Transcript Quiz-3 Review - Communications Systems Center

Chapters:
4b-Routing, 5-LAN, 6-Wireless
Review - Quiz-3
Nov. 12, 2015
Some material copyright 1996-2006
J.F Kurose and K.W. Ross, All Rights Reserved
Computer Networking:
A Top Down Approach
Featuring the Internet,
5th edition.
Jim Kurose, Keith Ross
Addison-Wesley
11/10/15
Q3-1
Chapter 4: Network Layer
 4. 1 Introduction
 4.2 Virtual circuit and
datagram networks
 4.3 What’s inside a
router
 4.4 IP: Internet
Protocol




Datagram format
IPv4 addressing
ICMP
IPv6
 4.5 Routing algorithms
 Link state (OSPF)
 Distance Vector (RIP)
 Hierarchical routing (BGP)
 4.6 Routing in the
Internet



RIP
OSPF
BGP
 4.7 Broadcast and
multicast routing
Networ
4-2
k Layer
Distance Vector: link cost changes
Link cost changes:
 good news travels fast
 bad news travels slow -
“count to infinity” problem!
 44 iterations before
algorithm stabilizes: see
text
Poisoned reverse:
 If Z routes through Y to
get to X :

Z tells Y its (Z’s) distance
to X is infinite (so Y won’t
route to X via Z)
 will this completely solve
count to infinity problem?
60
x
4
y
50
1
z
Y advertises X in 4 hops
Z sends datagrams for X to Y
Z advertises "X in 5 hops".
Y-X link cost goes to 60
Y thinks Z can route in 5 hops,
so Y advertises "X in 6", sends
datagrams back to Z.
Z sends datagrams back to Y,
advertises "X in 7".
Y sends datagrams back to Z,
advertises "X in 8".
Q3-3
RIP (Distance-Vector Algorithm)
B
M
128.230.0.0/16
Y
Router A Table
Prefix
Distance Port
128.230.
2
X
130.207.
6
N
209.196.
7
X
24.56.
9
X
A
X
C
Router B Table
Prefix
Distance Port
128.230.
2
X
130.207.
6
X
209.196.
5
M
24.56.
11
X
Z
Router C Table
Prefix
Distance Port
128.230.
2
X
130.207.
4
X
209.196.
7
X
24.56.
11
P
Construct the Routing Table for Router X. Use "L" for the port to the local LAN.
Router X Table
Prefix
Distance Port
128.230.
1
L
130.207.
5
C
209.196.
6
B
24.56.
10
A
Using Poison Reverse, construct the Updates sent from Router X to A, B, and C. (infinity -> 15).
Update X to A Table
Prefix
Distance
128.230.
1
130.207.
5
209.196.
6
24.56.
15
Update X to B Table
Prefix
Distance
128.230.
1
130.207.
5
209.196.
15
24.56.
10
“Poison Reverse” prevents “ping-pong” routes.
Update X to C Table
Prefix
Distance
128.230.
1
130.207.
15
209.196.
6
24.56.
10
Q3-4
Graph abstraction
5
2
u
2
1
Graph: G = (N,E)
v
x
3
w
3
1
5
z
1
y
2
N = set of routers = { u, v, w, x, y, z }
E = set of links ={ (u,v), (u,x), (v,x), (v,w), (x,w), (x,y), (w,y), (w,z), (y,z) }
Remark: Graph abstraction is useful in other network contexts
Example: P2P, where N is set of peers and E is set of TCP connections
Q3-5
Graphical Method
(animated - keep clicking)
5
2 2
5
3
v
w
5
4
3
u
3
2
1
x
2
1
8
1
3
1
5
z
4
y
2
Next Slide
Q3-6
Hierarchical OSPF
Only this router
seen outside Area 1,
where it represents
all the subnets in
Area 1
Q3-7
Hierarchical Routing (BGP)
Our routing study thus far - idealization
 all routers identical
 network “flat”
… not true in practice
scale: with 200 million
destinations:
 can’t store all dest’s in
routing tables!
 routing table exchange
would swamp links!
administrative autonomy
 internet = network of
networks (Autonomous
Systems)
 each network admin may
want to control routing in its
own network
Q3-8
Interconnected ASes
3c
3a
3b
AS3
1a
2a
1c
1d
1b
Intra-AS
Routing
algorithm
2c
AS2
AS1
Inter-AS
Routing
algorithm
Forwarding
table
2b
 Forwarding table is
configured by both
intra- and inter-AS
routing algorithm


Intra-AS sets entries
for internal dests
Inter-AS & Intra-As
sets entries for
external dests
Q3-9
Broadcast Routing
 Deliver packets from source to all other nodes
 Source duplication is inefficient:
duplicate
duplicate
creation/transmission
R1
R1
duplicate
R2
R2
R3
R4
source
duplication
R3
R4
in-network
duplication
 Source duplication: how does source
determine recipient addresses?
Q3-10
In-network duplication
 Flooding: when node receives brdcst pckt,
sends copy to all neighbors

Problems: cycles & broadcast storm
 Controlled flooding: node only brdcsts pkt
if it hasn’t brdcst same packet before
Node keeps track of pckt ids already brdcsted
 Or reverse path forwarding (RPF): only forward
pckt if it arrived on shortest path between
node and source

 Spanning tree
 No redundant packets received by any node
Q3-11
Chapter 5 - Link and Physical Layers
 5.1 Introduction and




services
5.2 Error detection
and correction
5.3Multiple access
protocols
5.4 Link-Layer
Addressing
5.5 Ethernet
 5.6 Hubs and switches
 5.7 PPP
 5.8 Link Virtualization:
ATM and MPLS
Q3-12
Link Layer: Introduction
Some terminology:
“link”
 hosts and routers are nodes
 communication channels that
connect adjacent nodes along
communication path are links



wired links
wireless links
LANs
 layer-2 packet is a frame,
encapsulates datagram
data-link layer has responsibility of
transferring datagram from one node
to adjacent node over a link
Q3-13
Adaptors Communicating
datagram
sending
node
frame
adapter
rcving
node
link layer protocol
physical layer
media and signals
frame
adapter
 link layer implemented in  receiving side
“adaptor” (aka NIC)
 looks for errors, rdt, flow
control, etc
 Ethernet (802.11) card,
 extracts datagram, passes
serial card, WiFi card
to receiving node
 sending side:


encapsulates datagram in
a frame
adds error checking bits,
rdt, flow control, etc.
 adapter is semi-
autonomous
 link & physical layers
Q3-14
Parity Checking - Forward Error Correction (FEC)
Single Bit Parity:
Detect single bit errors
Two Dimensional Bit Parity:
Detect and correct single bit errors
Odd Parity = choose parity
bit so that there is an odd
number of 1's.
1
Even Parity = choose parity
bit so that there is an even
number of 1's.
0
0
Q3-15
Checksumming: Cyclic Redundancy Check
 view data bits, D, as a binary number
 choose r+1 bit pattern (generator), G
 goal: choose r CRC bits, R, such that



<D,R> exactly divisible by G (modulo 2)
receiver knows G, divides <D,R> by G. If non-zero remainder:
error detected!
can detect all burst errors less than r+1 bits
 widely used in practice (ATM, HDLC)
Q3-16
This is for a 3-bit CRC. 3 bits are added to
the data bit stream (D), to be replaced by the
CRC (R) when the data bit stream is sent.
The "Generator Polynomial" (G=1001) must
have 1 more bit than the CRC, and the first
and last bits must be "1".
CRC Example
Want:
D.2r XOR R = nG
equivalently:
D.2r = nG XOR R
equivalently:
if we divide D.2r by G,
want remainder R
R = remainder[
D.2r
G
The "division" uses a
bit-wise XOR logic op
rather than an
arithmetic subtract
operation (no carry).
0
0
0
0
]
Bits sent: 101110 011
Q3-17
MAC Protocols: a taxonomy
Three broad classes:
 Channel Partitioning


divide channel into smaller “pieces” (time slots,
frequency, code)
allocate piece to node for exclusive use
 Random Access
 channel not divided, allow collisions
 “recover” from collisions
 “Taking turns”
 Nodes take turns, but nodes with more to send can take
longer turns
"MAC" – Multiple Access Control protocol
Allows multiple hosts to use (access) the same
communication channel.
Q3-18
Channel Partitioning MAC protocols: TDMA
TDMA: time division multiple access
 access to channel in "rounds"
 each station gets fixed length slot (length = pkt trans time) in each
round
 unused slots go idle
 example: 6-station LAN, 1,3,4 have pkt, slots 2,5,6 idle
 TDM (Time Division Multiplexing): channel divided into N time slots,
one per user; inefficient with low duty cycle users and at light load.
 FDM (Frequency Division Multiplexing): frequency subdivided.
Code Division M. A. – later in Wireless section
Q3-19
Channel Partitioning MAC protocols: FDMA
frequency bands
FDMA: frequency division multiple access
 channel spectrum divided into frequency bands
 each station assigned fixed frequency band
 unused transmission time in frequency bands go idle
 example: 6-station LAN, 1,3,4 have pkt, frequency bands 2,5,6 idle
TDM (Time Division Multiplexing): channel divided into N time slots, one per
user; inefficient with low duty cycle users and at light load.
 FDM (Frequency Division Multiplexing): frequency subdivided.

Q3-20
Slotted ALOHA
Pros
 single active node can
continuously transmit at full
rate of channel
 highly decentralized: only
slots in nodes need to be in
sync
 simple
Cons
 collisions, wasting slots
 idle slots
 nodes may be able to detect
collision in less than time to
transmit packet
 clock synchronization
Maximum Network Efficiency – 37%
Q3-21
Pure (unslotted) ALOHA
 unslotted Aloha: simpler, no synchronization
 when frame first arrives from Network Layer
 transmit immediately
 collision probability increases:
 frame sent at t0 collides with other frames sent in [t0-1,t0+1]
Maximum Network Efficiency – 18%
Q3-22
CSMA/CD (Collision Detection)
CSMA/CD: carrier sensing, deferral as in CSMA
collisions detected within short time
 colliding transmissions aborted, reducing channel
wastage

 collision detection:
 easy in wired LANs: measure signal strengths,
compare transmitted, received signals
 difficult in wireless LANs: receiver shut off while
transmitting
 human analogy: the polite conversationalist
Q3-23
“Taking Turns” MAC protocols
Polling:
 master node
“invites” slave
nodes to transmit in
turn
 concerns:



polling overhead
latency
single point of
failure (master)
Token passing:
 control token passed from
one node to next
sequentially.
 token message
 concerns:



token overhead
latency
single point of failure (token)
Q3-24
MAC Addresses and ARP
 32-bit IP address:


network-layer address
used to get datagram to destination IP subnet
 MAC (or LAN or Physical or Ethernet) address:


used to get frame from one interface to another physically-connected
interface (same network)
48 bit MAC address (for most LANs)
burned in the adapter ROM (can be changed by OS software).
Q3-25
LAN Addresses and ARP
Each adapter on LAN has unique LAN address (written as hex)
1A-2F-BB-76-09-AD
71-65-F7-2B-08-53
LAN
(wired or
wireless)
Broadcast address =
FF-FF-FF-FF-FF-FF
(all 1's)
= adapter
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
Q3-26
ARP: Address Resolution Protocol
Question: how to determine
MAC address of B
knowing B’s IP address?
137.196.7.78
 Each IP node (Host, Router)
on LAN has ARP table
 ARP Table: IP/MAC address
mappings for some LAN
nodes
1A-2F-BB-76-09-AD
137.196.7.23
137.196.7.14
LAN
71-65-F7-2B-08-53
137.196.7.88
< IP address; MAC address; TTL>

TTL (Time To Live): time
after which address
mapping will be forgotten
(typically 20 min)
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
Q3-27
DHCP: Dynamic Host Configuration Protocol
Goal: allow host to dynamically obtain its IP
address from network server when it joins
network
Can renew its lease on address in use
Allows reuse of addresses (only hold address while
connected an “on”
Support for mobile users who want to join network.
Q3-28
DHCP client-server scenario
DHCP server: 223.1.2.5
Notes:
1. Server listens on
UDP port 67
2. Client listens on
UDP port 68
3. Both use IP
Broadcast
Address for
destination addr.
4. Client uses 0.0.0.0
for source IP
time
addr.
DHCP discover
arriving
client
src : 0.0.0.0, 68
dest.: 255.255.255.255,67
yiaddr: 0.0.0.0
transaction ID: 654
DHCP offer
src: 223.1.2.5, 67
dest: 255.255.255.255, 68
yiaddrr: 223.1.2.4
transaction ID: 654
Lifetime: 3600 secs
DHCP request
src: 0.0.0.0, 68
dest:: 255.255.255.255, 67
yiaddrr: 223.1.2.4
transaction ID: 655
Lifetime: 3600 secs
DHCP ACK
Newer “service discovery” protocols
(e.g., “Bonjour”,"ZeroConf") allow a host to
randomly self-assign a “local link” address
from the reserved range 169.254.0.0/16 to use
while contacting a local “configuration server”
using a mDNS multicast address, 224.0.0.251.
src: 223.1.2.5, 67
dest: 255.255.255.255, 68
yiaddrr: 223.1.2.4
transaction ID: 655
Lifetime: 3600 secs
Q3-29
Ethernet
“dominant” wired LAN technology:
 cheap $20 for 100Mbs!
 first widely used LAN technology
 Simpler, cheaper than token LANs and ATM
 Kept up with speed race: 10 Mbps – 10 Gbps
Metcalfe’s Ethernet
sketch
Q3-30
Star topology
 Bus topology popular through mid 90s
 Now star topology prevails
 Connection choices: hub or switch (more later)
hub or
switch
Q3-31
Ethernet Frame Structure
 Addresses: 6 bytes (3 bytes=vendor, 3=serial no.)
 if adapter receives frame with matching destination
address, or with broadcast address (eg ARP packet), it
passes data in frame to net-layer protocol
 otherwise, adapter discards frame
 Type: 2 bytes - indicates the higher layer protocol
(mostly IP but others may be supported such as
ARP, Novell IPX, and AppleTalk)
 CRC: checked at receiver, if error is detected, the
frame is simply dropped
"Data" includes Network, Transport, and Application headers
and application data (if any).
Q3-32
Unreliable, connectionless service
 Connectionless: No handshaking between sending
and receiving adapter.
 Unreliable: receiving Ethernet adapter doesn’t
send acks or nacks to sending adapter



stream of datagrams passed to network layer can have
gaps
gaps will be filled if app is using TCP
otherwise, app will see the gaps
Q3-33
Ethernet uses CSMA/CD
 No slots (asynchronous)
 Multiple access (MA) to a common
media (coax, hub,…)
 Adapter doesn’t transmit if it
senses that some other adapter is
transmitting, that is, carrier sense
(CS)
 Transmitting adapter aborts when
it senses that another adapter is
transmitting, that is, collision
detection (CD)
 Before attempting a
retransmission, adapter waits
a random time, that is, random
access
 exponential backoff: after
the mth collision, adapter
chooses a K at random from
{0,1,2,…,2m-1}. Adapter waits
K·(512 bit-times) ("time slot"
or "contention time") and tries
again.
 The "time slot" is 51.2 us for
10 Mbps, 5.12 us for 100
Mbps, 512 ns for Gbps.
Q3-34
CSMA/CD efficiency
 Tprop = max prop between 2 nodes in LAN
 ttrans = time to transmit max-size frame
= 96%
 Efficiency goes to 1 as tprop goes to 0
 Goes to 1 as ttrans goes to infinity
( 1us for 200 m)
( 120 us for 1500 bytes
at 100 Mbps)
 Much better than ALOHA, but still decentralized, simple,
and cheap.
Q3-35
10BaseT - 1000BaseT
 10/100/1000 Mbps rate; latter called “Giga Ethernet”
 T stands for Twisted Pair (F stands for Optical Fiber)
 Nodes connect to a Hub: “star topology”; 100 meter (330 ft) max
distance between nodes and Hub
twisted pair
hub
Q3-36
Hubs
Hubs are essentially physical-layer repeaters:
 bits coming from one link go out all other links
 at the same rate
 no frame buffering
 no CSMA/CD at hub: adapters detect collisions
 provides net management functionality
twisted pair
hub
Q3-37
Interconnecting with hubs
 Backbone hub interconnects LAN segments
 Extends max distance between nodes
 But individual segment collision domains become one large collision
domain, since datagrams immediately "repeated" out every port
(except arrival port)
 Can’t interconnect 10BaseT & 100BaseT.
hub
hub
hub
hub
Q3-38
Switch
 Link layer device
stores and forwards Ethernet frames
 examines frame header and selectively
forwards frame based on MAC dest address
 when frame is to be forwarded on segment,
uses CSMA/CD to access segment
 transparent
 hosts are unaware of presence of switches
 plug-and-play, self-learning
 switches do not need to be configured

Today, most small boxes sold as "Hubs" are actually "Switches". This
is definitely true if they work automatically with mixed 10 Mbps and
100 Mbps connections.
Q3-39
Self learning
 A switch has a switch table
 entry in switch table has three fields:
(MAC Address, Interface, Time Stamp)
 stale entries in table dropped (TTL can be 60 min)
 switch learns which hosts can be reached through
which interfaces
 when frame received, switch “learns” location of
sender and incoming LAN segment (physical port)
 records sender/port pair in switch table

Q3-40
Switch example
Suppose C sends frame to D
1
3
2
B
C
hub
hub
hub
A
address interface
switch
I
D
E
F
G
A
B
E
G
1
1
2
3
C
1
H
 Switch receives frame from from C
 notes in switch table that C is on interface 1
 If C were in table, the time stamp (TTL) would be updated.
 because D is not in table, switch forwards frame into interfaces
2 and 3 (Floods).
 frame received by D (and E & F, which ignore it)
Q3-41
Switch: traffic isolation
 switch installation breaks subnet into LAN
segments
 switch filters packets:
 same-LAN-segment frames not usually
forwarded onto other LAN segments
 segments become separate collision domains
switch
collision
domain
hub
collision domain
hub
collision domain
hub
Q3-42
Switches: dedicated access
 Switch with many interfaces
A
 Hosts have direct connection to
switch
 No collisions; full duplex
Switching: A-to-A’ and B-to-B’
simultaneously, no collisions
C’
B
switch
C
B’
A’
Q3-43
Switches vs. Routers
 both store-and-forward devices


routers: network layer devices (examine network layer headers)
switches are link layer devices
 routers maintain routing tables, implement routing algorithms
 switches maintain switch tables, implement filtering, learning
algorithms
Application
Transport
Network
MAC
Physical
5 HTTP
4 TCP
3 IP
2 Eth-MAC
1 Eth-Phys.
1
Hub
Switch
Q3-44
Institutional network
to external
network
mail server
web server
router
switch
IP subnet
hub
hub
hub
Q3-45
VLANs
Port-based VLAN: switch ports grouped (by
switch management software) so that
single physical switch ……
Virtual Local
Area Network
Switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure.
1
7
9
15
2
8
10
16
…
…
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
… operates as multiple virtual switches
1
7
9
15
2
8
10
16
…
Electrical Engineering
(VLAN ports 1-8)
…
Computer Science
(VLAN ports 9-16)
Q3-46
Port-based VLAN
router
 traffic isolation: frames
to/from ports 1-8 can only
reach ports 1-8

can also define VLAN based on
MAC addresses of endpoints,
rather than switch port
 dynamic membership:
ports can be dynamically
assigned among VLANs
1
7
9
15
2
8
10
16
…
Electrical Engineering
(VLAN ports 1-8)
…
Computer Science
(VLAN ports 9-15)
 forwarding between VLANS:
done via routing (just as with
separate switches)

in practice vendors sell combined
switches plus routers
Q3-47
Chapter 6 outline – Quiz 3
6.1 Introduction
Wireless
 6.2 Wireless links,
characteristics

CDMA (Carrier Detect)
 6.3 IEEE 802.11
wireless LANs (“wifi”)
 6.4 Cellular Internet
Access


architecture
standards (e.g., GSM)
Mobility
 6.5 Principles:
addressing and routing
to mobile users
 6.6 Mobile IP
 6.7 Handling mobility in
cellular networks
 6.8 Mobility and higherlayer protocols
6.9 Summary
Elements of a wireless network
wireless link




network
infrastructure
base station
 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
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
Elements of an ad hoc wireless 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 characteristics
Multiple wireless senders and receivers create
additional problems (beyond multiple access):
C
A
B
Hidden terminal problem
 B, A hear each other
 B, C hear each other
B
A
 A, C can not hear each other
C
C’s signal
strength
A’s signal
strength
space
Signal fading:
 B, A hear each other
 B, C hear each other
means A, C unaware of their interference  A, C can not hear each other
interfering at B
at B
IEEE 802.11 Wireless LAN
 802.11a
 802.11b
 5-6 GHz range
 2.4-5 GHz unlicensed spectrum
 up to 54 Mbps
 up to 11 Mbps
 802.11g
 direct sequence spread
spectrum (DSSS) in physical
 2.4-5 GHz range
layer
 up to 54 Mbps
• all hosts use same chipping
 802.11n: multiple antennae
code
 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
Internet
AP
BSS
1
hub, switch
or router
AP
BSS 2
with base station
 base station = access
point (AP)
 Basic Service Set (BSS)
(aka “cell”) in
infrastructure mode
contains:
 wireless hosts
 access point (AP or
WAP): 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 (only 1, 6, 11 don't
overlap)
 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
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)
A
C
A
B
B
C
C’s signal
strength
A’s signal
strength
space
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 back-off 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)
sender
receiver
DIFS
data
SIFS
ACK
Collision Avoidance: RTS-CTS exchange
A
AP
B
reservation collision
DATA (A)
time
defer
802.11 frame: addressing
R1 router
H1
Internet
AP
R1 MAC addr H1* MAC addr
dest. address
AP MAC addr H1 MAC addr R1 MAC addr
address 1
address 2
address 3
802.11 frame
WiFi
source address
802.3 frame
Ethernet
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
router
hub or
switch
BBS 1
AP 1
AP 2
H1
BBS 2
802.15: personal area network - Blue Tooth
 less than 10 m diameter
 replacement for cables
(mouse, keyboard,
headphones)
 ad hoc: no infrastructure
 master/slaves:


Bluetooth specification


P
2.4-2.5 GHz radio band
up to 721 kbps
radius of
coverage
M
S
slaves request permission to
send (to master)
master grants requests
 802.15: evolved from
P
S
P
M
Master device
S
Slave device
P
S
P
Parked device (inactive)
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 wide area net
 manages call setup (more later!)
 handles mobility (more later!)
Mobile
Switching
Center
Public telephone
network, and
Internet
Mobile
Switching
Center
wired network
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
frequency
bands
 CDMA: code division
multiple access
time slots
CDMA: two-sender interference
Orthogonal Codes
A
B
1
1
1
-1
1
-1
-1
-1
---0
Sum over i
Ai = 0
Bi = 0
Ai x Bi = 0
Ai x(-Bi) = 0
AxB
1
-1
1
1
1
-1
1
1
---0
1
-1
1
-1
1
1
-1
-1
---0
Cellular standards: Generations
1G systems: FM, voice, 1 mobile per frequency
2G systems: Digital, voice, many mobiles per freq.
3G systems: Digital, voice & data
Universal Mobile Telecommunications Service (UMTS)
 GSM next step, but using CDMA
CDMA-2000: CDMA in TDMA slots
(HSPA) 28 Mbps (AT&T)

1xEvolution Data Optimized
(1xEVDO) 14 Mbps (Verizon)
4G systems: Higher speed, like 802.17 WiMAX
As of August 2011, U.S. carriers currently use the following (most
will go to LTE):

AT&T = HSPA+
(63 Mbps) (2011 -> LTE)

Sprint = WiMax
(47 Mbps) (2013 -> LTE)

Verizon = LTE
(93 Mbps)

T-Mobile = HSPA+ (75 Mbps) (2013 -> LTE)

LTE
– 300 Mbps max. (700 MHz band, OFDMA)
LTE Advanced – 1000 Mbps max. (8x8 MIMO and 128 QAM)
Handoff with common MSC
 Handoff goal: route call via
new base station (without
interruption)
 reasons for handoff:

VLR Mobile
Switchin
g
Center
old
routing
old BSS

new
routing

new BSS
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