Cellular Network
이동 무선 통신의 초석, 유선 네트워크로 지원하지 못하는 지역에서도 사용 가능
challenges(most important)
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wireless: communication over wireless link
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wireless links and NW characteristics
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WIFI: 802.11 wireless LANs
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Cellular NW: 4G & 5G
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mobility: handling the mobile user who changes point of attachment to NW
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mobility management: principles
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mobility management: 4G/5G NW, Mobile IP
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mobility: impact on higher-layer protocols
Elements
wireless hosts
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laptop, phone…
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run applications
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may be stationary or mobile
base station
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typically connected to wired NW
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relay - responsible for sending packets between wired NW and wireless host(s) in its area
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ex) cell towers, 802.11 access points
wireless link
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typically used to connect mobiles to base station, also used as backbone link
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multiple access protocol coordinates link access
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various transmission rates and distances, frequency bands
infrastructure mode
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base station connects mobiles into wired NW
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handoff: mobile changes base station providing connection into wired NW
ad hoc mode
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no base stations
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nodes can only transmit to other nodes within link coverage
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nodes organize themselves into a NW: route among themselves
wireless network taxonomy
single hop | multiple hops | |
infrastructure | host connects to base station(WIFI, cellular) which connects to larger internet | host may have to relay through several wireless nodes to connect to larger Internet(mesh net) |
no infra | no base station,
no connection to larger Internet(Bluetooth, ad hoc nets) | no base station, |
Cellular Wireless Networks
Wireless Link characteristics
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important differences from wired link
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decreased signal strength
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interference from other sources
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multipath propagation
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SNR : signal to noise ration / larger SNR → easier to extract signal from noise
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SNR versus BER tradeoffs
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given physical layer : increase power → increase SNR → decrease BER
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given SNR : choose physical layer that meets BER requirement, giving highest throughput
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Hidden terminal problem, Signal attenuation
Code Division Multiple Access
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all users share same frequency, but each user has own chipping sequence to encode data (orthogonal)
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encoding : original data * chipping sequence
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decoding :encoded data * chipping sequence
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계산법 HW04, SBY SHEET 10-2 참고
WiFi : 802.11 wireless LANs
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All use CSMA/CA for multiple access, and have base-station and ad-hoc network versions
Architecture
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wireless host communicates with base station
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base station = access point
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Basic Service Set(BSS) (aka “cell”) in infrastructure mode contains :
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wireless hosts, access point(AP):base station, ad hoc mode : host only
Channels, association
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spectrum divided into channels at different frequencies
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AP admin chooses frequency for AP
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interference possible : channel can be same as that chosen by neighboring AP
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arriving host : must associate with an AP
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scans channels, listening for beacon frames containing AP’s name (SSID) and MAC address
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selects AP to associate with
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then may perform authentication
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then typically run DHCP to get IP address in AP’s subnet
Passive / Active scanning
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passive scanning
(1) beacon frames sent from APs
(2) association Request frame sent : H1 to selected AP
(3) association Response frame sent from selected AP to H1
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active scanning
(1) Probe Request frame broadcast from H1
(2) Probe Response frames sent from APs
(3) Association Request frame sent : H1 to selected AP
(4) Association Response frame sent from selected AP to H1
Multiple access
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avoid collisions : 2+ nodes transmitting at same time
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802.11 : CSMA - sense before transmitting : don’t collide with detected ongoing transmission by another node
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802.11 : no collision detection
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difficult to sense collisions : high transmitting signal, weak received signal due to fading
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can’t sense all colisions in any case : hidden terminal, fading
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goal : avoid collisions : CSMA/CA
MAC Protocol : CSMA/CA
+Avoiding collisions
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idea : sender “reserves” channel use for data frames using small reservation packets
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sender first transmits small request-to-send packet to BS using CSMA
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-RTSs may still collide with each other (but they’re short)
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BS broadcasts clear-to-send CTS in response to RTS
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CTS heard by all nodes
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sender transmits data frame
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other station defer transmissions
Addressing
Mobility within same subnet
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H1 remains in same IP subnet : IP address can remain same
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switch : which AP is associated with H1?
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self-learning (Ch.6) : switch will see frame from H1 and remember which switch port can be used to reach H1
Advanced capabilites
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Rate adaption
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base station, mobile dynamically change transmission rate(physical layer modulation technique) as mobile moves, SNR varies
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SNR decreases, BER increase as node moves away from base station
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When BER becomes too high, switch to lwer transmission rate but with lower BER
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Power management
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node-to-AP : I am going to sleep until next beacon frame
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AP knows not to transmit frames to this node
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node wakes up before next beacon frame
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beacon frame : contains list of mobiles with AP-to-mobile frames waiting to be sent
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node will stay awake if AP-to-mobile frames to be sent; otherwise sleep again until next beacon frame
Personal area networks : Bluetooth
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less than 10m, replacement for cables, ad hoc : no infrastructure, 2.4~2.5GHz up to 3Mbps
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master controller / clients devices : master polls clients, grants requests for client transmissions
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TDM / FDM : sender uses 79 frequency channels in known, pseudo-random order slot-to-slot
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parked mode, bootstrapping
Cellular networks : 4G and 5G
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the solution for wide area mobile internet
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transmission rates up to 100’s Mbps
Similarities to wired Internet
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edge/core distinction, but both below to same carrier
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global cellular network : a network of networks
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widesparead use of protocols we’ve studied : HTTP, DNS, TCP, UDP, IP, NAT, separation of data/control planes, SDN, Ethernet, tunneling
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interconnected to wired Internet
Differences from wired Internet
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different wireless link layer
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mobility as a first class service
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user identity : via SIM card
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business model : users subscribe to a cellular provider
Elements
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Mobile devices, Base station, Home Subscriber Service, Serving Gateway, PDN Gateway, Mobility Management Entity
LTE : data plane control plane separation
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LTE radio access network
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downstream channel: FDM, TDM within
frequency channel (OFDM - orthogonal
frequency division multiplexing)
• “orthogonal”: minimal interference
between channels
• upstream: FDM, TDM similar to OFDM
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each active mobile device allocated two or
more 0.5 ms time slots over 12 frequencies
• scheduling algorithm not standardized
– up to operator
• 100’s Mbps per device possible
LTE data plane protocol stack : packet core
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tunneling :mobile datagram encapsulated using GPRS Tunneling Protocol, sent inside UDP datagram to S-GW
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S-GW re-tunnels datagrams to P-GW
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supporting mobility : only tunneling endpoints change when mobile user moves
LTE data plane : associating with a BS
1) BS broadcasts primary synch signal every 5 ms on all frequencies
- BSs from multiple carriers may be broadcasting synch signals
2) mobile finds a primary synch signal, then locates 2nd synch signal on this freq.
- mobile then finds info broadcast by BS: channel bandwidth, configurations; 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 needed to authenticate, establish state, set up data plane
LTE mobiles : sleep modes
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light sleep : 100’s msec
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deep sleep : 5-10sec
Global cellular network : a network of IP networks
On to 5G
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goal: 10x increase in peak bitrate, 10x decrease in latency, 100x
increase in traffic capacity over 4G
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5G NR (new radio)
- MIMO: multiple directional antennae
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millimeter wave frequencies: much higher data rates, but over shorter distances
- pico-cells: cells diameters: 10-100 m
- massive, dense deployment of new base stations required
Mobility
Mobility management : principles
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home network, visited network, ISP/WiFi : no notion of global home
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generic
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home needs to know where you are
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Mobility with indirect routing
- triangle routing : inefficient when correspondent and mobile are in same network
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overcomes triangle routing inefficiencies
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non-transparent to correspondent: correspondent must get
care-of-address from home agent
Mobility management : practice
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4G/5G networks
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Mobile IP
