Transcript Ethernet (Switched)

Media access sub-layer
OSI
Application
Presentation
Session
Transport
Network
Data Link
Physical
LOGICAL LINK sublayer
Framing
Error
control
Flow
control
MEDIA ACCESS sublayer
Transmission/reception
of frames
Single communication channel
that is shared by all the
machines on the network
Broadcast
Networks
computer
1
2
3
4
5
General Rule:
Smaller,
geographically
localized
networks
cable
Short messages sent by
any machine are received
by all others
Packets
Address
Quick Review…
Fields
Packets
1
2
3
4
5
3
ALL machines receive it, but one
processes it
Also possible to address a packet to ALL
machines (special code in the address
field)
Mode of operation:
Broadcasting
Also possible to address a packet to a
SUBSET of machines
(group number code in the address field)
Quick Review…
Mode of operation:
Multicasting
The Medium Access Sublayer
deals with
BROADCAST NETWORKS AND
THEIR PROTOCOLS
4.1 THE CHANNEL ALLOCATION
PROBLEM
4.2 MULTIPLE ACCESS
PROTOCOLS
IEEE STANDARD 802 FOR LANs
DATALINK
SWITCHING
VLANs
The Channel Allocation Problem


Central theme  How to allocate a
single broadcast channel among
competing users?
Static

FDM /TDM (Frequency/Time Division
Multiplexing)
FDM : Radio/TV broadcasts
 TDM : POTS (Plain Old Telephone System)
 GSM uses both (Global System for Mobile

Communications)


Wasteful of bandwidth
Dynamic
Pure/ Slotted ALOHA
 Carrier Sense Multiple Access (CSMA)
Protocols
 Collision free protocols

Dynamic Channel Allocation
Technologies
1. Pure ALOHA
2. Slotted ALOHA
3. CSMA
4. CSMA/CD (old ETHERNET)
5. Switching (Fast ETHERNET)
6. Token passing (Token Ring )
ALOHA Protocols
Back in 1970, the University of Hawaii
built a network out of radios that
broadcast signals. Basic idea:



Anyone may transmit whenever they
want.
(Continuous time model.)
Each radio detects collisions by listening
to its own signal. A collision is detected
when a sender doesn't receive the
signal that it just sent.
After a collision, wait a random amount
of time and transmit the same frame
again. This technique is known as
backoff.
Pure ALOHA
Slotted ALOHA


Time is divided into slots… can
only transmit at start of slot
Vulnerable period halved => max.
efficiency is doubled

Requires sync of clocks

Still poor at hi-loads
Carrier Sense, Multiple Access
(CSMA)
Communication Link

We can improve the performance of our
simple network greatly if we introduce
carrier sensing (CS). With carrier
sensing, each host listens to the data
being transmitted over the cable.
A host will only transmit its own frames
when it cannot hear any data being
transmitted by other hosts.
 When a frame finishes, an interframe gap
of about 9.6sec is allowed to pass before
another host starts transmitting its frame.

Carrier Sense Multiple Access
(CSMA)


Improves performance when higher
medium utilisation
When a node has data to transmit, the
node first listens to the cable (using a
transceiver) to see if a carrier (signal) is
being transmitted by another node.
CSMA with
Collision
Detection
CSMA/CD can be in one of three
states: contention, transmission, or
idle.
IEEE 802.3: CSMA/CD Bus LAN

The 802.3 standard describes the
operation of the MAC sub-layer in a bus
LAN that uses carrier sense, multiple
access with collision detection
(CSMA/CD).
Beside carrier sensing, collision detection
and the binary exponential back-off
algorithm, the standard also describes the
format of the frames and the type of
encoding used for transmitting frames.
 The minimum length of frames can be
varied from network to network. This is
important because, depending on the size
of the network, the frames must be of a
suitable minimum length.
 The standard also makes some
suggestions about the type of cabling that
should be used for CSMA/CD bus LANs.


The CSMA/CD Bus LAN is also widely
called Ethernet.
Ethernet MAC
Sublayer Protocol
Frame formats. (a) DIX Ethernet,
(b) IEEE 802.3.
IEEE 802.3: MAC Addresses

Every network card in the world has a
unique 46-bit serial number called a MAC
address. The IEEE allocates these
numbers to network card manufacturers
who encode them into the firmware of
their cards.
The destination and source address fields
of the MAC frame have 48 bits set aside
(the standard also allows for 16-bit
addresses but these are rarely used).
 The most significant bit is set to 0 to
indicate an ordinary address and 1 to
indicate a group address (this is for
multicasting, which means that frames are
sent to several hosts). If all 48 bits are set
to 1 then frames are broadcast to all the
hosts.
 If the two most significant bits are both
zero then the 46 least significant bits
contain the MAC addresses of the source
and destination hosts.

IEEE 802.3: Minimum Frame Length



When a host transmits a frame, there is a
small chance that a collision will occur.
The first host to detect a collision
transmits a 48-bit jam sequence.
To ensure that any hosts involved with the
collision realise that the jam sequence is
associate with their frame, they must still
be transmitting when the jam sequence
arrives. This means that the frame must
be of a minimum length.
The worse case scenario is if the two
hosts are at far ends of the cable. If host
A’s frame is just reaching host B when it
begins transmitting, host B will detect the
collision first and send a jam signal back
to host A.
CSMA/CD  Minimum Ethernet
Frame Size
To ensure that no node may completely
receive a frame before the transmitting
node has finished sending it, Ethernet
defines a minimum frame size (i.e. no
frame may have less than 46 bytes of
payload).

The minimum frame size is related to the
distance which the network spans, the
type of media being used and the number
of repeaters which the signal may have to
pass through to reach the furthest part of
the LAN.

Together these define a value known as
the Ethernet Slot Time, corresponding to
512 bit times at 10 Mbps.

IEEE 802.3: Minimum Frame Length

The longest time between starting to
transmit a frame and receiving the first bit
of a jam sequence is twice the
propagation delay from one end of the
cable to the other.
A
(a)
A
Packet starts at
time 0
B
A
Packet at time tp-
(b)
Collision occurs
at time tp
(c)
B
A
Jam sequence gets
back to A at 2tp
(d)

Jam sequence

Jam sequence
This means that a frame must have
enough bits to last twice the propagation
delay.
 The 802.3 CSMA/CD Bus LAN transmits
data at the standard rate of r = 10Mbps.
 The speed of signal propagation is about v
= 2108m/s.

B
B
IEEE 802.3: Minimum Frame Length
In order to calculate the minimum frame
length, we must first work out the
propagation delay from one end of the
cable to the other.
IEEE 802.3: Minimum Frame Length

The standard frame length is at least 512
bits (64 bytes) long, which is much longer
than our minimum requirement of 64 bits
(8 bytes).


We only have to start worrying when the
LAN reaches lengths of more than 2.5km.
802.3 CSMA/CD bus LANs longer than
500m are usually composed of multiple
segments joined by in-line passive
repeaters, which output on one cable the
signals received on another cable.

When we work out the minimum frame
length for these longer LANs, we also
have to take the delays caused by the
passive repeaters (about 2.5sec each)
into account as well.
Shortest Ethernet Frame
Why specify a shortest frame of 64byte?



64 bytes sent at 10Mbps  51.2sec
500m/segment, 4 repeaters between
nodes 2500m 25 sec propagation
delay
The frame should be longer enough for
sender to detect the collision(2x25 or
about 50 sec )
Node
A
R1
R2
R3
R4
500m 25 sec propagation
delay
Node
B
IEEE 802.3: Non-Deterministic

The 802.3 CSMA/CD bus LAN is said to
be a non-deterministic network. This
means that no host is guaranteed to be
able to send its frame within a reasonable
time (just a good probability of doing so).


When the network is busy, the number of
collisions rises dramatically and it may
become very difficult for any hosts to
transmit their frames.
A real-time computing application (such
as an assembly line) will demand that
data is transmitted within a specified time
period.

Since the 802.3 bus LAN cannot
guarantee this, its use for real-time
applications may not only be undesirable
but potentially dangerous in some
situations.
Ethernet 10Base-T
& 100Base-TX

Wiring
 Unshielded
Twisted Pair
(UTP)
 Category
5 wiring is best
 Cat
3 and Cat 4 in
some older installations
 Bundle
of eight wires
(only uses four)
 Terminates
connector
in RJ-45
10Base-T &
100Base-TX hubs

UTP-based networks use hubs to
interconnect NICs
 each
UTP cable runs directly from a
NIC to a hub
10Base-T & 100Base-TX
hubs
Hubs have many ports, each of which
has one incoming network cable

Hubs are usually located in computer
rooms, or network distribution
cupboards


a patch panel (or patch bay) is used to
connect between hubs and the wall
sockets throughout a building
10Base-T & 100Base-TX
wiring

Wiring
 100
meters maximum distance hubto-station
 Can
use multiple hubs (max 4) to
increase the distance between any
two stations
200 m
100 m
100 m
10Base-T to
100Base-TX
Upgrading from 10Base-T to
100Base-TX


Need new hub



Need new NICs


May have some 10 Mbps ports to
handle 10Base-T NICs
May have autosensing 10/100 ports
that handle either
Only for stations that need more speed
No need to rewire

This would be expensive
Multiple Hubs in 10Base-T

Farthest stations in 10Base-T can be
five segments (500 metres apart)

100 metres per segment

Separated by four hubs
100m
100m
10Base-T hubs
100m
500m, 4 hubs
100m
100m
Multiple Hubs in 100BaseTX

Limit of Two Hubs in 100Base-TX
 Must
be within a few metres of each
other
 Maximum span ~200 metres
 Shorter distance span than 10Base-T
2 Co-located
Hubs
100Base-TX
Hubs
100m
100m
Latency and Congestion
with hubs

Ethernet is a shared media LAN
 Only
one station can transmit at a
time
 Even in multi-hub LANs
 Others must wait
 This causes delay
All Other
Stations
Must Wait
One Station Sends
IEEE 802.2:
Logical Link
Control
(a) Position of LLC. (b)
Protocol formats.
Repeaters
Regenerate
Provide
the signal
more flexibility in network
design
Extend
the distance over which a
signal may travel down a cable
Example
 Ethernet HUB
Ethernet Repeaters and Hubs
 Connect
together one or more
Ethernet cable segments of any
media type
 If
an Ethernet segment were
allowed to exceed the maximum
length or the maximum number of
attached systems to the segment,
the signal quality would deteriorate.
Ethernet Repeaters and Hubs

Used between a pair of segments
Provide signal amplification and
regeneration to restore a good signal level
before sending it from one cable segment
to another
Ethernet Bridge
 Join
two LAN segments (A,B),
constructing a larger LAN
 Filter
traffic passing between the
two LANs and may enforce a
security policy separating different
work groups located on each of the
LANs.
Local Internetworking
Ethernet Bridges



Simplest and most frequently used 
Transparent Bridge (meaning that the
nodes using a bridge are unaware of its
presence).
Bridge could forward all frames, but then it
would behave rather like a repeater
Bridges are smarter than repeaters!
Ethernet
Bridges
A bridge stores
the hardware
addresses
observed from
frames received
by each interface
and uses this
information to
learn which
frames need to
be forwarded by
the bridge.
Ethernet Switch 
Modern LANs

Fundamentally similar to a bridge

Supports a larger number of connected
LAN segments

Richer management capability.

Logically partition the traffic to travel only
over the network segments on the path
between the source and the destination
(reduces the wastage of bandwidth)
Ethernet Switch 
Benefits
 Improved

security
users are less able to tap-in into other
user's data
 Better
management
control who receives what information
(i.e. Virtual LANs)
 limit the impact of network problems

 Full

duplex
rather than half duplex required for
shared access
Switched LAN
• Hub and Switched LAN
– hub simulates a single shared medium
– switch simulates a bridged LAN with
one computer per segment
Ethernet Switches
Highly Scalable
10Base-T switches

Competitive with 100Base-TX
hubs in both cost and
throughput
 Increasingly used to desktops


100Base-TX switches


Higher performance (and
price)
Gigabit Ethernet switches

Very expensive
Ethernet Switches

No limit on number of Ethernet
switches between farthest stations
 So
no distance limit on size of
switched networks

Ethernet Switches must be
Arranged in a Hierarchy (or daisy
chain)
 Only
one possible path between
any two stations, switches
1
Path=4,5,2,1,3
2
3
4
5
6
Repeaters, Hubs,
Bridges, Switches,
Routers and
Gateways
(a) Which device is in which layer.
(b) Frames, packets, and
headers.
Repeaters, Hubs,
Bridges,
Switches, Routers
and Gateways
(a) A hub. (b) A bridge. (c)
a switch.
Switches Versus Routers





Switches
Fast
Inexpensive
No benefits of
alternative routing
No hierarchical
addressing





Routers
Slow
Expensive
Benefits of
alternative routing
Hierarchical
addressing
“Switch where you can; route
where you must”
IEEE 802.11 – Wireless Ethernet
Two configurations:
Ad-hoc. No central control, no
connection to the outside world
Infrastructure. Uses fixed network
Access Point to connect to the
outside world

IEEE 802.11 – Wireless Ethernet
Uses CSMA/CA protocol. CSMA part is
the same as in 802.3 Ethernet
CA stands for Collision Avoidance and
works as follows:
If the carrier is present for a specific
time period, transmitter sends a frame
If no collision receiver send ack
Transmitter can also reserve the
channel by sending Request to Send
(RTS)

IEEE 802.11 – Wireless Ethernet
IEEE 802.11 does not implement
Collision Detection because it cannot
detect collisions at the receiver end
(hidden terminal problem)
To avoid collisions the frame contains
field indicating the length of transmission
Other stations defer transmission
The 802.11
Protocol Stack
Where does 802.11 live in the OSI?
Telnet, FTP, Email, Web,
etc.
Application
Presentation
Session
TCP, UDP
IP, ICMP, IPX
Logical Link Control 802.2
(Interface to the upper layer
protocols)
MAC
Wireless lives at
Layers 1 & 2
only!
Transport
Network
Data Link
802.3, 802.5, 802.11
LAN: 10BaseT, 10Base2,
10BaseFL
WLAN: FHSS, DSSS, IR
Physical
The 802.11 MAC
Sublayer Protocol
(a) The hidden
station problem.
(b) The exposed
station problem.
CSMA-CA + Acknowledgement
Carrier Sense Multiple Access with Collision
Avoidance
How CSMA-CA works:
• Device wanting to transmit senses the
medium (Air)
• If medium is busy - defers
• If medium is free for certain period (DIFS) transmits frame
Latency can increase if “air” is very busy!
Device has hard time finding “open air” to
send frame!
* DIFS - Distributed Inter-Frame Space
*(approx 128 µs)
The 802.11 MAC
Sublayer Protocol
The use of virtual channel
sensing using CSMA/CA.
The 802.11 Frame
Structure
The 802.11 data frame.
Summary



IEEE 802.11b (WiFi) is a
wireless LAN technology
that is rapidly growing in
popularity
Convenient, inexpensive,
easy to use
Growing number of “hot
spots” everywhere
 airports,
hotels,
bookstores, Starbucks,
etc

Estimates: 70% of
WLANs are insecure!
IEEE 802.5 and Token
Ring
FDDI  Fiber Distributed
Data Interface
– data rate 100Mbps, use as a backbone
– With multi-mode fiber any given ring
segment can be up to 200 km in length.
A total of 500 stations can be connected
with a maximum separation of 2 km.
– two complete rings to overcome failures
High Speed LANs
• FDDI: Fiber Distributed Data Interface
• 100Mbps, distance up to 200km, 100
hosts mainly used as a backbone
Bandwidth
Scaling
Mbps
1000
900
800
700
600
500
400
300
200
100
0
Gigabit Ethernet
(Switched)
Ethernet
ATM OC-12
(Switched)
ATM OC-3
(Switched)
Fast Ethernet
(Switched)
FDDI
(Switched)
Token Ring
(Switched)
Ethernet
(Switched)
FE
Switched LAN Type
Cisco Systems