Medium Access Sublayer (5.3) Multiple access layer. Medium Access Sublayer (cont d) Contents
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1 Medium Access Sublayer (5.3) Multiple access layer Network Layer Medium Access Sublayer Data Link Layer Physical Layer 1 2 Medium Access Sublayer (cont d) Contents Medium access (MAC) sublayer is not important on point-to-point links The MAC sublayer is only used in broadcast or shared channel networks Examples: Satellite, Ethernet, Wireless, Cellular Channel partitioning protocols Cellular networks Random access protocols IEEE 802 LANs, IEEE Wireless LANS 3 4
2 Channel partitioning Protocols Multiple Access Techniques TDMA/FDMA/CDMA Static and predetermined allocation of channel access: independent of user activity Idle users may be assigned to the channel, in which case channel capacity is wasted Examples: TDMA, FDMA, CDMA Allow multiple users to share a common transmission medium Techniques TDMA: Time division Multiple Access The spectrum usage is divided in the time domain People take turns using the spectrum (amount of time allocated is the slot time) FDMA: Frequency division multiple access The spectrum usage is divided in the frequency domain People are assigned portions of the spectrum for their own use (portion of the spectrum is the channel) CDMA: Code Division Multiple Access The entire spectrum is used by everyone but in a coded format The signal is spread and only the receiver who knows the code can recover the signal Analogy: people talking in different languages all at the same time 5 6 Spread Spectrum Techniques (Types of CDMA) Example Two types of techniques exist DS-CDMA and Frequency hopping CDMA Direct sequence CDMA (DS-CDMA or DSSS) Each bit of the signal is replaced by a code (longer bit sequence) A narrow band signal (R bps) is multiplied by a wideband signal (W bps) Receiver who knows the code will recover the signal; for the rest it appears as random noise Used in IEEE Wavelan Data Code Output Receiver uses the code and the received signal to recover Original data 8
3 Frequency hopping Random access protocols (5.3.2) Frequency sequence CDMA (FS-CDMA or FSSS) A single user s signal is spread out over a number of channels (1011) is transmitted as f1, f9, f11, f13 The receiver who tunes to f1, f9, f11 and f13 in sequence will be able to recover the bit stream Used in Blue tooth wireless technology Single channel shared by a large number of hosts No coordination between hosts Control is completely distributed Examples: ALOHA, CSMA, CSMA/CD 9 10 Contention Access (cont d) Contention Access (cont d) Advantages: Short delay for bursty traffic Simple (due to distributed control) Flexible to fluctuations in the number of hosts Fairness Disadvantages: Low channel efficiency with a large number of hosts Not good for continuous traffic (e.g., voice) Cannot support priority traffic High variance in transmission delays 11 12
4 Contention Access Methods Pure ALOHA Pure ALOHA Slotted ALOHA CSMA 1-Persistent CSMA Non-Persistent CSMA P-Persistent CSMA CSMA/CD Originally developed for ground-based packet radio communications in 1970 Goal: let users transmit whenever they have something to send The Pure ALOHA Algorithm Pure ALOHA (cont d) 1. Transmit whenever you have data to send 2. Listen to the broadcast Because broadcast is fed back, the sending host can always find out if its packet was destroyed just by listening to the downward broadcast one round-trip time after sending the packet 3. If the packet was destroyed, wait a random amount of time and send it again The waiting time must be random to prevent the same packets from colliding over and over again Note that if the first bit of a new packet overlaps with the last bit of a packet almost finished, both packets are totally destroyed. t : one packet transmission time Vulnerable period: 2t t 15 t 0 t 0 +t t 0 +2t t 0 +3t Vulnerable 16
5 Pure ALOHA (cont d) Due to collisions and idle periods, pure ALOHA is limited to approximately 18% throughput in the best case Can we improve this? Slotted ALOHA Slotted ALOHA cuts the vulnerable period for packets from 2t to t. This doubles the best possible throughput from 18.4% to 36.8% How? Time is slotted. Packets must be transmitted within a slot The Slotted ALOHA Algorithm CSMA 1. If a host has a packet to transmit, it waits until the beginning of the next slot before sending 2. Listen to the broadcast and check if the packet was destroyed 3. If there was a collision, wait a random number of slots and try to send again We could achieve better throughput if we could listen to the channel before transmitting a packet This way, we would stop avoidable collisions. To do this, we need Carrier Sense Multiple Access, or CSMA, protocols 19 20
6 Assumptions with CSMA Networks CSMA (cont d) 1. Constant length packets 2. No errors, except those caused by collisions 3. No capture effect 4. Each host can sense the transmissions of all other hosts 5. The propagation delay is small compared to the transmission time There are several types of CSMA protocols: 1-Persistent CSMA Non-Persistent CSMA P-Persistent CSMA Persistent CSMA 1-Persistent CSMA (cont d) Sense the channel. If busy, keep listening to the channel and transmit immediately when the channel becomes idle. If idle, transmit a packet immediately. If collision occurs, Wait a random amount of time and start over again. The protocol is called 1-persistent because the host transmits with a probability of 1 whenever it finds the channel idle
7 The Effect of Propagation Delay on CSMA Propagation Delay and CSMA A packet B carrier sense = idle Transmit a packet Contention (vulnerable) period in Pure ALOHA two packet transmission times Contention period in Slotted ALOHA one packet transmission time Contention period in CSMA up to 2 x end-to-end propagation delay Collision 25 Performance of CSMA > Performance of Slotted ALOHA > Performance of Pure ALOHA 26 1-Persistent CSMA with Satellite Systems 1-Persistent CSMA (cont d) Satellite system: long prop. delay (270 msec) Carrier sense makes no sense It takes 270 msecs to sense the channel, which is a really long time Vulnerability time = 540 msec (1/2 a second is forever in a network!) 27 Even if prop. delay is zero, there will be collisions Example: If stations B and C become ready in the middle of A s transmission, B and C will wait until the end of A s transmission and then both will begin transmitted simultaneously, resulting in a collision. If B and C were not so greedy, there would be fewer collisions 28
8 Non-Persistent CSMA Tradeoff between 1- and Non- Persistent CSMA Sense the channel. If busy, wait a random amount of time and sense the channel again If idle, transmit a packet immediately If collision occurs wait a random amount of time and start all over again If B and C become ready in the middle of A s transmission, 1-Persistent: B and C collide Non-Persistent: B and C probably do not collide If only B becomes ready in the middle of A s transmission, 1-Persistent: B succeeds as soon as A ends Non-Persistent: B may have to wait P-Persistent CSMA P-Persistent CSMA (cont d) Optimal strategy: use P-Persistent CSMA Assume channels are slotted One slot = contention period (i.e., one round trip propagation delay) 1. Sense the channel If channel is idle, transmit a packet with probability p if a packet was transmitted, go to step 2 if a packet was not transmitted, wait one slot and go to step 1 If channel is busy, wait one slot and go to step Detect collisions If a collision occurs, wait a random amount of time and go to step
9 P-Persistent CSMA (cont d) Comparison of CSMA and ALOHA Protocols Consider p-persistent CSMA with p=0.5 When a host senses an idle channel, it will only send a packet with 50% probability If it does not send, it tries again in the next slot. The average number of tries is: Σ i (1-p) i p = 8 i=0 1 p (Number of Channel Contenders) 34 CSMA/CD CSMA/CD In CSMA protocols If two stations begin transmitting at the same time, each will transmit its complete packet, thus wasting the channel for an entire packet time In CSMA/CD protocols The transmission is terminated immediately upon the detection of a collision CD = Collision Detect Sense the channel If idle, transmit immediately If busy, wait until the channel becomes idle Collision detection Abort a transmission immediately if a collision is detected Try again later after waiting a random amount of time 35 36
10 CSMA/CD (cont d) Collision detection time Carrier sense reduces the number of collisions Collision detection reduces the effect of collisions, making the channel ready to use sooner How long does it take to realize there has been a collision? Worst case: 2 x end-to-end prop. delay packet A B Ethernet Physical Layer Ethernet Physical Configuration (for thick coaxial cable) Transceiver Transceiver Cable 4 Twisted Pairs 15 Pin Connectors Channel Logic Manchester Phase Encoding 64-bit preamble for synchronization Segments of 500 meters maximum Maximum total cable length of 1500 meters between any two transceivers Maximum of 2 repeaters in any path Maximum of 100 transceivers per segment Transceivers placed only at 2.5 meter marks on cable 39 40
11 Manchester Encoding Ethernet Synchronization Data stream Encoded bit pattern bit = high/low voltage signal 0 bit = low/high voltage signal 64-bit frame preamble used to synchronize reception 7 bytes of followed by a byte containing Manchester encoded, the preamble appears like a sine wave Ethernet: MAC Layer MAC Layer Ethernet Frame Format Data encapsulation Frame Format Addressing Error Detection Link Management CSMA/CD Backoff Algorithm Multicast bit Destination (6 bytes) Source (6 bytes) Length (2 bytes) Data ( bytes) Pad Frame Check Seq. (4 bytes) 43 44
12 Ethernet MAC Frame Address Field Ethernet MAC Frame Other Fields Destination and Source Addresses: 6 bytes each Two types of destination addresses Physical address: Unique for each user Multicast address: Group of users First bit of address determines which type of address is being used 0 = physical address 1 = multicast address Length Field 2 bytes in length determines length of data payload Data Field: between 0 and 1500 bytes Pad: Filled when Length < 46 Frame Check Sequence Field 4 bytes Cyclic Redundancy Check (CRC-32) Ethernet Backoff Algorithm: Binary Exponential Backoff Binary Exponential Backoff (cont d) If collision, Choose one slot randomly from 2 k slots, where k is the number of collisions the frame has suffered. One contention slot length = 2 x end-to-end propagation delay This algorithm can adapt to changes in network load. 47 slot length = 2 x end-to-end delay = 50 µs A t=0µs: Assume A and B collide (k A = k B = 1) A, B choose randomly from 2 1 slots: [0,1] Assume A chooses 1, B chooses 1 t=100µs: A and B collide (k A = k B = 2) A, B choose randomly from 2 2 slots: [0,3] Assume A chooses 2, B chooses 0 t=150µs: B transmits successfully t=250µs: A transmits successfully B 48
13 Binary Exponential Backoff (cont d) Ethernet Performance In Ethernet, Binary exponential backoff will allow a maximum of 15 retransmission attempts If 16 backoffs occur, the transmission of the frame is considered a failure Ethernet Features and Advantages Ethernet Disadvantages 1. Passive interface: No active element 2. Broadcast: All users can listen 3. Distributed control: Each user makes own decision Lack of priority levels Cannot perform real-time communication Security issues Simple Reliable Easy to reconfigure 51 52
14 Interconnecting LANS Hubs Why not just one big LAN? Limited amount of supportable traffic: on single LAN, all stations must share bandwidth limited length: specifies maximum cable length limited number of stations: 802.4/5 have token passing delays at each station Physical layer extension Repeaters copies (amplifies, regenerates) bits between LAN segments Link Layer extensions Bridges receives, stores, forward (when appropriate) packets between LANs has two layers of protocol stack: physical and link-level (media access) Acts as a physical layer repeater Frame on a port broadcast on all other ports All ports see the same traffic Hubs used to connect many devices to extend a single ethernet tap Smart hub: can shut down misbehaving ports Bridges (forwarding) Forwarding Algorithm Techniques for forwarding packets flood packets (obvious drawbacks) router-discovery-like protocol Bridge "observes" traffic and "learns" which stations are attached allows bridge to identify hosts on LAN segment drawback? 1. bridge receives every packet transmitted on every attached LAN 2. bridge stores for each packet physical address of sender port (incoming LAN segment) on which packet was received 3. for each packet received on any port: lookup destination. physical address in table if not found, flood onto all attached LANs if found, forward only out to specified LAN 4. forwarding table deleted if not refreshed 55 56
15 Limitations of Bridges Ethernet Switching Do not scale Requires careful placement of nodes broadcast does not scale Learning time Do not accommodate heterogeneity Different networking technologies cannot be bridged Connect many Ethernet segments or subnets through an Ethernet switch Every port acts as a bridge The switch maintains a list of devices connected to its ports Allows simultaneous communication between non-overlapping ports More expensive than hubs or routers (cost is proportional to number of ports) No collisions to segment 4 to segment 3 to segment 1 to segment Why Ethernet switching? LANs may grow very large The switch has a very fast backplane It can forward frames very quickly to the appropriate subnet Cheaper than upgrading all host interfaces to use a faster network A Brief Note: Fast Ethernet IEEE 802.3u 100 Mbps Ethernet The MAC sublayer for Fast Ethernet is the same as for normal Ethernet Physical layer is slightly different: no more Manchester encoding (4B5B, 8B6T coding are used instead) Gigabit ethernet already exists, 10 Gpbs ethernet (coming) 59 60
16 Wireless Networking (Chapter 5.7.1) Physical Layer Basically wireless Ethernet Connects a number of computers in a wireless LAN Ad-hoc mode (AHM) as well as Access Point mode (APM) supported AHM - Only direct communication, no routing functionality APM - Computers connected to the Internet via an AP Typical mode of operation Access point name refers to a channel; a host connected to an AP tunes to the same channel as the AP Operates in the ISM band LBand 915 to 928 MHz and the SBand 2.4 to Ghz band are used for Uses Direct Sequence Spread Spectrum (DSSS) Signal is sent in a coded form Topic of a course in communications Initial versions were 1 to 2 Mbps. Now 11 Mbps (802.11b) available Future a can provide up to 50 Mbps Expected to operate in 5.7 Ghz band DSSS Access Control 83 MHz divided into eleven 22 MHz wide stationary channels At any point only 3 non-overlapping channels available Spreading sequence is 11 bit barker word 1 is mapped to 1, -1, 1, 1, -1, 1, 1, 1, -1, -1, -1 0 is mapped to -1, 1, -1, -1, 1, -1, -1, -1, 1, 1,1 Same sequence is used by all hosts Multiple access problem needs to be solved Input signal is spread to 22 MHz wide spectrum Different modulation schemes used to obtain different rates BPSK (gives 1 Mbps), QPSK (gives 2 Mbps), QPSK with CCK (gives 11 Mbps at 8 bits per symbol and chip rate of Mcps) Carrier sensing Is the medium idle? Wait for an amount of time (IFS), if still idle transmit IFS = inter frame spacing Is the medium busy? Wait until current txm ends, wait (IFS), if idle wait for random amount of time, else wait until current txm ends and repeat (exponential backoff for collisions) ACKs and immediate response actions can be sent after SIFS (Short IFS) < IFS value used in multiple access control Have Data Y Idle? Y Wait IFS Y Idle? Y Txmt Data N N Wait EOT Wait IFS N Idle? decr backoff 63 64
17 Problems with Carrier Sensing Use of RTS, CTS X Y Z Hidden Terminal Problem Hidden terminal problem Z does not hear X; hence transmits to Y and collides with transmission from X No carrier does not imply send Exposed terminal problem W hears Y but can safely transmit to X Carrier may not imply don t send X W Y Exposed Terminal Problem Z Sender sends a small packet RTS (request to send) before sending data Receiver sends CTS (clear to send) All potential senders hearing RTS waits until a CTS is heard from some receiver If no CTS, transmit If CTS, wait for a time for sender to send data Hear RTS, but no CTS, then send Exposed terminal case Don t hear RTS, but CTS receiver is close, don t send Hidden terminal case Issues Bluetooth Very popular in buildings, public spaces Tremendous opportunities Free/unlicensed spectrum interference issues Security, privacy, authentication being added Nice features to have Roaming across networks Remote authentication Mobile access A cable replacement technology Operates in the ISM band (2.4Ghz to 2.8 Ghz) Range is 10 cm to 10 meters can be extended to 100 meters by use of power control Data rates up to 1 Mbps (721Kbps) Supposed to be low cost, single chip radio Ideal for connecting devices in close proximity (piconet) Phone and earpiece Computer and printer Camera and printer/fax etc Can form personal area networks (piconet and scatternet) 67 68
18 Personal area networks Bluetooth Radio Link Piconet Master/slave nodes Master and up to 7 slaves Master allocates channels Scatternet Node may be master in one network and slave in another network Allows devices to be shared in different networks IEEE operates in the same band using DSSS Bluetooth uses Frequency Hopping 83.5 MHz channel is divided into 79 1-MHz channels 1600 hops per second (stays at one frequency for 625 microsecs) Hopping sequence is 16 or 32 Selected by the master based on its MAC address Master can connect up to seven slaves to form a piconet All members of the piconet use the same hopping sequence Bluetooth Packet Format Connection establishment 72 bits Access Code 54 bits Header Payload (2744 bits) Inquiry broadcast Inquiry scan Access code identifies control packet type Channel access code, device access code, inquiry access code Header contains address (3 bits) and packet types (4 bits) Voice packets with different FEC rates Data packets with low bit rate and high bit rate (with varying FEC as well) Page Master response Response-FHS Unicast-DAC ACK-DAC FHS-sender Inquiry response Page scan Slave response 71 Connection ACK-DAC Connection 72
19 piconet Bluetooth Link Formation A set of bluetooth devices connected to a master Scatternet: a set of piconets 73 Master inquires who is around Active slaves respond and the master learns who is around Master pages slaves and informs them of hopping sequence, active member address Active slaves get packets when header matches active member address A link is formed between master and each slave Inactive slaves can go into park state and give up address Master Slave Parked slave 74
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