无线网络和移动网络(Wireless and Mobile Networks)

Background

number of wireless (mobile) phone subscribers now exceeds number of wired phone subscribers (5-to-1)!
number of wireless Internet-connected devices equals number of wireline Internet-connected devices
- laptops, Internet-enabled phones promise anytime untethered Internet access
two important (but different) challenges
- wireless: communication over wireless link
- mobility: handling the mobile user who changes point of attachment to network

Elements of a wireless network

wireless hosts
- laptop, smartphone
- run applications
- may be stationary (non-mobile) or mobile: wireless does not always mean mobility
base station(基站)
- typically connected to wired network
- relay - responsible for sending packets between wired network and wireless host(s) in its “area”
- - e.g., cell towers, 802.11 access points
wireless link
- typically used to connect mobile(s) to base station
- also used as backbone link
- multiple access protocol coordinates link access
- various data rates, transmission distance
Characteristics of selected wireless links

infrastructure mode
- base station connects mobiles into wired network
- handoff: mobile changes base station providing connection into wired network
ad hoc mode
- no base stations
- nodes can only transmit to other nodes within link coverage
- nodes organize themselves into a network: route among themselves
Wireless network taxonomy(无线网络分类)

Wireless links, characteristics

Wireless Link Characteristics

important differences from wired link make communication across (even a point to point) wireless link much more “difficult”:
- decreased signal strength: radio signal attenuates as it propagates through matter (path loss)
- interference from other sources: standardized wireless network frequencies (e.g., 2.4 GHz) shared by other devices (e.g., phone); devices (motors) interfere as well
- multipath propagation: radio signal reflects off objects ground, arriving ad destination at slightly different times
SNR: signal-to-noise ratio
- larger SNR – easier to extract signal from noise (a “good thing”)
SNR versus BER tradeoffs
- given physical layer: increase power -> increase SNR->decrease BER
- given SNR: choose physical layer that meets BER requirement, giving highest thruput
- - SNR may change with mobility: dynamically adapt physical layer (modulation technique, rate)

Wireless network characteristics

Multiple wireless senders and receivers create additional problems (beyond multiple access):

Hidden terminal problem
- B, A hear each other
- B, C hear each other
- A, C can not hear each other means A, C unaware of their interference at B

Signal attenuation:
- B, A hear each other
- B, C hear each other
- A, C can not hear each other interfering at B

Code Division Multiple Access (CDMA)

unique “code” assigned to each user; i.e., code set partitioning
- all users share 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”)
encoded signal = (original data) X (chipping sequence)
decoding: inner-product of encoded signal and chipping sequence
CDMA encode/decode:

two-sender interference:

IEEE 802.11 wireless LANs (“Wi-Fi”)

802.11b
- 2.4-5 GHz unlicensed spectrum
- up to 11 Mbps
- direct sequence spread spectrum (DSSS) in physical layer
- - all hosts use same chipping code
802.11a
- 5-6 GHz range
- up to 54 Mbps
802.11g
- 2.4-5 GHz range
- up to 54 Mbps
802.11n: multiple antennae
- 2.4-5 GHz range
- up to 200 Mbps

all use CSMA/CA for multiple access
all have base-station and ad-hoc network versions

802.11 LAN architecture

wireless host communicates with base station
- base station = access point (AP)
Basic Service Set (BSS) (aka “cell”) in infrastructure mode contains:
- wireless hosts
- access point (AP): base station
- ad hoc mode: hosts only

802.11: Channels, association

802.11b: 2.4GHz-2.485GHz spectrum divided into 11 channels at different frequencies
- AP admin chooses frequency for AP
- interference possible: channel can be same as that chosen by neighboring AP!
host: must associate with an AP
- scans channels, listening for beacon frames containing AP’s name (SSID) and MAC address
- selects AP to associate with
- may perform authentication [Chapter 8]
- will typically run DHCP to get IP address in AP’s subnet

802.11: passive/active scanning

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

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

IEEE 802.11: multiple access

avoid collisions: 2+ nodes transmitting at same time
802.11: CSMA - sense before transmitting
- don’t collide with ongoing transmission by other node
802.11: 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/C(ollision)A(voidance)

IEEE 802.11 MAC Protocol: CSMA/CA

802.11 sender
- (1) if sense channel idle for DIFS then transmit entire frame (no CD)
- (2) if sense channel busy then
- - start random backoff time
- - timer counts down while channel idle
- - transmit when timer expires
- - if no ACK, increase random backoff interval, repeat 2
802.11 receiver
- if frame received OK
- - return ACK after SIFS (ACK needed due to hidden terminal problem)

Avoiding collisions

idea: allow sender to “reserve” channel rather than random access of data frames: avoid collisions of long data frames

sender first transmits small request-to-send (RTS) packets to BS using CSMA
- RTSs may still collide with each other (but they’re short)
BS broadcasts clear-to-send CTS in response to RTS
CTS heard by all nodes
- sender transmits data frame
- other stations defer transmissions

avoid data frame collisions completely using small reservation packets!

Collision Avoidance: RTS-CTS exchange:

802.11 frame: addressing

802.11: mobility within same subnet

H1 remains in same IP subnet: IP address can remain same
switch: which AP is associated with H1?
- self-learning (Ch. 5): switch will see frame from H1 and “remember” which switch port can be used to reach H1

802.11: advanced capabilities

Rate adaptation
- base station, mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves, SNR varies

SNR decreases, BER increase as node moves away from base station
When BER becomes too high, switch to lower transmission rate but with lower BER

Cellular Internet access

Components of cellular network architecture

cell
- covers geographical region
- base station (BS) analogous to 802.11 AP
- mobile users attach to network through BS
- air-interface: physical and link layer protocol between mobile and BS
MSC
- connects cells to wired tel. net.
- manages call setup (more later!)
- handles mobility (more later!)

Cellular networks: the first hop

Two techniques for sharing mobile-to-BS radio spectrum
- combined FDMA/TDMA: divide spectrum in frequency channels, divide each channel into time slots
- CDMA: code division multiple access

2G (voice) network architecture :

3G (voice+data) network architecture :

Key insight: new cellular data network operates in parallel (except at edge) with existing cellular voice network
- voice network unchanged in core
- data network operates in parallel

3G versus 4G LTE network architecture :

4G: differences from 3G

all IP core: IP packets tunneled (through core IP network) from base station to gateway
no separation between voice and data – all traffic carried over IP core to gateway

Functional split of major LTE components

Radio+Tunneling: UE – eNodeB – PGW

Quality of Service in LTE

QoS from eNodeB to SGW: min and max guaranteed bit rate
QoS in radio access network: one of 12 QCI values

Principles: addressing and routing to mobile users

What is mobility?
spectrum of mobility, from the network perspective:

Mobility: vocabulary

home network : 归属地网络

home agent : 归属代理

permanent address : 永久地址

foreign agent : 外部代理

visited network : 被访网络

correspondent : 通信者

care-of address(COA) : 转交地址

Mobility: approaches

let routing handle it: routers advertise permanent address of mobile-nodes-in-residence via usual routing table exchange.
- routing tables indicate where each mobile located
- no changes to end-systems

not scalable to millions of mobiles

let end-systems handle it:
- indirect routing: communication from correspondent to mobile goes through home agent, then forwarded to remote
- direct routing: correspondent gets foreign address of mobile, sends directly to mobile

Mobility: registration

end result:
- foreign agent knows about mobile
- home agent knows location of mobile

Mobility via indirect routing :

Indirect Routing: comments

mobile uses two addresses:
- permanent address: used by correspondent (hence mobile location is transparent to correspondent)
- care-of-address: used by home agent to forward datagrams to mobile
foreign agent functions may be done by mobile itself
triangle routing: correspondent-home-network-mobile
- inefficient when correspondent, mobile are in same network

Indirect routing: moving between networks

suppose mobile user moves to another network
- registers with new foreign agent
- new foreign agent registers with home agent
- home agent update care-of-address for mobile
- packets continue to be forwarded to mobile (but with new care-of-address)
mobility, changing foreign networks transparent: on going connections can be maintained!

Mobility via direct routing :

direct routing: comments

overcome triangle routing problem
non-transparent to correspondent: correspondent must get care-of-address from home agent
- what if mobile changes visited network?

Accommodating mobility with direct routing

anchor foreign agent: FA in first visited network
data always routed first to anchor FA
when mobile moves: new FA arranges to have data forwarded from old FA (chaining)

Mobile IP

RFC 3344
has many features we’ve seen:
- home agents, foreign agents, foreign-agent registration, care-of-addresses, encapsulation (packet-within-a-packet)
three components to standard:
- indirect routing of datagrams
- agent discovery
- registration with home agent

indirect routing :

agent discovery :

agent advertisement: foreign/home agents advertise service by broadcasting ICMP messages (typefield = 9)

registration example :

Handling mobility in cellular networks

home network: network of cellular provider you subscribe to (e.g., Sprint PCS, Verizon)
- home location register (HLR): database in home network containing permanent cell phone #, profile information (services, preferences, billing), information about current location (could be in another network)
visited network: network in which mobile currently resides
- visitor location register (VLR): database with entry for each user currently in network
- could be home network

GSM: indirect routing to mobile

GSM: handoff with common MSC

handoff goal: route call via new base station (without interruption)
reasons for handoff:
- stronger signal to/from new BSS (continuing connectivity, less battery drain)
- load balance: free up channel in current BSS
- GSM doesn’t mandate why to perform handoff (policy), only how (mechanism)
handoff initiated by old BSS

1. old BSS informs MSC of impending handoff, provides list of 1+ new BSSs
1. MSC sets up path (allocates resources) to new BSS
1. new BSS allocates radio channel for use by mobile
1. new BSS signals MSC, old BSS: ready
1. old BSS tells mobile: perform handoff to new BSS
1. mobile, new BSS signal to activate new channel
1. mobile signals via new BSS to MSC: handoff complete. MSC reroutes call
1. MSC-old-BSS resources released

GSM: handoff between MSCs

anchor MSC: first MSC visited during call
- call remains routed through anchor MSC
new MSCs add on to end of MSC chain as mobile moves to new MSC
optional path minimization step to shorten multi-MSC chain

Handling Mobility in LTE

Paging: idle UE may move from cell to cell: network does not know where the idle UE is resident
- paging message from MME broadcast by all eNodeB to locate UE
handoff: similar to 3G:
- preparation phase
- execution phase
- completion phase

Mobility: cellular versus Mobile IP

Wireless, mobility: impact on higher layer protocols

logically, impact should be minimal …
- best effort service model remains unchanged
- TCP and UDP can (and do) run over wireless, mobile
… but performance-wise:
- packet loss/delay due to bit-errors (discarded packets, delays for link-layer retransmissions), and handoff
- TCP interprets loss as congestion, will decrease congestion window un-necessarily
- delay impairments for real-time traffic
- limited bandwidth of wireless links