Download Ethernet Switches

Ethernet
Dominant LAN technology:
 cheap $$ for 100Mbs or even 1Gbps!
 first widely used LAN technology
 Simpler, cheaper than token LANs and ATM
 Kept up with speed race: 10, 100, 1000 Mbps
Metcalfe's Ethernet
sketch
Bus: Thin coaxial cable
5: LANs, ARP,Hubs etc,
Today, Ethernet installations use a Star topology with a hub or
switch at the center
1
Star topology
 Bus topology popular through mid 90s
 Now star topology prevails
 Connection choices: hub or switch (more later)
hub or
switch
5: LANs, ARP,Hubs etc,
2
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network
layer protocol packet) in Ethernet frame
If there’s a match, pass
frame’s data field to
Network Layer;
otherwise, discard
[46, 1500 bytes]
If over, fragment;
else if less than 46,
have it stuffed
Preamble:
 7 bytes with pattern 10101010 followed by one byte with
pattern 10101011
 used to synchronize receiver, sender clock rates
5: LANs, ARP,Hubs etc,
3
Ethernet Frame Structure
(more)
 Addresses: 6 bytes, frame is received by all
adapters on a LAN and dropped if address does
not match
 Type: indicates the higher layer protocol, mostly
IP but others may be supported such as Novell
IPX and AppleTalk)
 CRC: checked at receiver, if error is detected, the
frame is simply dropped
5: LANs, ARP,Hubs etc,
4
Unreliable, connectionless service
 Connectionless: No handshaking between sending
and receiving adapter.
 Unreliable: receiving 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
5: LANs, ARP,Hubs etc,
5
Ethernet uses CSMA/CD
• No slots
• adapter doesn’t transmit if it
senses that some other adapter
is transmitting, that is, carrier
sense
• transmitting adapter aborts
when it senses that another
adapter is transmitting, that is,
collision detection
• Before attempting a
retransmission, adapter
waits a random time, that
is, random access
5: LANs, ARP,Hubs etc,
6
Ethernet CSMA/CD algorithm
=min(10, num of collisions)
1. Adaptor receives datagram
4. If adapter detects another
from net layer & creates frame
transmission while
transmitting, aborts and sends
2. If adapter senses channel idle,
jam signal
it starts to transmit frame. If it
senses channel busy, waits
5. After aborting, adapter enters
until channel idle and then
exponential backoff: after the
transmits
mth collision, adapter chooses
a K at random from
3. If adapter transmits entire
m
{0,1,2,…,2
-1}. Adapter
frame without detecting
waits K·512 bit times and
another transmission, the
returns to Step 2
adapter is done with frame !
No signal energy in
channel for 96 bit
times
5: LANs, ARP,Hubs etc,
7
Ethernet’s CSMA/CD (more)
Jam Signal: make sure all
other transmitters are
aware of collision; 48 bits
Bit time: .1 microsec for 10
Mbps Ethernet ;
for K=1023, wait time is
about 50 msec
Exponential Backoff:
• Goal: adapt retransmission
attempts to estimated current
load
– heavy load: random wait
will be longer
• first collision: choose K from
{0,1}; delay is K· 512 bit
transmission times
• after second collision: choose K
from {0,1,2,3}…
• after ten collisions, choose K
from {0,1,2,3,4,…,1023}
5: LANs, ARP,Hubs etc,
8
CSMA/CD efficiency
Long-run fraction of time during which frames are being transmitted on the channel
without collisions when there is a large number of active nodes, with each node having
a large number of frames to send.
 Tprop = max prop between 2 nodes in LAN
 ttrans = time to transmit max-size frame
efficiency 
1
1  5t prop / ttrans
 Efficiency goes to 1 as tprop goes to 0
 Goes to 1 as ttrans goes to infinity
 Much better than ALOHA, but still decentralized,
simple, and cheap
5: LANs, ARP,Hubs etc,
9
Ethernet Technologies: 10Base2 (1990s)
 10: 10Mbps; 2: under 200 meters max cable length
 Max distance bet. 2 nodes without a repeater in between
 thin coaxial cable in a bus topology (broadcast
technology)
 repeaters used to connect up to multiple segments
 repeater repeats bits it hears on one interface to
its other interfaces: physical layer device only!
5: LANs, ARP,Hubs etc,
13
10BaseT and 100BaseT (802.3 LAN)
Standardized by IEEE 802.3
 10/100 Mbps rate; latter called ''fast ethernet''
 T stands for Twisted Pair
 There is a Hub (broadcast technology) to which
nodes are connected by 2 pairs of twisted pair
(Category 5 with RJ-45 connector), thus
Transmit, receive
''star topology''
 CSMA/CD not implemented at hub; adapter sense
the channel and detect collision during
transmission
5: LANs, ARP,Hubs etc,
Adapter has a point-to-point connection to the hub
14
10BaseT and 100BaseT (more)
 Max distance from node to Hub is 100 meters
Management Features
 Hub can internally disconnect jabbering adapter
 Hub can gather monitoring information, statistics
for display to LAN administrators

Bandwidth usage, collision rates, average frame sizes,
etc. – for network debugging, correction, future planning
5: LANs, ARP,Hubs etc,
15
Gbit Ethernet (IEEE 802.3z)
 use standard Ethernet frame format
 allows for point-to-point links and shared




broadcast channels
in shared mode, CSMA/CD is used; short distances
between nodes to be efficient
uses Star topology with hub, called here
''Buffered Distributor'‘ or switch at center
Full-Duplex at 1 Gbps for point-to-point links
Serves as a backbone for interconnecting multiple
10Mbps, 100 Mbps Ethernet LANs
10Gbit (802.3ae) extends Ethernet technology to point-to-point WAN
16
5: LANs, ARP,Hubs etc,
links
Interconnecting LANs
Q: Why not just one big LAN?
 Limited amount of supportable traffic: on single
LAN, all stations must share bandwidth
 limited length: 802.3 specifies maximum cable
length
 Large ''collision domain'' (can collide with many
stations)
 limited number of stations: 802.5 have token
passing delays at each station
5: LANs, ARP,Hubs etc,
17
Definition of Terms
Hubs
 Physical Layer devices: essentially repeaters
operating at bit levels: repeat received bits on one
interface to all other interfaces
 Hubs can be arranged in a hierarchy (or multi-tier
design), with backbone hub at its top
5: LANs, ARP,Hubs etc,
18
Hubs (more)
 Each connected LAN referred to as LAN segment
 Hubs do not isolate collision domains: node may collide with any
node residing at any segment in LAN
 Hub Advantages:
 simple, inexpensive device
 Allows Inter-LAN segment communication
 As a multi-tier, it provides graceful degradation: portions of
the LAN continue to operate if one hub malfunctions
 extends maximum distance between node pairs (100m per
Hub)
5: LANs, ARP,Hubs etc,
19
Hub limitations
 single collision domain results in no increase in max
throughput
 multi-tier throughput same as single segment
throughput
 individual LAN restrictions pose limits on number
of nodes in same collision domain and on total
allowed geographical coverage
 cannot connect different Ethernet types (e.g.,
10BaseT and 100baseT)
Constraints:
• Total number of hosts in a multi-tier LAN
• Geographical reach of multi-tier LAN
5: LANs, ARP,Hubs etc,
20
END OF SESSION
5: LANs, ARP,Hubs etc,
21
Bridges
 Link Layer devices: operate on Ethernet
frames, examining frame header and
selectively forwarding frame based on its
destination
 Bridge isolates collision domains since it
buffers frames and uses LAN destination
addresses
 When a frame is to be forwarded on a LAN
segment, a bridge uses CSMA/CD to access
segment and transmit
5: LANs, ARP,Hubs etc,
22
Bridges (more)
 Bridge advantages:
 Isolates
collision domains resulting in higher
total max throughput, and does not limit the
number of nodes nor geographical coverage
 Can
connect different type Ethernet since it is
a store and forward device
 Transparent:
no need for any change to host’s
LAN adapter configuration when connecting to
a bridge
5: LANs, ARP,Hubs etc,
23
Bridges: frame filtering, forwarding
 bridges filter packets
 same-LAN
-segment frames are not forwarded
onto other LAN segments
 forwarding:
 how
to know which LAN segment on which to
forward frame?
 looks like a routing problem (more shortly!)
5: LANs, ARP,Hubs etc,
24
Backbone Bridge
5: LANs, ARP,Hubs etc,
25
Interconnection Without Backbone
 Not recommended for two reasons:
- single point of failure at Computer Science hub
- all traffic between EE and SE must path over
CS segment
5: LANs, ARP,Hubs etc,
26
Bridge Filtering
 bridges learn which hosts can be reached through
which interfaces by maintaining a filtering table
 when a frame received, the bridge ''learns'' the
location of the sender: incoming LAN segment
 records sender location in filtering table
 filtering table entry:
 (Node LAN Address, Bridge Interface, Time Stamp)
 stale entries in the Filtering Table are dropped
(TTL can be 60 minutes)
5: LANs, ARP,Hubs etc,
27
Bridge Filtering
 filtering procedure:
if destination is on LAN on which frame was received
then drop the frame
else { lookup filtering table
if entry found for destination
then forward the frame on interface indicated;
else flood; /* forward on all but the interface
on
which the frame
arrived*/
}
5: LANs, ARP,Hubs etc,
28
Bridge Learning: example
Suppose C sends a frame to D and D replies back
with a frame to C
 C sends the frame to the bridge, but the bridge
has no info. about D, so it floods both LANs



bridge notes that C is on port 1
frame ignored on upper LAN
frame received by D
5: LANs, ARP,Hubs etc,
29
Bridge Learning: example
 D generates a reply to C, sends
 bridge
sees frame from D
 bridge notes that D is on interface 2
 bridge knows C on interface 1, so it selectively
forwards the frame out via interface 1
5: LANs, ARP,Hubs etc,
30
Bridges Spanning Tree
 for increased reliability, it is desirable to have
redundant, alternate paths from source to dest
 with multiple simultaneous paths, cycles result bridges may multiply and forward frame forever
 solution: organize bridges in a spanning tree by
disabling subset of interfaces
Disabled
5: LANs, ARP,Hubs etc,
31
Bridges vs. Routers
 both store-and-forward devices
 routers: network layer devices (examine network layer
headers)
 bridges are Link Layer devices
 routers maintain routing tables, implement routing
algorithms
 bridges maintain filtering tables, implement
filtering, learning and spanning tree algorithms
5: LANs, ARP,Hubs etc,
32
Routers vs. Bridges
Bridges + and + Bridge operation is simpler requiring less
processing bandwidth
- Topologies are restricted with bridges: a spanning
tree must be built to avoid cycles
- Bridges do not offer protection from broadcast
storms (endless broadcasting by a host will be
forwarded by a bridge)
5: LANs, ARP,Hubs etc,
33
Routers vs. Bridges
Routers + and + arbitrary topologies can be supported, cycling is
limited by TTL counters (and good routing protocols)
+ provide firewall protection against broadcast storms
- require IP address configuration (not plug and play)
- require higher processing bandwidth
 bridges do well in small (few hundred hosts) while
routers used in large networks (thousands of hosts)
5: LANs, ARP,Hubs etc,
34
Interconnection Devices
COMPARISON OF FEATURES
HUBS
BRIDGES
ROUTERS
ETHERNET
SWITCHES
Traffic
Isolation
No
Yes
Yes
Yes
Plug-andPlay
Yes
Yes
No
Yes
Optimal
Routing
No
No
Yes
No
Yes
No
No
Yes
Cut-through
5: LANs, ARP,Hubs etc,
35
Ethernet Switches
 Full-fledged packet switch

layer 2 (frame) forwarding,
filtering using LAN addresses
 Each LAN segment – in an
isolated collision domain
 Switching: A-to-B and A'to-B' simultaneously, no
collisions
 large number of interfaces
 often: individual hosts,
star-connected into switch
 Ethernet, but no
Provides direct upstream &
collisions!
downstream connections;
collision detection & carrier
5: LANs, ARP,Hubs etc,
sensing are not needed
36
Ethernet Switches
 Operate in full-duplex
 cut-through switching: frame forwarded
from input to output port without awaiting
for assembly of entire frame
 slight reduction in latency
 combinations of shared/dedicated,
10/100/1000 Mbps interfaces
For as long as the packet’s
destination is known, switch
transmits packet (with carrier
sensing)
5: LANs, ARP,Hubs etc,
37
Ethernet Switches (more)
Institutional Network using a combination of hubs, Ethernet switches,
router
Dedicated
Shared
5: LANs, ARP,Hubs etc,
38
END OF SESSION
5: LANs, ARP,Hubs etc,
39
Extra Topics
Wireless Networking is not going to be
included in the Finals.
5: LANs, ARP,Hubs etc,
40
IEEE 802.11 Wireless LAN
 wireless LANs: untethered (often mobile) networking
 IEEE 802.11 standard:
 MAC protocol
 unlicensed frequency spectrum: 900Mhz, 2.4Ghz
 Basic Service Set (BSS)
(a.k.a. cell) contains:
 wireless hosts
 access point (AP): base
station
 BSSs combine to form
distribution system (DS)
5: LANs, ARP,Hubs etc,
41
Elements of a Wireless Network
Wireless host
Network
Infrastructure
Wireless Access Point
Coverage area
5: LANs, ARP,Hubs etc,
42
Elements of a Wireless Network
Wireless Hosts
• end-system devices that run applications
• e.g. laptop, palmtop, PDA, phone, desktop computer
Wireless Links
• connects hosts to base station or another wireless host
• e.g. 802.11a, 802.11g, 802.11b, UMTS/WCDMA, GSM, etc.
Base Station
• sends/receives packets to and from a wireless host associated with base
station; coordinates multiple transmissions of hosts
• e.g. cell towers (cellular networks), Access Points (802.11 wireless
LANs)
Network Infrastructure
Network Infrastructure
5: LANs, ARP,Hubs etc,
• larger network with which a wireless host connects
to
43
From wired to wireless
Replacing a simple wired home network with a wireless 802.11 net
• Wireless NIC replaces wired Ethernet card at Hosts
• Access Point replaces Ethernet switch
• Virtually no changes needed at the network layer or above
• Main focus of system changes: link-layer
5: LANs, ARP,Hubs etc,
44
Wireless Design Considerations
Problem: Decreasing signal strength
• radio signal passing through wall
• signal in free space - disperses
Results in decreased
Signal strength
(or even path loss)
• electromagnetic radiation attenuates as it passes through matter
5: LANs, ARP,Hubs etc,
45
Wireless Design Considerations
Problem: Interference from other sources
• 2.4 GHz wireless phones
• 802.11b wireless LAN
• microwave
• nearby motor
Same frequency!
Electromagnetic noise
• radio sources transmitting in the same frequency band interferes with
each other
5: LANs, ARP,Hubs etc,
46
Wireless Design Considerations
Problem: Multipath Propagation
•Portions of electromagnetic wave reflect off objects and the ground –
results in blurring of received signal at receiver
5: LANs, ARP,Hubs etc,
47
Wireless Design Considerations
Problem: Multipath Propagation
It is also sometimes possible to mount the antenna so that the mounting structure
screens it from the reflections but not from the wanted signal. Changing the antenna
height can effectively reduce or eliminate the multipath signals by dispersing the signals
away from the receiving antenna
48
5: LANs, ARP,Hubs etc,
Wireless Links
• High and time-varying bit error rates will be more common
• 802.11 employs CRC error detection codes
• 802.11 uses link-level ARQ protocols that retransmit corrupted frames
• Broadcasting problem: Undetectable collisions
• hidden terminal problem – occurs when physical obstructions in the
environment prevent hosts from detecting each other.
hidden terminals: A, C
cannot hear each other
• obstacles
• signal attenuation
• collisions at B
5: LANs, ARP,Hubs etc,
49
Broadcasting problem: Undetectable collisions
• Fading of a signal’s strength causes undetectable collisions
•A and C are placed such that their
signals are not strong enough to
detect each other’s transmissions,
yet strong enough to interfere with
each other at Host B.
• goal: avoid collisions at B
• CSMA/CA: CSMA with Collision Avoidance
5: LANs, ARP,Hubs etc,
50
IEEE 802.11 Standards
SUMMARY
Standard
Frequency
Range
Data Rate
802.11b
2.4-2.485 GHz
Up to 11 Mbps
802.11a
5.1-5.8 GHz
Up to 54 Mbps
802.11g
2.4-2.485 GHz
Up to 54 Mbps
Operating at higher frequency results in
shorter transmission distance for a given
power level and suffer more from
multipath propagation
ISM bands in the United States.
5: LANs, ARP,Hubs etc,
51
IEEE 802.11 Architecture
Ad hoc Network
• network with no central control and with no connections to the outside
world
• formed “on the fly” – mobile devices in proximity communicates with
each other in the absence of a centralized AP
5: LANs, ARP,Hubs etc,
52
IEEE 802.11 Architecture
Infrastructure Wireless LAN
Wireless host
Network
Infrastructure
Wireless Access Point
Basic Service Set (BSS)
5: LANs, ARP,Hubs etc,
53
IEEE 802.11 Architecture
INTRODUCTION
BSS contains:
• 1 or more wireless stations – has an 802.11 NIC – contains MAC
address
• central base station (AP)
• has a unique MAC address (in it’s firmware)
• assigned with one or two-word Service Set Identifier (SSID)
• assigned also with channel number
5: LANs, ARP,Hubs etc,
54
IEEE 802.11 Architecture
Within the 85 MHz band (2.4-2.485 GHz), there are 11 overlapping
channels
• channels 1, 6, 11 are usually assigned to the APs, and
• each AP may be interconnected with a switch
5: LANs, ARP,Hubs etc,
55
Wi-Fi jungle
Any physical location where a wireless station receives a strong signal from 2 or more
APs
How does an incoming station connect to an AP?
SSID
Service Set Identifier (SSID) is the name of the wireless LAN network. It is also called the ESS-ID or, simply, the network name. A device cannot
connect to the network if the SSID on the device does not match the SSID of the network.
Once AP is selected, host dialogues with AP using 802.11 association
Station
sends
message into subnet to get its IP
protocol;
joinsDHCP
subnetdiscovery
if successful
Station scans 11 channels for beacon frames from any AP out there
address
56
5: LANs, ARP,Hubs etc,
• beacon frame – AP’s SSID & MAC address
Wi-Fi jungle
AUTHENTICATION
To create association with AP, station may be required to
authenticate itself to the AP.
Access may be permitted based on station’s MAC address
Or, User name and password may be required from the station
• AP typically communicates with an authentication server using a
protocol named RADIUS.
5: LANs, ARP,Hubs etc,
57
802.11 MAC Protocol
CSMA/CA
Why not detect collision?
Hidden terminal problem
Signal attenuation – received signal is typically very small compared
to the strength of transmitted signal
How frames are transmitted?
In its entirety; once a station begins to transmit, there’s no turning
back
5: LANs, ARP,Hubs etc,
58
802.11 MAC Protocol
LINK-LAYER ACKNOWLEDGEMENT SCHEME
DEST: After frame passes a CRC check,
it waits for SIFS time, then sends an
ACK frame
SOURCE: If sender does not receive an
ACK with a given amount of time, it
retransmits frame (using CSMA/CA to
access channel) and counts number of
retransmissions (if this exceeds max.
value, it discards frame)
Short Inter-frame Spacing (SIFS)
Distributed Inter-frame Spacing (DIFS)
5: LANs, ARP,Hubs etc,
59
802.11 MAC Protocol
LINK-LAYER TRANSMISSION SCHEME
1 If channel is idle, wait for DIFS time,
then transmit
Else
choose random back-off value and
count down this value when channel is
sensed idle. If channel is sensed busy,
retain value.
2 When counter = 0 (channel is idle),
station transmits entire frame and waits
for an ACK.
3 If ACK is received and there are other
Short Inter-frame Spacing (SIFS)
Distributed Inter-frame Spacing (DIFS)
frames to send,
station begins CSMA/CA in step 1
Else reenter back-off phase in step 1
using random value chosen for larger
60
5: LANs, ARP,Hubs etc,
interval
802.11 MAC Protocol
RESERVATION SCHEME for COLLISION AVOIDANCE
H1
AP
H2
When sender wants to send a data
frame, RTS is sent to AP
• includes total time required to
send data frame and an ACK
frame
AP responds by broadcasting a
CTS frame
• permission to send
• tells other stations to wait for
reserved duration
HIDDEN TERMINALS: H1 is hidden from H2, and vice versa
For channel reservation: RTS, CTS frames
5: LANs, ARP,Hubs etc,
61
802.11 MAC Protocol
RESERVATION SCHEME for COLLISION AVOIDANCE
1 H1 broadcasts RTS frame and is heard by
AP
2 AP responds with a CTS frame, and is
heard by H1 & H2
• After hearing CTS, H2 refrains from
transmitting for the time specified in the
CTS frame
RTS/CTS introduces delay and
consumes channel resources – used
only for transmitting long data frames
5: LANs, ARP,Hubs etc,
Used only when frame > RTS threshold (usually set > max frame length)
62
Collision Avoidance: RTS-CTS
exchange
 RTS and CTS short:
 collisions
less likely, of
shorter duration
 end result similar to
collision detection
 IEEE 802.11 allows:
 CSMA
 CSMA/CA: reservations
 polling from AP
5: LANs, ARP,Hubs etc,
63
802.11 MAC Operation
Data Frames and their ACK
DIFS
Data
Src
SIFS
Ack
Dest
DIFS
Contention Window
Next MPDU
Other
Defer Access
Backoff after Defer
 Acknowledgment should arrive within SIFS
 Senders wait for DIFS no-carrier time, then
exponential backoff delay [slot=Tprop]
5: LANs, ARP,Hubs etc,
64
Problems with 802.11 MAC as above
 Technical problems:

`code` not precise, esp. re backoff, count-down
 Missing elements…
Spec also allows PCF (Point Coordination Function):
polling to coordinate senders to ensure QoS
 SIFS < PIFS < DIFS (priorities!)

 Can’t detect collision while sending…
Wasteful – esp. for long packets
 Idea for long packets: reserve channel to avoid
collisions – RTS/CTS [optional] mechanism…

5: LANs, ARP,Hubs etc,
65
RTS/CTS [optional in 802.11 MAC]
 Sender sends small request-to-send (RTS) to AP
RTSs may collide with each other (but are short)
 Include indication of length of packet transmission
 Receiver broadcasts clear-to-send CTS in response to
RTS
 CTS heard by all nodes
 sender transmits data frame
 other stations defer transmissions for time
specified in CTS

Avoid data frame collisions completely
using small reservation packets!
5: LANs, ARP,Hubs etc,
66
Collision Avoidance: RTS-CTS exchange
A
B
AP
reservation collision
DATA (A)
defer
time
5: LANs, ARP,Hubs etc,
67
Chapter 5: Summary
 principles behind data link layer services:
 error detection, correction
 sharing a broadcast channel: multiple access
 link layer addressing, ARP
 various link layer technologies
 Ethernet
 hubs, bridges, switches
 IEEE 802.11 LANs
 journey down the protocol stack now OVER!
 Next
chapter: security
5: LANs, ARP,Hubs etc,
68