each user generate per second? Also compute the transmit time of each packet. d. Suppose that the link is shared by three users based on statistical (asynchronous) TDM. Compute the delay including the transmit time assuming the M/M/1 queueing. (Note that the system cannot be analyzed via M/M/1 system since the service time is not Exponential, but Deterministic) SNU SCONE lab. 25 SNU SCONE lab. 26 Quiz Solutions Today s News Topics Layer 2 Data Link (Sept. 11 supp.) PD: 2.1 Error control (Sept. 13, Sept. 18) PD: 2.5 MAC (Sept. 18 Supp, Sept. 20) PD: 2.6 WLAN (Sept. 25, Sept 25 Supp) PD: 2.7 Supplement class at 7:00 PM today SNU SCONE lab. 27 SNU SCONE lab. 28

8 Go-Back-N Example (N=4 case) [0, 3] [1, 4] [2, 5] SN TO ACK Semantics Cumulative ACK Compare to Point ACK ACK (RN) acknowledges packets up to (RN-1) Lost ACKs may be recovered RN Properties of Go-Back-N Properly designed, Go-Back-N achieves 100% utilization Right size of N? In case of a single packet loss/error Retransmit packets [MinSN, SN] How about an ACK loss/error? How about out-of-sequence packets? SNU SCONE lab. [0, 3] [2, 5] [4, 7] [5, 8] SN TO RN SNU SCONE lab. Link Utilization of Go-Back-N Utilization : U is a function of a and N Case 1 : N > 1 + 2a Frame 1 ACK reaches to the sender before transmission of N th frame continuous transmission possible Case 2 : N < 1 + 2a : U = N / (1+2a) Wasted time between N and 1 + 2a SNU SCONE lab. 31 SNU SCONE lab. 32

9 YA ARQ: Selective Repeat Survey the Internet and learn Selective Repeat ARQ Medium Access Control Basic & ALOHA Chong-kwon Kim SNU SCONE lab. 33 Taxonomy of Medium Access Methods MAC protocols can be considered as distributed ATDM Contention Based Contention Based Method Every station is a peer No master-slave relation Fully distributed, non-coordination method Judge & behave based on locally available information Stations use care not to interfere other But should be aggressive to earn the fair share Collisions still can occur CSMA/CD Aloha CSMA CSMA/CA SNU SCONE lab. 35 SNU SCONE lab. 36

10 ALOHA - 1 Proposed by Abramson in 1970 Implemented and used at the Univ. of Hawaii U. Hawaii campuses ALOHA - 2 Transmit a frame and check if the frame is transmitted correctly ACK Retransmit in case of no ACK Example SNU SCONE lab. 37 SNU SCONE lab. 38 Today s News Topics Layer 2 Data Link (Sept. 11 supp.) PD: 2.1 Error control (Sept. 13, Sept. 18) PD: 2.5 MAC (Sept. 18 Supp) PD: 2.6 WLAN (Sept. 20, Sept 25) PD: 2.7 Selected problems will be posted on the class homepage tomorrow ALOHA Performance Analysis Tfr = 1(unit time) A is successful if No attempt in (t-1, t) && (t, t+1) n stations each with p attempt rate Total attempt rate = n p A frame sent at time t is successful if only one attempt in (t, t+1) and no attempt in (t-1, t) Binomial distribution, Bin(x; n, p) = ncx 1 P(success) = b(1; n, p) b(0; n, p) = nc1 1 nc0 1 = 1 SNU SCONE lab. 39 SNU SCONE lab. 40

11 ALOHA Performance Analysis Note b(x; n, p) Po(x; G) where G = n p if n is large and p is small The throughput for pure ALOHA is S = G e 2G. The maximum throughput S max = when G = (1/2) ALOHA - 3 Procedure What happens if G=n p is too small OR too large? SNU SCONE lab. 41 SNU SCONE lab. 42 Problem of ALOHA Problem with Pure (Unslotted) Aloha An attempt may be interfered by later attempts Vulnerable time is long Slotted ALOHA & CSMA Chong-kwon Kim SNU SCONE lab. 44

12 Slotted Aloha - 1 Solution Synchronize the transmissions How to synchronize stations? (know the beginning of slots) Slotted Aloha - 2 A station that has a packet to send, waits until the next slot Success Idle Idle Collision Time A slot is either Idle, Success or Collision Idle: No arrival during the previous slot time Success: Exactly one arrival Collision: More than one arrival SNU SCONE lab. 45 SNU SCONE lab. 46 Slotted Aloha Performance Analysis A slot is successful if There is exactly one attempt during the previous slot Pr(1 arrival in a unit time) = G e -G Slotted Aloha - Problem Slotted Aloha is unrealistic Synchronization is a big problem One solution is to have a control node and the node transmits beacons that announce the beginning of slots Or adopt NTP (Network Time Protocol) A centralized controller brings out other problems The throughput for slotted ALOHA is S = G e G. The maximum throughput S max = when G = 1 SNU SCONE lab. 47 SNU SCONE lab. 48

13 CSMA (Carrier Sense Multiple Access) - 1 Success Idle Idle Collision Time Two problems of slotted Aloha Long Idle slot Long Collision slot How to shorten the Idle slot? Pure Aloha blindly accesses the medium even though there are ongoing transmissions On the other hand, slotted Aloha blindly postpones transmission even though the medium is idle Check the medium and transmit frames immediately if the medium is idle, not waiting until the next slot Carrier sensing SNU SCONE lab. 49 CSMA - 2 How to determine if the medium is idle or not? Measuring the signal strength, voltage level CSMA Listen-before-talk Success Idle Idle CCA: Clear Channel Assessment RSSI: Received Signal Strength Ind. Collision Time Time SNU SCONE lab. 50 Performance P-Persistent CSMA CSMA/CD Chong-kwon Kim SNU SCONE lab. 51

14 CSMA Collision CSMA still suffers from collisions CSMA/CD (Collision Detection) Recall the second problem of Slotted Aloha Long Collision Slot How to shorten the length of collision slots? Method While transmitting a frame, listen if there is a collision If collision, terminate the transmission immediately How to detect collisions? In the case of a single transmission, the voltage level confined within a certain level Two or more concurrent transmissions cause higher than the normal voltage level How about Wireless link? 53 SNU SCONE lab. 54 CSMA/CD CD Duration How long should we listen for collision? Two times of the maximum propagation delay CSMA/CD Procedure SNU SCONE lab. 55 Failed transmission SNU SCONE lab. 56

15 Further (personal) investigations Various persistence methods Non/1/p-persistent The speed of Ethernet increased from 10 Mbps 100 Mbps 1 Gbps 10 Gbps 100 Gbps. Problem of high speed Ethrrnets. Solutions. Today s News Topic MAC (Sept. 18, Sept.) PD: 2.6 WLAN (Sept. 20, Oct. 25) PD: 2.7 Layer 3 Inter-networking, Bridge (Oct. 25 Supp.) PD: 3.1 Modern layer 2 switches no longer use CSMA/CD. They use switch fabrics that enable multi-point to multi-point dedicated full-duplex connections. SNU SCONE lab. 57 SNU SCONE lab. 58 Wireless Characteristics Performance of wireless links is much lower than that of wired links Wireless LAN WHY? Attenuation Chong-kwon Kim Root cause: Attenuation of wireless medium is much higher than that of wired medium SNU SCONE lab. 60

16 IEEE WLAN Standard IEEE a/b/g (1997) 2.4 GHz DS 1 & 2 Mbps1 & 2 Mbps.11b CCK 5.5 & 11 Mbps 2.4 GHz FH IR 5 GHz.11a OFDM 6~54 Mbps 1999 (up to 11 20MHz) TCP IP LLC MAC PLCP PMD PHY Three physical layer specifications operating at 1 and 2 Mbps Two additional parts in a and b IEEE b 2.4-GHz band at 5.5 and 11 Mbps Complementary code keying (CCK) modulation Input data treated in blocks of 8 bits at MHz 8 bits/symbol MHz = 11 Mbps.11g OFDM 6~54 Mbps 2003 (Both 2.4 GHz & 5 GHz) SNU SCONE lab. 61 SNU SCONE lab PHY a 5-GHz band up to 54 Mbps Uses orthogonal frequency division multiplexing (OFDM) Multiple carrier signals at different frequencies Data rates 6, 9, 12, 18, 24, 36, 48, and 54 Mbps Transmission Rates AMC: Adaptive Modulation & Coding Learn: Performance Anomaly Problem IEEE g (2002) Extends IEEE b to higher data rates Combines a and b IEEE n (2006) 100 Mbps 63 SNU SCONE lab. 64

17 Topology Infrastructure Mode All packets pass through the Access Point (AP) No direct communications between Mobile Stations (MS, or client/host) Ad-hoc mode Peer-to-peer (station-to-station) direct communications IEEE Architecture AP AP AP SNU SCONE lab. 65 SNU SCONE lab. 66 IEEE BSS / ESS Smallest building block is Basic Service Set (BSS) One AP and multiple stations Same MAC protocol Competing for access to same shared wireless medium BSSID (= MAC addr. of the AP) ESS (Extended Service Set) Two or more BSSes interconnected by DS Appears as a single logical LAN to LLC AP Logic within station that provides access to DS Provides DS services in addition to acting as station Portal Integrate IEEE architecture with wired LAN Wireless LAN - Signal & Interference Chong-kwon Kim SNU SCONE lab. 67

18 Basic Operation In CSMA/CD, we can be almost sure that a frame is delivered successfully in case of no collision No longer true in wireless networks How about CS Collision Detection is impossible In wireless links? 1. How do you know a frame is successfully transmitted? Use ACK Data 2. No ACK Retransmission A ACK should be transmitted before any other frames SNU SCONE lab. 69 Wireless Characteristics - CS Carrier sense Detect EM signal strength In wired networks, Sender s CS Receiver s CS O R T In wireless networks, CS at sender CS at receiver O R How do you know the receiver s channel status? T SNU SCONE lab. 70 Hidden / Exposed Terminal Hidden Terminal Problem C cannot sense BA transmission and attempts to send a frame RTS/CTS & Virtual CS Request To Send (RTS) & Clear To Send (CTS) Short control frames that specifies the intention to transmit (RTS) and willingness to receive (CTS) Specifies the remaining time of the transaction in RTS/CTS frames Network Allocation Vector(NAV) Exposed Terminal Problem C can send to D without disturbing AB transmission But, C thinks the medium is busy NAV NAV SNU SCONE lab. 71 SNU SCONE lab. 72

19 RTS/CTS & Hidden Terminal RTS/CTS & Exposed Terminal RTS/CTS can mitigate the hidden terminal problem SIFS Suppose you hear an RTS but not a CTS, what will you do? How long should you wait before sending your frame? Suppose you hear a CTS without RTS, what will you do? SNU SCONE lab. 73 RTS/CTS exchange cannot solve the exposed terminal problem SNU SCONE lab. 74 Today s News Topic MAC (Sept. 18, Sept.) PD: 2.6 WLAN (Sept. 20, Oct. 25) PD: 2.7 Layer 3 Inter-networking, Bridge (Oct. 25 Supp.) PD: 3.1 Supplementary class 7:00 PM this evening (Slightly) Revised note will be post today Wireless LAN -Protocol Chong-kwon Kim SNU SCONE lab. 75

20 Inter- Frame Space (IFS) An ACK should be immediately delivered IFS: Minimum time that the medium should be idle to transmit a frame SIFS (short IFS): For all immediate response actions RTS-CTS, DATA-ACK, Poll response PIFS (point coordination function IFS): Used by the centralized controller in PCF scheme when issuing polls DIFS (distributed coordination function IFS): Used as minimum delay for asynchronous frames contending for access Media Access Control Distributed wireless foundation MAC (DWFMAC) Distributed access control mechanism Lower sublayer is distributed coordination function (DCF) Contention algorithm to provide access to all traffic Point coordination function (PCF) Centralized MAC algorithm Contention free If SIFS < DIFS, then data frames cannot intervene DATA-ACK transaction SNU SCONE lab. 77 SNU SCONE lab. 78 DCF Basics DCF Basic DATA The DATA frame should wait at least DIFS to protect ACK, CTS and etc A frame that arrives while the medium is busy should wait extra in addition to DIFS The extra waiting time should be randomized In the case of collision, should reduce attempt rate SNU SCONE lab. 79 New packet Medium = Idle DATA New packet Medium = Busy DIFS Idle for DIFS Backoff counter = Rand[0, CW] Random Backoff DATA Collision? Slot, Δ Backoff counter = 0 DIFS After DIFS, decrease backoff Idle for DIFS counter by one if the medium is idle for Δ If medium is busy, freeze countdown SNU SCONE lab. 80

21 BEB (Binary Exp. Backoff) Example Collision = Overloaded Contention Window (CW) Backoff delay = Random (0.. CW) STA A Data A Data At the beginning, CW = CWmin For each unsuccessful transmission, double CW up to CWmax STA B BC=2 Data BC=1 STA C A SIFS STA D DIFS BC=5 DIFS DIFS SNU SCONE lab. 81 SNU SCONE lab. 82 DCF CSMA/CA Frame Format RTS/CTS exchange is optional BC=0 &!TxPend TxPend & CCA.busy CA (Collision Avoidance) Access Control Tx Idle TxPend & CCA>DIFS PLCP Header - DSSS Preamble (srt/long) PLCP header Sync 128 (56) SFD 16 Signal 8 Service 8 Length 16 CRC 16 MPDU 192 us Rate Indication 1 Mbps 1/2/5.5/11 Mbps No. of Bytes!(CCA>DIFS) Slot & CCA>DIFS Wait DIFS CCA>DIFS GetBC Access Control Back-off CCA>DIFS & BC=0 & TxPend TxSuccess Access Control Tx & Wait Ack TxFail BC=BC-1 CW=CWmin DoubleCW SNU SCONE GetBC lab. 83

22 MAC Frame Fields (1) Frame Control: Frame type/sub-type Control, management, or data To DS/From DS More fragment Retry Power Mgmt More data WEP Order Duration/Connection ID: Indicates time (in s) for successful transmission of MAC frame May contain AID (Association ID) MAC Frame Fields (2) Sequence Control: 4-bit fragment number subfield For fragmentation and reassembly 12-bit sequence number Number frames between given transmitter and receiver Frame Body: MSDU (or a fragment of) LLC PDU or MAC control information Frame Check Sequence: 32-bit CRC(Cyclic Redundancy Check) Address Fields To DS From DS Address 1 Address 2 Address 3 Address DA SA BSSID DA BSSID SA BSSID SA DA RA TA DA SA Address 1: All stations filter on this address Address 2: Transmitter address (TA) - Identify transmitter to address the ACK frame to Address 3: Dependent on To and From DS bits Address 4 : Only needed to identify the original source of WDS(Wireless DS) frames Wireless LAN - Other Features Chong-kwon Kim

23 AP Discovery To use AP 1. Discovery 2. Authentication 3. Association Scan Discover APs in transmission range Active scan, Passive scan Passive scan Client listen Beacon frames that AP transmits periodically Active scan Client probe the existence of APs by sending Probe request frame AP replies with Probe response frame Probe response frame SNU is SCONE similar lab. to Beacon frame 89 Authentication Open system and Shared key system Authentication Auth. Req Challenge Answer WEP Key SNU SCONE lab. 90 Association State 1 Unauthenticated Unassociated Authentication State 2 Authenticated Unassociated Association State 3 Authenticated Associated Deauthentication Notification Deassociation Notification Class 1 Frames Class 1 & 2 Frames Class 1,2 &3 Frames Association Request Allocate Association ID Association Response SNU SCONE lab. 91 Power Saving Energy is very scarce resource in mobile devices Network interfaces consume sizable amount of energy How to Conserve Energy? Turn off network interfaces when not in use PSM (Power Saving Mode) Sleep state Listen To sleep Signals to the AP not to send frames How to resume communication (Wakeup)? Transmission (Station AP) Reception (AP Station) SNU SCONE lab. 92

24 AP Packet Reception - PSM Beacon(TIM) Interval Station DTIM wakeups DTIM Interval DTIM Broadcast AP transmits beacons every beacon interval (~100msec) - Defer if channel is busy - PS nodes wake up for beacon reception TIM(Traffic Indication Map) - Indicate the existence of backlogged unicast traffic to PS nodes - PS node may request packet transmission Broadcast packets are announced by a Delivery TIM(DTIM) and are sent immediately afterwards - DTIM interval is a multiple of TIM interval AP Station Delivery in PS Mode If TIM indicates frames buffered STA sends PS-Poll to AP and stays awake to receive data DATA PS-Poll Learn U-APSD (Unscheduled Automatic Power Saving Delivery) SNU SCONE lab e QoS Priority and Parameterized QoS Priority scheme EDCA (Enhanced DCF Channel Access) Four access classes Different CW and AIFS Access Classes AC Description CW Min. AIFS 0 Background 15(31) 7 1 Best Effort 15(31) 3 2 Video 7(15) 2 or 1(AP) 3 Voice 3(7) 2 or 1(AP) Parameterized QoS scheme HCCA(Hybrid Coordination Function Channel Access) PCF based resource allocation AC3 AC2 AC1 AC0 2 0~3 slots 2 0~7 slots 3 0~15 slots 7 0~15 slots SIFS AIFS Random BC SNU SCONE lab. 95 SNU SCONE lab. 96