Computer Networks Lecture #12 PDF

Computer Networks Lecture #12 In the last lecture Link virtualization - MPLS Data center networking Today Introduction to wireless / mobile networks Wireless Wireless links and network characteristics WiFi: IEEE 802.11 wireless LANs Cellular networks: 4G and 5G Introduction to...

Computer Networks Lecture #12 In the last lecture Link virtualization - MPLS Data center networking Today Introduction to wireless / mobile networks Wireless Wireless links and network characteristics WiFi: IEEE 802.11 wireless LANs Cellular networks: 4G and 5G Introduction to wireless / mobile networks Trends of wireless and mobile networks More wireless (mobile) phone subscribers than xed (wired) phone subscribers 10 : 1 in 2019 More mobile-broadband-connected devices than xed-broadband-connected devices 5 : 1 in 2019 4G/5G cellular networks now embracing Internet protocol stack, including SDN Two important (but di erent) challenges Wireless: communication over wireless link Mobility: handling the mobile user who changes a point of attachment to the network ff fi fi Elements of a wireless network Elements of a wireless network Elements of a wireless network Elements of a wireless network Characteristics of selected wireless links Elements of a wireless network Elements of a wireless network Wireless network taxonomy Single hop Multi hop Hosts may have to relay through Hosts connect to the base Infrastructure several wireless nodes to station (WiFi, cellular) which (e.g., APs) connect to larger Internet connects to larger Internet - e.g., mesh networks No base station, no connection No base station, no to larger Internet No connection to larger Internet May have to relay to reach infrastructure - e.g., Bluetooth, ad hoc others (wireless nodes) networks - e.g., MANET, VANET Wireless links and network characteristics  Characteristics of wireless links Decreased signal strength Radio signal attenuates as it propagates through matter (path loss) Multipath propagation Radio signal is re ected o objects or ground, arriving at the destination at slightly di erent times Interference from other sources Wireless network frequency (e.g., 2.4GHz) shared by many devices (e.g., WiFi, cellular, motors) → signal from other devices can be interference ⇒ make communication across (even a point-to-point) wireless link much more “di cult” ff fl ffi ff Wireless link characteristics: fading (attenuation) Wireless radio signal attenuates (loses power) as it propagates (free space “path loss”) Wireless link characteristics: multipath Multipath propagation Radio signal re ects o objects ground, built environment, arriving at destination at slightly di erent times ff fl ff  Wireless link characteristics: multipath Multipath propagation Radio signal re ects o objects ground, built environment, arriving at destination at slightly di erent times Coherence time Amount of time data is present in channel to be received In uences maximum possible transmission rate, since coherence times cannot overlap Inversely proportional to - Frequency & receiver velocity fl ff fl ff Characteristics of wireless links Signal-to-Noise Ratio (SNR) Important metric on the link quality Larger SNR → easier to extract signal from noise (a “good thing”) SNR vs. BER tradeo s Given a physical layer (parameter) - Increase power → increase SNR → decrease BER Given SNR - Choose a physical layer that meets BER requirement, giving highest throughput - SNR may change with mobility - Dynamically adapt physical layer (modulation technique, rate) ff Characteristics of wireless links Multiple wireless senders and receivers create additional problems (beyond multiple access) Hidden node (terminal) problem Due to obstacles or signal attenuation Node A and B can hear each other Node B and C can hear each other Node A and C cannot hear each other - They can send packets simultaneously (hard to coordinate), resulting in collision, interference Code Division Multiple Access (CDMA) Unique “code” assigned to each other (i.e., code set partitioning) All users share the same frequency, but each user has own “chipping” sequence (i.e., code) to encode data Allows multiple users to “coexist” and transmit simultaneously with minimal interference (if codes are “orthogonal”) Encoding Original data × chipping sequence (inner product operation) Decoding Encoded data × chipping sequence CDMA encode/decode Sender Receiver … but this isn’t really useful yet! CDMA encode/decode Sender1 Sender2 … now that’s useful! WiFi: IEEE 802.11 wireless LANs IEEE 802.11 Wireless LAN IEEE 802.11 standard Year Max data rate Range Frequency 802.11b 1999 11 Mbps 30 m 2.4 Ghz 802.11g 2003 54 Mbps 30m 2.4 Ghz 802.11n (WiFi 4) 2009 600 Mbps 70m 2.4, 5 Ghz 802.11ac (WiFi 5) 2013 3.47 Gpbs 70m 5 Ghz 802.11ax (WiFi 6/6E) 2020 600 Mbps ~ 9.6 Gbps 70m 2.4, 5 Ghz 802.11be (WiFi 7) 2024 (exp) 46 Gbps 2.4, 5, 6 Ghz unused TV bands 802.11af 2014 35~560 Mbps 1 Km (54-790 MHz) 802.11ah 2017 347Mbps 1 Km 900 Mhz All use CSMA/CA for multiple access, and have base-station and ad-hoc network versions IEEE 802.11 LAN architecture Wireless hosts communicate with a base station Base station - Access point (AP) Basic Service Set (BSS) (a.k.a “cell”) in infrastructure mode contains Wireless hosts Access point (AP): base station Ad hoc mode: hosts only IEEE 802.11: channels Frequency bands 2.4GHz : ISM band 5GHz, 6GHz : unlicensed national information infrastructure (UNII) band Spectrum divided into channels at di erent frequencies AP admin chooses frequency for AP Interference possible: channels can be the same as that chosen by neighboring APs ff IEEE 802.11: channels IEEE 802.11: channels 2.4 GHz band - overlapping channels 3 non-overlapping channels are allowed at the same time IEEE 802.11: channels 5 GHz band - 25 non-overlapping 20MHz-wide channels (in terms of OFDM) IEEE 802.11: association Arriving host: must associate with an AP Scans channels, listening for beacon frames containing AP’s name (SSID) and MA address Select AP to associate with May perform authentication Typically run DHCP to get IP address in the AP’s subnet IEEE 802.11: passive / active scanning Passive scanning Active scanning Beacon frames : sent from APs - Probe Request Frame : broadcast from H1 Association Request frame : From H1 to - Probe Response frames : sent from APs selected AP - Association Request frame : From H1 to Association Response frame : From selected selected AP AP to H1 - Association Response frame : From selected AP to H1  IEEE 802.11: multiple access Need to avoid collisions when two or more nodes transmit at the same time 802.11: basically, CSMA (sense before transmitting) Do not collide with detected ongoing transmission by another node But no collision detection is allowed Di cult to sense collisions - high transmitting signal, weak received signal due to fading Can’t sense all collision in any case - hidden node issue, fading ⇒ Try to avoid collision CSMA/CA (collision avoidance) ffi IEEE 802.11: multiple access Point Coordination Function (PCF → HCCA) Polling-based channel access : contention free High complexity : need central coordination (not widely used in practice) IEEE 802.11: multiple access Distributed Coordination Function (PCF → EDCA) Contention-based channel access Carrier-Sense Multiple Access / Collision Avoidance (CSMA/CA) - Transmission defers until the current transmission ends Simple, widely used in practice CSMA/CA Sender If idle channel is sensed for DIFS, transmit a frame If busy channel is sensed, 1. Start random backo timer 2. Timer counts down while channel is idle 3. Transmit when the timer expires 4. If ACK is not returned, increase the random backo interval, go to 2. Receiver If a frame is correctly received - Return ACK after SIFS (ACK needed due to hidden node problem) ff ff Channel access example in CSMA/CA If channel is idle during DIFS, all nodes select a random backo counter (CW) When the backo counter becomes 0, node A tries accessing the channel Node A has the smallest CW Node A Transmission Busy 3 2 1 0 11 10 9 8 8 7 6 5 Node B Busy Busy 10 9 8 7 7 6 5 4 4 3 2 1 A Node C Busy Transmission B C 6 5 4 3 3 2 1 0 15 14 13 12 * Transmission (DATA + ACK + SIFS + DIFS) ff ff Channel access example in CSMA/CA While the current transmission is going on, other nodes stop decreasing the backo counter, called “freezing the counter” Node A Transmission Busy 3 2 1 0 11 10 9 8 8 7 6 5 Node B Busy Busy 10 9 8 7 7 6 5 4 4 3 2 1 A Node C Busy Transmission B C 6 5 4 3 3 2 1 0 15 14 13 12 * Transmission (DATA + ACK + SIFS + DIFS) ff Channel access example in CSMA/CA When the ongoing transmission ends, the backo counter resumes decreasing More precisely, after idle time during DIFS If Node A has a new frame to send, it chooses a new random backo counter for the next transmission Node A Transmission Busy 3 2 1 0 11 10 9 8 8 7 6 5 Node B Busy Busy 10 9 8 7 7 6 5 4 4 3 2 1 A Node C Busy Transmission B C 6 5 4 3 3 2 1 0 15 14 13 12 * Transmission (DATA + ACK + SIFS + DIFS) ff ff Channel access example in CSMA/CA The backo counter of Node C becomes 0 rst, and Node C starts to transmit a frame Others do freezing while the current transmission is in progress Node A Transmission Busy 3 2 1 0 11 10 9 8 8 7 6 5 Node B Busy Busy 10 9 8 7 7 6 5 4 4 3 2 1 A Node C Busy Transmission B C 6 5 4 3 3 2 1 0 15 14 13 12 * Transmission (DATA + ACK + SIFS + DIFS) ff fi RTS/CTS in CSMA/CA For avoiding collisions, sender can “reserves” channel use for data frames using small reservation packets Sender rst transmits small Request-to-Send (RTS) packet to the receiver using CSMA RTSs can still be collided with each other (but they are short) The receiver broadcast clear-to-send (CTS) in response to RTS CTS is heard by all neighbor nodes (I will receive a frame soon. So, be quite) Other stations can defer transmissions fi Throughput issue in CSMA/CA Too small number of active nodes Low collision rate → CW can be a waste of channel resource Too large number of active node High collision rate → Collision results in a waste of channel resource Optimize IFS and CW depending on the number of active nodes To mitigate collision To enhance throughput IEEE 802.11 enhancement: jumbo frame To decrease control overhead, frame aggregation is introduced Smaller per frame overhead is achieved IEEE 802.11 enhancement: block ACK Frame aggregation Multiple frames are aggregated in a jumbo frame Low overhead to access the channel But, higher error rate Block ACK IEEE 802.11 enhancement: reverse direction Reverse direction Sending & receiving at once Used for bidirectional service such as VOIP, video conferencing, online games. Channel access overhead is further minimized Minimizing delay Increasing channel utilization  IEEE 802.11 enhancement: QoS Fairness vs. Quality of Service (QoS) IEEE 802.11 DCF is known to be long-term fair → An equal opportunity for channel access is provided in a long-term view Various services may require di erentiated channel access to meet their requirement IEEE 802.11e (for di erentiated service) EDCA / HCCA Di erent IFSs and CWs are used for each priority class TXOP can be dynamically set ff ff ff IEEE 802.11: more issues to be considered What if channel status is di erent to each node? STA-1’s exclusive access → 1Mbps STA-2’s exclusive access → 11Mbps Equal channel access - 11Mbits / 11sec , 11Mbits / 1sec → 22Mbits / 12sec Access with proportional fairness - 1Mbits / 1sec, 11Mbps / 1sec → 12Mbps / 2sec ff IEEE 802.11: more issues to be considered AP is contending for channel access with mobile nodes Mobile nodes → access for uplink tra c AP → access for downlink tra c Unfairness in channel access between uplink / downlink ffi ffi IEEE 802.11 frame structure Addressing in IEEE 802.11 frame IEEE 802.11: frame structure IEEE 802.11: mobility within same subnet H1 remains in the same IP subnet IP address can remain the same Switches Which AP is associated with H1? Self-learning can handle this issue - Switch will see frame from H1, and remember which switch port can be used to reach H1 IEEE 802.11: rate adaptation Nodes (base station, mobile) dynamically change transmission rate (physical layer modulation technique) as mobile moves (i.e., SNR varies) SNR decreases, BER increases as a node moves away from the base station When BER becomes too high, switch to lower transmission rate but with lower BER IEEE 802.11: power management Node-to-AP “I am going to sleep until the next beacon frame” AP would not transmit frames to this node (in the sleep state) Node wakes up before the next beacon frame Beacon frame Contains a list of nodes with AP-to-node frames waiting to be sent Node will stay away if there is AP-to-node frames to be delivered to itself Otherwise, sleep again until the next beacon frame Personal area networks: Bluetooth Coverage (range) Less than 10m diameter (indoor), mourned 100m (outdoor) In Bluetooth 5, up to 400m (indoor), around 1000m (outdoor) Replacement for cables (mouse, keyboard, headphone, etc.) Ad hoc mode - no infrastructure 2.4 ~ 2.5 GHz ISM radio band, up to 3Mbps Personal area networks: Bluetooth Master controller / client devices Master polls clients, grants requests for client transmissions TDM (625 μsec slot) FDM - Sender uses 79 frequency channels in known, pseudo-random order slot-to-slot (spread spectrum) Other devices/equipment not in piconet only interfere in some slots Parked mode: Clients can “go to sleep” (park) and later wakeup (to preserve battery) Bootstrapping: nodes self-assemble (plug-and-play) into piconet Example service: Pandemic + Bluetooth Cellular networks: 4G / 5G 4G/5G cellular networks The solution for wide-area mobile Internet Widespread deployment/use More mobile-broadband-connected devices than xed-broadband-connected devices (5:1 in 2019) Transmission rate 4G: up to 100Mbps 5G: up to 10~20Gbps Technical standards rd 3 Generation Partnership Project (3GPP) (www.3gpp.org) fi 4G/5G cellular network Similarity to wired Internet Edge/core distinction, but both below to the same carrier Global cellular network: a network of networks Widespread use of protocols we’ve studied - HTTP, DNS, TCP, UDP, IP, NAT, separate of data/control planes, SDN, Ethernet, tunneling Interconnected to wired Internet  4G/5G cellular network Di erences from wired Internet Di erent wireless link layer st Mobility as the 1 class service User “identity” (via SIM card) Business model - Users subscribe to a cellular provider - Strong notion of “home network” vs roaming on visited nets. - Global access, with authentication infrastructure, and inter-carrier settlements ff ff  Elements of 4G LTE architecture Mobile device: Smartphone, tablet, laptop, IoT, … with 4G LTE radio Identity Module (SIM) card 64-bit International Mobile Subscriber Identity (IMSI), stored on Subscriber Identity Module (SIM) card LTE jargon: User Equipment (UE)  Elements of 4G LTE architecture Base station: At “edge” of carrier’s network Manages wireless radio resources, mobile devices in its coverage area (“cell”) Coordinates devices authentication with other elements Similar to WiFi AP but, Active role in user mobility Coordinates with nearby base stations to optimize radio use LTE jargon: eNode-B Radio Access Network: 4G radio Connects device (UE) to a base station (eNode-B) Multiple devices connected to each base station Many di erent possible frequency bands, multiple channels in each band Popular band: 600, 700, 850, 1500, 1700, 1900, 2100, 2600, 3500 MHz Separate upstream and downstream channels Sharing 4G radio channel among users OFDM: Orthogonal Frequency Division Multiplexing Combination of FDM and TDM 100’s Mbps possible per user/device ff OFDMA: time division (LTE) OFDMA: time division (LTE) Physical Resource Block (PRB) : blocks of 7×12=84 resource elements Unit of transmission scheduling  OFDMA: time division (LTE) Transmission scheduling example: send to 7 UEs in 7 blocks of REs in one PBR  Elements of 4G LTE architecture Home Subscriber Service: Stores info about mobile devices for which the HSS’s network is their “home network” Works with MME in device authentication  Elements of 4G LTE architecture Serving Gateway (S-GW), PDN Gateway (P-GW): Lie on data path from mobile to/from Internet P-GW Gateway to mobile cellular network Looks like any other Internet gateway router Provide NAT services Other routers Extensive use of tunneling  Elements of 4G LTE architecture Mobility Management Entity: Device authentication (device- to-network, network-to-device) coordinated with mobile home network HSS Mobile device management Device handover between cells Tracking/paging device location Path (tunneling) setup from mobile device to P-GW LTE: data plane control separation Control plane New protocols for mobility management, security, authentication (later) Data plane New protocols at link, physical layers Extensive use of tunneling to facilitate mobility LTE data plane protocol stack: first hop LTE link layer protocols: Packet Data Convergence: header compression, encryption Radio Link Control (RLC) protocol: fragmentation/reassembly, reliable data transfer Medium Access: requesting, use of radio transmission slots (OFDM) LTE data plane protocol stack: packet core Tunneling: Mobile datagram is encapsulated using GPRS Tunneling Protocol (GTP), sent inside UDP datagram to S-GW S-GW re-tunnels datagrams to P-GW Supporting mobility: only tunneling endpoints change when mobile user moves GTP tunneling UE to eNodeB eNB to S-GW S-GW to P-GW P-GW to PDN LTE data plane: associating with a BS 1 BS broadcasts primary synch signal every 5ms on all frequencies - BSs from multiple carriers may be broadcasting synch signals nd 2 Mobile nds a primary synch signal, then locates 2 synch signal on this freq. - Mobile then nds info broadcast by BS: channel bandwidth, con guration; BS’s cellular carrier info - Mobile may get info from multiple base stations, multiple cellular networks 3 Mobile selects which BS to associate with (e.g., preference for home carrier) 4 More steps still are needed to authenticate, establish state, set up data plane fi fi fi LTE data plane: sleep modes As in WiFi, Bluetooth: LTE mobile may put radio to sleep to conserve battery Light sleep: after 100’s msec of inactivity - Wake up periodically (100’s msec) to check for downstream transmissions Deep sleep: after 5~10 secs of inactivity - Mobile may change cells while deep sleeping - need to re-establish association Global cellular network: a network of IP network Home network HSS: Identity & services info, while in home network and roaming All IP: Carriers interconnect with each other, and public Internet at exchange points Legacy 2G, 3G: not all IP, handled otherwise On to 5G: motivation On to 5G: motivation “Initial standards and launches have mostly focused on enhanced Mobile Broadband, 5G is expected to increasingly enable new business models and countless new use cases, in particular those of massive Machine Type Communication and Ultra-reliable and Low Latency Communications On to 5G: motivation eMBB Industry vertical: - Manufacturing - Constructions - Transport - Health - Smart communities - Education - Tourism mMTC URLLC - Agriculture - Finance On to 5G: Radio ×10 increase in peak bitrate, ×10 decrease in latency, ×10 increase in tra c capacity over 4G 5G NR (new radio) Two frequency bands: FR1 (450MHz ~ 6GHz) and FR2 (24GHz ~ 52GHz): millimeter wave frequencies Not backwards-compatible with 4G MIMO: multiple directional antennae Millimeter wave frequencies: much higher data rates, but over shorter distances Pico-cells: cells diameters (10 ~ 100m) Massive, dense deployment of new base stations required ffi On to 5G: SDN-like architecture Functional elements: communication, computation, data Control plane: resource control Control plane: resources, as used by users Beyond 5G? “6G” not obviously next: “NextG” and “Beyond 5G” heard more often than “6G” 5G on an evolutionary path (like the Internet) Agility: cloud technologies (SDN) mean that new features can be introduced rapidly, deployed continuously Customization: change can be introduced bottom-up (e.g., by enterprises and edge cloud partners with Private 5G) - No need to wait for standardization - No need to reach agreement (among all incumbent stakeholders) Questions?