Download Lecture 22

Switch Table
 Q: how does switch know that
A’ reachable via interface 4,
B’ reachable via interface 5?
 A: each switch has a switch
table, each entry:

C’
B
6
 Q: how are entries created,
maintained in switch table?
something like a routing
protocol?
1
5
(MAC address of host, interface
to reach host, time stamp)
 looks like a routing table!

A
2
3
4
C
B’
A’
switch with six interfaces
(1,2,3,4,5,6)
5: DataLink Layer
5-1
Switch: self-learning
 switch learns which hosts
can be reached through
which interfaces


Source: A
Dest: A’
A A A’
C’
when frame received,
switch “learns” location of
sender: incoming LAN
segment
records sender/location
pair in switch table
B
1
6
5
2
3
4
C
B’
A’
MAC addr interface TTL
A
1
60
Switch table
(initially empty)
5: DataLink Layer
5-2
Switch: frame filtering/forwarding
When frame received:
1. record link associated with sending host
2. index switch table using MAC dest address
3. if entry found for destination
then {
if dest on segment from which frame arrived
then drop the frame
else forward the frame on interface indicated
}
else flood
forward on all but the interface
on which the frame arrived
5: DataLink Layer
5-3
Self-learning,
forwarding:
example
Source: A
Dest: A’
A A A’
C’
B
 frame destination
unknown: flood
A6A’
1
2
4
5
 destination A
location known:
selective send
C
A’ A
B’
3
A’
MAC addr interface TTL
A
A’
1
4
60
60
Switch table
(initially empty)
5: DataLink Layer
5-4
Interconnecting switches
 switches can be connected together
S4
S1
S2
A
B
S3
C
F
D
E
I
G
H
 Q: sending from A to G - how does S1 know to
forward frame destined to F via S4 and S3?
 A: self learning! (works exactly the same as in
single-switch case!)
5: DataLink Layer
5-5
Self-learning multi-switch example
Suppose C sends frame to I, I responds to C
S4
1
S1
S2
A
B
C
2
S3
F
D
E
I
G
H
 Q: show switch tables and packet forwarding in S1,
S2, S3, S4
5: DataLink Layer
5-6
Institutional network
to external
network
mail server
router
web server
IP subnet
5: DataLink Layer
5-7
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
5: DataLink Layer
5-8
Chapter 6: Wireless and Mobile Networks
Background:
 # wireless (mobile) phone subscribers now
exceeds # wired phone subscribers!
 computer nets: laptops, palmtops, PDAs,
Internet-enabled phone 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
5: DataLink Layer
5-9
Chapter 6 outline
6.1 Introduction
Wireless
 6.2 Wireless links,
characteristics

CDMA
 6.3 IEEE 802.11
wireless LANs (“wi-fi”)
 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
5: DataLink Layer
5-10
Elements of a wireless network
network
infrastructure
wireless hosts
 laptop, PDA, IP phone
 run applications
 may be stationary
(non-mobile) or mobile

wireless does not
always mean mobility
5: DataLink Layer
5-11
Elements of a wireless network
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
5: DataLink Layer
5-12
Elements of a wireless network
network
infrastructure
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
5: DataLink Layer
5-13
Characteristics of selected wireless link
standards
Data rate (Mbps)
200
54
5-11
802.11n
802.11a,g
802.11b
4
1
802.11a,g point-to-point
data
802.16 (WiMAX)
3G cellular
enhanced
UMTS/WCDMA-HSPDA, CDMA2000-1xEVDO
802.15
.384
UMTS/WCDMA, CDMA2000
.056
3G
2G
IS-95, CDMA, GSM
Indoor
Outdoor
10-30m
50-200m
Mid-range
outdoor
Long-range
outdoor
200m – 4 Km
5Km – 20 Km
5: DataLink Layer
5-14
Elements of a wireless network
network
infrastructure
infrastructure mode
 base station connects
mobiles into wired
network
 handoff: mobile
changes base station
providing connection
into wired network
5: DataLink Layer
5-15
Elements of a 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
5: DataLink Layer
5-16
Wireless network taxonomy
single hop
infrastructure
(e.g., APs)
no
infrastructure
host connects to
base station (WiFi,
WiMAX, cellular)
which connects to
larger Internet
no base station, no
connection to larger
Internet (Bluetooth,
ad hoc nets)
multiple hops
host may have to
relay through several
wireless nodes to
connect to larger
Internet: mesh net
no base station, no
connection to larger
Internet. May have to
relay to reach other
a given wireless node
MANET, VANET
5: DataLink Layer
5-17
802.16: WiMAX
 like 802.11 & cellular:
point-to-point
base station model
transmissions to/from
base station by hosts
with omnidirectional
antenna
 base station-to-base
station backhaul with
point-to-point antenna

point-to-multipoint
 unlike 802.11:
 range ~ 6 miles (“city
rather than coffee
shop”)
 ~14 Mbps
5: DataLink Layer
5-18
Chapter 6 outline
6.1 Introduction
Wireless
 6.2 Wireless links,
characteristics

CDMA
 6.3 IEEE 802.11
wireless LANs (“wi-fi”)
 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
5: DataLink Layer
5-19
Components of cellular network architecture
MSC
cell
 connects cells to wide area net
 manages call setup (more later!)
 handles mobility (more later!)
 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
Mobile
Switching
Center
Public telephone
network, and
Internet
Mobile
Switching
Center
wired network
5: DataLink Layer
5-20
Cellular standards: brief survey
2G systems: voice channels
 IS-136 TDMA: combined FDMA/TDMA (north
america)
 GSM (global system for mobile communications):
combined FDMA/TDMA

most widely deployed
 IS-95 CDMA: code division multiple access
GSM
Don’t drown in a bowl
of alphabet soup: use this
for reference only
5: DataLink Layer
5-21
Cellular standards: brief survey
2.5 G systems: voice and data channels
 for those who can’t wait for 3G service: 2G extensions
 general packet radio service (GPRS)
 evolved from GSM
 data sent on multiple channels (if available)
 enhanced data rates for global evolution (EDGE)
 also evolved from GSM, using enhanced modulation
 data rates up to 384K
 CDMA-2000 (phase 1)
 data rates up to 144K
 evolved from IS-95
5: DataLink Layer
5-22
Cellular standards: brief survey
3G systems: voice/data
 Universal Mobile Telecommunications Service (UMTS)
 data service: High Speed Uplink/Downlink packet
Access (HSDPA/HSUPA): 3 Mbps
 CDMA-2000: CDMA in TDMA slots
 data service: 1xEvlution Data Optimized (1xEVDO)
up to 14 Mbps
….. more (and more interesting) cellular topics due to mobility (stay
tuned for details)
5: DataLink Layer
5-23