Data link layer (layer 2)

Transcription

1 Data link layer (layer 2) Gruppo Reti TLC COMPUTER NETWORK DESIGN Review of layer 2 protocols - 1 Copyright Quest opera è protetta dalla licenza Creative Commons NoDerivs-NonCommercial. Per vedere una copia di questa licenza, consultare oppure inviare una lettera a: Creative Commons, 559 Nathan Abbott Way, Stanford, California 94305, USA. This work is licensed under the Creative Commons NoDerivs-NonCommercial License. To view a copy of this license, visit: or send a letter to Creative Commons, 559 Nathan Abbott Way, Stanford, California 94305, USA. COMPUTER NETWORK DESIGN Review of layer 2 protocols - 2 Pag. 1

2 Data link layer Identifies and manages frames/datagrams over a link Wired or wireless Point-to-point Broadcast (need also medium access control to regulate access) Datagrams are transferred by different link protocols over different links Unless a virtual circuit based layer 2 is adopted When adopting the virtual circuit service, a layer 2 network can be defined Data can be forwarded through several devices at layer 2 only! Tons of layer 2 protocols were defined Link layer protocols implemented in the network adaptor (NIC) as hardware, software and firmware COMPUTER NETWORK DESIGN Review of layer 2 protocols - 3 Data link layer functions Frame delineation Explicit delimiters (flag) before and after packet transmission Lenght indicator Fixed length Silence between packets Multiplexing (of higher layer protocols) Addressing Error detection (at least in the headers, typically on header plus data) Window protocol Flow control Sequence and error control through retransmission Multiple access protocols for shared media (broadcast channel) COMPUTER NETWORK DESIGN Review of layer 2 protocols - 4 Pag. 2

3 Data link layer protocols Several protocols belong to the variable packet size HDLC family LAP-B (Link Access Procedure Balanced) and LAP-D (Link Access Procedure D-Channel) ISDN LAP-F (Link Access Procedure to Frame Mode Bearer Service) Frame Relay LLC (Logical Link Control) Standard LANs (on top of MAC) PPP (Point-to-Point Protocol) Internet ATM (AAL+ATM) Fixed packet size MAC COMPUTER NETWORK DESIGN Review of layer 2 protocols - 5 Layer two architectures LAP-B standardized In the ISDN framework as an extension to the telephone network infrastructure to integrate packet services first attempt of network integration LLC standardized (today often unused) for Local Area Networks over the MAC layer Frame Relay standardized to create packet networks based on virtual circuits (normally permanent VCs) originally proposed within the ISDN framework today used to create VPNs (Virtual Private Networks) for companies to interconnect LANs to build logical topologies to interconnect Internet routers for ISP COMPUTER NETWORK DESIGN Review of layer 2 protocols - 6 Pag. 3

4 PPP Layer two architectures RFC 1661 defined in the Internet framework to provide access to the Internet from user premises ATM For B-ISDN (Broadband ISDN) first attempt to build a really integrated network support all type of services, with different transmission speeds and quality of service requirements over the same network infrastructure should have reached the user terminal today used as an alternative to Frame Relay Ethernet Commercial standard for Local Area Networks Access protocol to solve the distributed access problem COMPUTER NETWORK DESIGN Review of layer 2 protocols - 7 Reliability Frames can be lost on a link? Yes, for transmission errors Yes, in layer two protocols with virtual circuit service due to buffer overflow within switching nodes For error control Need either error detection or error correction capability Error correction Requires more overhead It is convenient only either for significant bit error rates or for very large delays When error detected on data, retransmission is employed COMPUTER NETWORK DESIGN Review of layer 2 protocols - 8 Pag. 4

5 Reliability: window protocols Send a packet, wait for an ACK When the ACK is received, send the next packet Packet (and ACK) can be lost Wait for ACK and then retransmit How long to wait? One timeout Timeout should be larger than the RTT (Round Trip Time) RTT includes Two way propagation delay Processing time at the receiver (boundable and negligible) Packet and ACK transmission time Can manage one packet at the time? Yes, but since the packets (and the ACK) can be lost we need to number it anyway Named stop&wait protocol COMPUTER NETWORK DESIGN Review of layer 2 protocols - 9 Reliability: window protocols Max bit rate of the stop&wait? One packet per RTT For large RTTs, it may be small Can increase the packet size? Yes but.. Increase the number of packets that can be sent without receiving an ACK Number of packets is named window Need to store many packets (for retransmission and reordering) Selective repeat TX and RX windows larger than one Typically set to the same value nstraint on the window size to ensure correct behaviour WT+WR 2 k where k is the number of bit used to number packets and ACKs COMPUTER NETWORK DESIGN Review of layer 2 protocols - 10 Pag. 5

6 Transmission windows Correctly Acked PDUs PDUs waiting for an ACK PDUs that can be transmitted PDUs that cannot be trasmitted W TX n PDUs to be passed to higher layer protocols PDU that were correctly ACKed and moved to higher layer protocols for further processing W RX PDUs that can be accepted n Out of sequence PDU s that cannot be accepted COMPUTER NETWORK DESIGN Review of layer 2 protocols - 11 Window protocols bit rate The maximum bit rate (in absence of errors) of a window protocol is ruled by transmission _ window min, link _ speed RTT For a given RTT, there is an optimal window size that maximizes the bit rate (equal to the link speed) Larger windows translate in an equivalent RTT increase with no bit rate increase Short connections get higher throughput for a given window size To control TX throughput, it is possible to act on RTT (e.g. delaying ACK transmission but it may induce retransmissions Window size COMPUTER NETWORK DESIGN Review of layer 2 protocols - 12 Pag. 6

7 Window protocol: flow control Flow control is needed to avoid to overwhelm a slow receiver (Rx) with a too fast transmitter (Tx) When a Rx becomes slow its buffer initially fills up Then bits must be dropped To avoid it, we can exploit the window size at the Tx to regulate its sending rate The Rx monitors its buffer size sends control messages to the Tx to reduce the transmitter window E.g. On/Off flow control (full/zero) window COMPUTER NETWORK DESIGN Review of layer 2 protocols - 13 Layered architecture: ISDN B channel L3 Packet layer Packet layer Packet layer L2 LAP-B LAP-B LAP-B LAP-B L1 User terminal Switching node User terminal Error control COMPUTER NETWORK DESIGN Review of layer 2 protocols - 14 Pag. 7

8 Frame relay core and edge approach L>= 3 Higher layer protocols DL-CONTROL Error control Higher layer protocols DL-CONTROL L2 DL-CORE DL-CORE DL-CORE DL-CORE L1 User terminal Frame Relay switching node User terminal COMPUTER NETWORK DESIGN Review of layer 2 protocols - 15 B-ISDN Core and edge approach L>=3 L2 Higher layers protocols AAL Error detection only on-demand for some AAL Higher layers protocols AAL L1 ATM ATM ATM ATM User terminal ATM switching node User terminal COMPUTER NETWORK DESIGN Review of layer 2 protocols - 16 Pag. 8

9 B-ISDN: reference model Control plane Higher layers Management plane User plane Higher layer AAL (ATM Adaptation Layer) ATM layer Layer management Plane management layer COMPUTER NETWORK DESIGN Review of layer 2 protocols - 17 Data format (many layer 2 protocols) address control data CRC /16 8/16 >= Flag field ( ) to delimit the packets Not in LLC Address field has different flavors Address in LAP-B multi-point configuration Virtual circuit identifier (DLCI) and some bits for congestion control and traffic control in Frame Relay Two fields in LLC for protocol multiplexing Not used in PPP COMPUTER NETWORK DESIGN Review of layer 2 protocols - 18 Pag. 9

10 Control field differentiates the PDUs LAP-B Data format (many layer 2 protocols) Packet and ACK number Error and on/off flow control Connection management LLC (Optional) packet and ACK number PPP Not used Frame relay (but only user to user) Packet and ACK number Error and flow control COMPUTER NETWORK DESIGN Review of layer 2 protocols - 19 Data format (many layer 2 protocols) CRC for error detection Not available in LLC (done by the MAC) Additional fields? Protocol multiplexing in PPP COMPUTER NETWORK DESIGN Review of layer 2 protocols - 20 Pag. 10

11 ATM cell format GFC VPI VCI VCI HEC VPI VCI PT CLP VPI VCI VPI VCI HEC PT VCI CLP 5 BYTE DATA DATA 48 BYTE UNI CELL NNI CELL COMPUTER NETWORK DESIGN Review of layer 2 protocols - 21 ATM cell header GFC (4 bit): Generic Flow Control GFC for flow control at the user to networkk interface VPI (8-12 bit): Virtual Path Identifier and VCI (16 bit): Virtual Circuit Identifier virtual circuit identifiers (labels) PT (3 bit): Payload Type classifyiescell type as user data, managment, traffic control CLP (1 bit) Provides two priority levels in tghe cell flow HEC (8 bit): Header Error Code permits to detect header errors and correct single errors COMPUTER NETWORK DESIGN Review of layer 2 protocols - 22 Pag. 11

12 AAL: ATM Adaptation Layer Integrates ATM transport to offer service to users Servide dependent layer It defines four classes of service Examples of AAL functions: Transmission errors detection Segmentation and reassembly Cell loss management Flow control Synchronization Timestamping COMPUTER NETWORK DESIGN Review of layer 2 protocols - 23 AAL service classes Synchronism required between source and dest Speed Connection type AAL type Class A Class B Class C Class D costant (CBR) required Connection oriented not required variable (VBR) AAL 1 AAL 2 AAL 3/4-5 connectionl ess Possible applications voice 64kbit/s video CBR video/audio VBR data data COMPUTER NETWORK DESIGN Review of layer 2 protocols - 24 Pag. 12

13 Lenght CRC - 32 COMPUTER NETWORKS Data link layer protocols AAL 3/4 CPCS PDU 1B 1B 2B 0-3B 1B 2B 2B CPI B Tag BA size AAL payload pad AL E Lenght Tag 2 byte 44 byte 2 byte SAR - PDU SAR SAR header trailer ST=BOM SAR header ST=COM SAR trailer SAR header ST=EOM SAR trailer SAR PDU ST SN MID LI CRC bit 6 bit 10 bit COMPUTER NETWORK DESIGN Review of layer 2 protocols - 25 AAL B 0-47B 2B 2B 4B CS Layer PDU CS Layer Payload SAR Layer PDU 48 bytes SAR payload 48 bytes SAR payload 48 bytes SAR payload End of segment = 1 COMPUTER NETWORK DESIGN Review of layer 2 protocols - 26 Pag. 13

14 LANs Small geographical extension Shared transmission medium (originally) only one node can transmit at a time Multiple access problem Motivation: bursty traffic Dedicated channel would be wasted When sending, each node would like a high tx speed Useful for broadcast-multicast transmission Need to use address to identify node for unicast traffic Many topologies bus,ring, star COMPUTER NETWORK DESIGN Review of layer 2 protocols - 27 Possible solutions for medium access Static channel division Fixed assignement of portion of channels Time Division Frequency Division Code Division Not suited to bursty traffic (N queues and servers at speed C are worse that 1 queue and server at speed NC) Could extend to a dynamic assignment scenario Suppose a centralized controller Need to collect node tx needs (according to which access scheme?) Need to send allocation decision to nodes (according to which access scheme?) Complexity and increase in delay Solution: rely on distributed, access protocols Goal: to emulate statistical multiplexing COMPUTER NETWORK DESIGN Review of layer 2 protocols - 28 Pag. 14

15 Access protocols for LANs: taxonomy Three main families: Random access (ALOHA, CSMA/CD, Ethernet) Ordered access (Token Ring, Token Bus, FDDI) Slotted, with reservation (DQDB) How to evaluate LAN access protocols performance Throughput Fairness Access delay Number of nodes, network size, reliability, ease of deployment COMPUTER NETWORK DESIGN Review of layer 2 protocols - 29 Random access protocols Optimistic approach Free access Each node send at the channel speed R No coordination among nodes If two concurrent transmissions collision MAC (Medium Access Control) random access protocols specify: How to detect a collision How to recover after a collision has been detected Randomly delayed retransmission COMPUTER NETWORK DESIGN Review of layer 2 protocols - 30 Pag. 15

16 Random access protocols SLOTTED ALOHA All packets same size Synchronization among nodes Time divided in to equal size slots Node tx begin only at slot boundaries If collision is detected, retransmit after a random delay (in slot) or retransmit in each subsequent slot with probability p COMPUTER NETWORK DESIGN Review of layer 2 protocols - 31 Random access protocols SLOTTED ALOHA All packets same size Synchronization among nodes Time divided in equal size slots Node tx begins only at slot boundaries If collision is detected, retransmit after a random delay (in slot) or retransmit in each subsequent slot with probability p Good Single node get the whole network capacity Bad Idle slots when retransmitting Need for synchronization Low efficiency (36% for infinite users) Collisions Vulnerability period 1 packet tx time Collisions last one packet tx time COMPUTER NETWORK DESIGN Review of layer 2 protocols - 32 Pag. 16

17 ALOHA Random access protocols All packets same size (only for comparison) No synchronization among nodes Node tx begin when the frame becomes availalble If collision is detected, retransmit after a random delay Good Single node get the whole network capacity Simpler than ALOHA Bad Even worse performance than SLOTTED ALOHA (18%) Higher collision probability (vulnerability period 2 packet tx time) Average collision duration 1.5 packet tx time COMPUTER NETWORK DESIGN Review of layer 2 protocols - 33 CSMA: Carrier Sense Multiple Access Listen before transmit! Sense the channel before transmission If the channel is sensed free, transmit a packet If the channel is busy, defer transmission to avoid collision 1-persistent CSMA: retry transmission as soon as channel sensed free 0-persistent CSMA : retry transmission after a random time p-persistent CSMA: with p behave as 1-persistent, with probability (1-p) behave as 0-persistent COMPUTER NETWORK DESIGN Review of layer 2 protocols - 34 Pag. 17

18 Collisions still occur due to propagation delay If a collision occurs, a full packet transmission time is wasted The propagation delay (distance) plays a fundamental role in collision probability Vulnerability period depends on propagation delay CSMA: collisions? COMPUTER NETWORK DESIGN Review of layer 2 protocols - 35 CSMA/CD (Collision Detection) Stop if speaking at the sam time! CSMA/CD adds to CSMA If a collision is (quickly) detected, packet transmission is suspended Reduce the waste due to useless transmission Collision detection: Compare the tx signal with the rx signal Easy in wired LANs: Almost impossible in wireless LANs: half duplex (when tx the rx is disbled) COMPUTER NETWORK DESIGN Review of layer 2 protocols - 36 Pag. 18

19 CSMA/CD: performance Throughput performance strongly depend on the end to end propagation delay More precisely, on the ratio between packet transmission time and the propagation delay Very good throughput performance on small size networks (with respect to packet size) and with relatively small transmission speed Large packets, much larger than network size! Constraint on the minimum packet size to detect collisions (a node must transmit when detecting a collision) COMPUTER NETWORK DESIGN Review of layer 2 protocols - 37 Random access protocols performance COMPUTER NETWORK DESIGN Review of layer 2 protocols - 38 Pag. 19

20 TDMA/FDMA Ordered access protocols Efficient and fair at high load At low loads Delay in access channel 1/N bit rate available to a single node Random access Efficient at low load Single node access the whole bit rate Low delays At high load Collisions reduce performance Can we get the best (and the worst :-) of the two? COMPUTER NETWORK DESIGN Review of layer 2 protocols - 39 Polling Ordered access protocols A Master node invites slaves to transmit in turn Issues Polling overhead Latency Single point of failure Token passing Control token (gives permission to transmit) passed among nodes Either physical or logical sequence Issue Token overhead Latency Single point of failure COMPUTER NETWORK DESIGN Review of layer 2 protocols - 40 Pag. 20

21 Slotted access protocols Slots propagate on the active bus (or ring) Need two bus in opposite directions to work Empty slot Check slot availability If empty transmit Otherwise wait for next available slot No throughput loss Unfair Need to reserve slots from upstream nodes to ensure fairness Empty slot with reservations Still some unfairness left Good access delay COMPUTER NETWORK DESIGN Review of layer 2 protocols - 41 Performance comparison Random access features Excellent access delays at low loads delay is significant at low loads Simple to implement If well designed to control collisions may be efficient for small networks Difficult to create class of traffic (priority) Ordered access features Highly efficient and fair at high load Loose one «network traversal per cycle» to transmit the token Token (or master) lost block the network Need to monitor token (master) generation process and to ensure uniqueness Slotted access (with reservation) features Maximum efficiency Some residual unfairness (delays) Need synchronization to identify slot boundaries COMPUTER NETWORK DESIGN Review of layer 2 protocols - 42 Pag. 21

22 LAN Interconnection Needed to Extend LAN physical size Increase the number of access nodes No need to modify protocol architecture May increase LAN throughput performance More space diversity Exploits traffic locality COMPUTER NETWORK DESIGN Review of layer 2 protocols - 43 Hub Application Application Presentation Presentation Session Session Transport Transport Network Network Data Link Hub Data Link COMPUTER NETWORK DESIGN Review of layer 2 protocols - 44 Pag. 22

23 Hub Multi-port device Operates at the bit level (layer one) Extend the cable lenght No increase in network capacity Regenerates strings of bit and forwards them on all the ports Shared bandwidth on all ports 3R: re-generation, re-shaping, re-timing May introduce delays Hubs Structured cabling (ease cabling and maintenance) On twisted-pair or fiber Star based topology COMPUTER NETWORK DESIGN Review of layer 2 protocols - 45 Switch Application Presentation Session Transport Network Data Link Switch Data Link Data Link Application Presentation Session Transport Network Data Link COMPUTER NETWORK DESIGN Review of layer 2 protocols - 46 Pag. 23

24 Layer 2 devices Switch Operate on layer 2 addresses From one segment LAN to extended LANs Interconnect segments of LANs Enable to increase the network size Store and forward devices Dedicated bandwidth per port Transparent to users (same behaviour with or withouth bridge/switch) Do not modify packet content Limited routing capability Backward learning algorithm COMPUTER NETWORK DESIGN Review of layer 2 protocols - 47 Modern LANs COMPUTER NETWORK DESIGN Review of layer 2 protocols - 48 Pag. 24

25 Switch operations Forwarding tables are initially empty! Three fundamentals functions: address learning: to dynamically create a routing (forwarding) table at the MAC layer (MAC Address, port_id) frame forwarding: forward packets depending on the outcome of the routing table look-up spanning tree algorithm execution to operate on a loopfree (tree) topology Build a logical tree topology among bridges/switches by activating/de-activating ports COMPUTER NETWORK DESIGN Review of layer 2 protocols - 49 Address learning Exploits the Backward learning algorithm For each received packet Read the source MAC address MAC_S to associate the address with the port PORT_X from which the packet has been received Update timer associated to the entry (MAC_S, PORT_X) Will later use PORT_X to forward packets to MAC_S Timer needed to automatically adapt to topology variations and to keep the table size small COMPUTER NETWORK DESIGN Review of layer 2 protocols - 50 Pag. 25

26 Frame forwarding When a correct packet (wrong packets are dropped) with a unicast MAC_D destination address is received on PORT_X Look for MAC-D in the table If found and associated to PORT_X, drop the packet If found and associated to port_y, forward to PORT_Y If not found, forward to any other output port except PORT_X If the packet has a multicast/broadcast address Forward to any port except PORT_X COMPUTER NETWORK DESIGN Review of layer 2 protocols - 51 Switch properties From a multiple-access network to a multiplexed network Reduce collision probability by partitioning the network in independent segments For a full duplex fully switched network Ethernet becomes a framing and transmission technique alternative to LAP-B, LAP-F, ATM The MAC layer becomes useless distance limitations induced only by the media transmission properties, not by the MAC Ease security and management Traffic separation COMPUTER NETWORK DESIGN Review of layer 2 protocols - 52 Pag. 26

27 Switch properties Throughput performance may increase More space diversity (higher capacity) Ned to exploit traffic locality Introduce store and forward (and queueing) delays Worse delays than hubs Stor and forward delay significant with respect to propagation delay Transmission time of a minimum packet size at least twice of the propagation delay Potential packet losses when queues are filled-up Unfairness in resource access COMPUTER NETWORK DESIGN Review of layer 2 protocols - 53 Data link layer in LANs IEEE data link layer in LANs has two sub layers LLC: Logical Link Control MAC: Medium Access Control network LLC MAC physical COMPUTER NETWORK DESIGN Review of layer 2 protocols - 54 Pag. 27

28 802.1 ARCHITECTURE COMPUTER NETWORKS Data link layer protocols Frame delineation LANs layer 2 functions MAC (silence among packets and SFD) Multiplexing (higher layer protocols) IEEE LLC, MAC Ethernet Error detection MAC Error correction (window protocol) LLC (optionale) Addressing MAC used to identify the NIC (Network Interface Card), LLC for higher layer protocol multiplexing COMPUTER NETWORK DESIGN Review of layer 2 protocols - 55 Standard IEEE INTERNETWORKING LOGICAL LINK CONTROL INTERNETWORKING LOGICAL LINK MEDIUM ACCESS MEDIUM ACCESS MEDIUM ACCESS MEDIUM ACCESS MEDIA ACCESS PHY PHY PHY PHY PHYSICAL COMPUTER NETWORK DESIGN Review of layer 2 protocols - 56 Pag. 28

29 Ethernet: packet format Preamble = SFD = MAC destination address MAC source address Higher layer protocol > 1500 BYTES D A T A FCS Inter Packet GAP (silence) 4 Equivalent to 12 COMPUTER NETWORK DESIGN Review of layer 2 protocols - 57 IEEE 802.3: packet format Preamble = SFD = MAC destination address MAC source address Length (<1500) BYTES D A T A Padding FCS Inter Packet GAP (silence) Equivalent to 12 COMPUTER NETWORK DESIGN Review of layer 2 protocols - 58 Pag. 29

30 Elements Laptops, smartphones (may be fixed, not mobile) Base station Typically connected to a wired network Covers a given area and acts as a relay Wireless links Need MAC to coordinate access Various bit rate and distance Wireless LANs network infrastructure COMPUTER NETWORK DESIGN Review of layer 2 protocols - 59 No base station Ad hoc wireless LANs Nodes transmito to other nodes within coverage Must create and ad hoc network among nodes Nodes router traffic among themselves COMPUTER NETWORK DESIGN Review of layer 2 protocols - 60 Pag. 30

31 Hidden Terminal Effect A and C cannot hear each other because of obstacles, signal attenuation A and C unaware of collision at B How to avoid it? COMPUTER NETWORK DESIGN Review of layer 2 protocols - 61 How to avoid collisions? Multiple access 2 or plus nodes transmitting at same time : CSMA - sense before transmitting don t collide with ongoing transmission by other node : no collision detection! difficult to receive (sense collisions) when transmitting due to weak received signals (fading) can t sense all collisions in any case: hidden terminal, fading goal: avoid collisions: CSMA/CA (Collision Avoidance) COMPUTER NETWORK DESIGN Review of layer 2 protocols - 62 Pag. 31

32 IEEE DCF Implementation mandatory DCF is based on the Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) scheme: stations that have data to transmit contend to access the channel a station has to repeat the contention procedure every time it has a new data frame to transmit Stations are half-duplex: they either transmit or receive (or sense the channel) Explicit ACKs sent by the receiver!!! To solve the hidden terminal problem COMPUTER NETWORK DESIGN Review of layer 2 protocols - 63 Sender: if sense channel idle for DIFS seconds then transmit entire frame (no collision detection) if sense channel busy Select a random backoff time and send If unsuccesfull (no ACK) exponential backoff (CA) Receiver: if data received correctly returns ACK after SIFS Collisions last a lot! DCF Overview COMPUTER NETWORK DESIGN Review of layer 2 protocols - 64 Pag. 32

33 DCF with Handshaking Explicit channel reservation to avoid collisions on long data frames Sender: send short RTS (Request To Send) Receiver (BS): reply with short CTS (Clear To Send) CTS reserves channel, notifying (possibly hidden) stations After the CTS reception, the sender node sends the data with no collision Avoids data frame collisions using short RTS and CTS collisions less likely of shorter duration performance result similar to collision detection COMPUTER NETWORK DESIGN Review of layer 2 protocols - 65 Layer 2 protocols: a comparison Protocol Packet delineation Layer 3 protocol multiplexing Error detection Error correction (window protocol) Sharing LAPB Delimiter Higher layer YES YES Multiplexing LAPF core + LAPF control ATM (core)+ AAL (edge) Delimiter Done at the physical layer Via virtual circuits Via virtual circuits YES in LAPF core YES in AAL (edge) Optional in LAPF control (edge) NO Multiplexing Multiplexing PPP Delimiter YES YES NO Multiplexing LLC NO (Done in MAC IEEE 802.3) YES Optional Optional Multiplexing Ethernet MAC Silence YES YES NO Multiplexing/mult iple access (Wi-FI) Silence Above YES NO Multiple access COMPUTER NETWORK DESIGN Review of layer 2 protocols - 66 Pag. 33