Transcript LAN Systems

Network Technology
CSE3020
Week 8
Network Technology CSE3020 - 2006
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Local Area Networks (LANs)
FDDI
Contention
Ethernet
token passing
polling
Token ring
Wireless LAN
MAC
protocols
Star
products
topologies
ring
tree
LAN
standard
bodies
bus
transmission
media
IEEE
fiber
twisted pair
coax
air (wireless)
ISO, ATM Forum
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Ethernet (CSMA/CD)
• Medium Access Control protocol is Carrier Sense Multiple
Access with Collision Detection (CSMA/CD).
Originally developed by Intel and Xerox as a part of Ethernet
LAN.
– Later used as a basis for the IEEE 802.3 LAN.
• IEEE 802.3 Medium Access Control:
– Random Access:
• Stations access medium randomly.
– Contention
• Stations content for time on medium.
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CSMA/CD Operation
• First listen for clear medium (carrier sense).
• If the medium is idle, transmit.
– If two stations start at the same instant, collision.
• If busy, listen for idle, then transmit immediately
– Stations continue to listen while transmitting
• If a collision is detected,
– transmit jamming signal then cease transmission.
– after jamming, wait a random amount of time then start again.
• Binary exponential back off: the mean value of the random delay is
doubled with each repetition (reduce chance of collision).
– After 16 unsuccessful attempts, the station gives up, and frame
is discarded
• Frame should be long enough to allow collision detection before
end of transmission.
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CSMA/CD Operation
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Collision Detection
• For twisted pair (extended star-topology) activity on more than
one port of the hub is a collision.
– Special collision presence signal is sent out to all stations to
inform of collision.
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IEEE 802.3 Frame Format
• Preamble: 7-octet pattern of alternating 0s and 1s for sync.
• SFD: Sequence 10101011 to indicate the start of the frame.
• DA and SA: Physical source and destination address.
• Length/type: Length of LLC data in octets.
• LLC PDU: supplied by LLC.
• Pad: Added octets to ensure frame length for proper collision
detection.
• FCS: 32-bit CRC.
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Ethernet Specification
• Concise notation of implementation (not always):
< data rate (Mbps) > < Signaling method > < Max segment length (in 100m) >
e.g.: 10Base5 & 10Base2
10Base5
10Base2
10/100Base-T
10Base-FP
Medium
Coaxial
Coaxial
UTP
Fiber
Signaling
Baseband
Baseband
Baseband
(Manchester) (Manchester) (Manchester)
Manchester
On/Off
Topology
Bus
Bus
Star
Star
Nodes
100
500
30
185
100
33
500
Max
Length (m)
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10/100BASE-T
hub
(or repeater)
max. 4 hubs
in a network
…
twisted
pair
…
100m
(max)
MAU integrated
into NIC
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Fast Ethernet (100Mbps)
100Base-TX
Transmission 2 pair STP
medium
Maximum
Segment
Length (m)
100
2 pair,
Cat. 5
UTP
100
Network
Span (m)
200
200
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100Base-FX
2 optical fibers
100
400
10
Gigabit Ethernet (1000 Mbps)
1000BASE-SX:
Short wavelength (770nm to 860nm),
multimode fiber.
1000BASE-LX:
Long wavelength (1270nm to 1355nm),
multi or single mode fiber.
1000BASE-CX:
Copper jumpers (specialized shielded
twisted-pair cable that spans no more than
25 m).
1000BASE-T:
4-pair cat. 5 UTP up to 100 m range.
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All Ethernet Network
Router
GE SW
GE SW
GE Hub
The
Internet
GE SW
1000/100
1000/10
1000BASE-X
Fast Ethernet
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10/100BASE-T
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LAN Generations
• First Generation:
– CSMA/CD and token ring.
– Terminal-to-host connectivity and client/server architecture.
– Moderate data rates.
• Second Generation:
– FDDI.
– Backbone LANs and high-performance workstations.
• Third Generation:
– ATM LANs.
– Aggregate throughputs and real-time support for multimedia
applications.
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Third Generation LANs
• Support for multiple guaranteed classes of service.
– Live video may need 2Mbps.
– File transfer can use background class of service.
• Scalable throughput.
– Both aggregate and per host.
• Facilitate LAN/WAN internetworking.
>> ATM is ideally suited to these requirements.
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Backbone Networks
• The role of a backbone network is to connect
individual LANs. This also allows all users to
access to a common resource (such as a printer, file
server, or the Internet)
• Issues to be considered:
 Unlike a LAN, a backbone network must deal
with a large number of users. Traffic is
aggregated from all LANs.
 Performance is important.
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Backbone Networks
• The 2nd generation LANs (Fast Ethernet, Gigabit
Ethernet) have higher data rate and can be used to
build the backbone network.
• Use asymmetric switches (operate at layer 2) to build
extended star topology. Traffic is aggregated from
bottom to top.
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Repeaters
• A certain type of cables can only transport signals to a
certain distance. Beyond the distance, the signals may be
too weak to be decoded accurately. This limits the
coverage of a LAN.
• Repeaters are used to extend the coverage of a LAN due
to its limitation in transmissions.
• A repeater is operate at physical layer, it mostly deals
with signal regeneration. They do not deal with the MAC
frame format.
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Repeaters
B
A
NET
DL
PHY
PHY
PHY
NET
DL
PHY
• Repeaters must forward signals to all output ports.
Unnecessary signal forwarding may occur.
• A certain MAC protocol may limit the number of
repeaters used in a LAN (eg. Ethernet allows for only
up to 4 repeaters).
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Bridges
A
D
C
Network
LLC
MAC
Physical
Relay
MAC
Physical
MAC
Physical
Network
LLC
MAC
Physical
• Unlike repeaters, bridges are operated in Layer-2.
They understand MAC frames.
• Bridges are capable of:
• Frame conversion (from one type to another).
• Frame forwarding (to the destination).
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Multi-site LAN interconnection
GW
LAN
Intersite
Communication
Facility
GW
LAN
GW
LAN
• Gateway (GW) router operates at layer 3, can provide
DHCP, DNS, Routing and Access control services.
• Each LAN is a subnet with Multi-site Network (Intranet)
•Inter-site Communication Facility – WAN Services provide
by Telecommunications company
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Multisite LAN interconnection
• WAN Services:
• ISDN
- Used for small enterprise networks (64 kbps) but mediumsized network requires multiple 64 kbps channels.
• Frame relay
- Multiplexing & routing of frames at link layer.
- Supports up to 34 or 45 Mbps.
• ATM
•High bit rate leased lines
-Supports intersite telephony traffic as well.
- Either DS1/T1(1.5 Mbps) or E1 (2 Mbps) or higher.
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Required Reading
• W. Stallings, Data and Computer Communications
Prentice-Hall.
Chapter 14 6E
Chapter 16 7E
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The Following Slides are for Interest
Only
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Precursors: ALOHA
• Developed for packet radio networks.
• When station has frame, it sends.
– Station listens (for max round trip time)plus small increment.
– If received ACK, fine. If not, retransmit the frame.
– If no ACK after repeated transmissions, give up.
• Frames includes frame check sequence (as in HDLC).
– If frame OK and address matches receiver, send ACK.
– Frame may be damaged by noise or by another station
transmitting at the same time (collision) – frame is ignored.
• Any overlap of frames causes collision, which increases rapidly
with load.
• Maximum utilization of the channel is 18%.
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Precursors: Slotted ALOHA
• Time in uniform slots equal to frame transmission time.
• Need central clock (or other sync mechanism) for all
stations.
• Transmission begins at slot boundary.
• Frames either miss or overlap totally.
• Maximum utilization of the channel is 37%.
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CSMA
• Propagation time is much less than transmission time.
– All stations know that a transmission has started almost immediately.
• First listen for clear medium (carrier sense).
• If the medium is idle, transmit.
– If two stations start at the same instant, collision.
– Wait reasonable time (round trip plus ACK contention) and if no ACK
then retransmit.
• Max utilization depends on propagation time (medium length)
and frame length.
– Longer frame and shorter propagation gives better utilization.
• If the medium is busy:
– Listen for idle medium then transmit immediately.
– If two stations are waiting, a collision will occur.
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10BASE-5
Up to 4 repeaters
thick
coax
segment length = 500m (max)
MAU
AUI cable
(50m max)
MAU
2.5m (min)
repeater
MAU
…
MAU = Medium Attachment Unit
AUI = Attachment Unit Interface
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10BASE-2 (Cheapernet)
thin
coax
segment length = 185m (max)
for 30 nodes (max)
repeater
BNC
connector
MAU integrated
into NIC
0.5m (min)
MAU = Medium Attachment Unit
AUI = Attachment Unit Interface
NIC = Network Interface Card
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Token Passing LANs
BUS
STAR SHAPED RING
wire center
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RING
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Token Ring (802.5)
• Most commonly used MAC protocol for ring topology
LAN.
• Small frame (token) circulates when stations are idle.
– Station waits for the token.
– Changes one bit in token to make it a start-of-frame sequence
for data frame.
– Appends the rest of the data frame.
– Frame makes round trip and is absorbed by transmitting
station.
– Station then inserts new token when transmission has
finished and leading edge of the returning frame arrives.
– Once a new token inserted, the next station downstream may
transmit.
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Token Ring (802.5)
• Under light loads, some inefficiency: must wait for token.
• Under heavy loads, efficient and fair: round robin fashion.
• Advantages: flexible and fair control, can provide priority
and guaranteed bandwidth services.
• Disadvantages: Token maintenance (to prevent loss or
duplication of token).
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Token Ring Operation
• A sends a packets to C
(1-5).
• C then sends packets to A
and D (6-9).
• In this example, packets
are shorter than link
length. A new token is
released when packet
arrives back at the
transmitter.
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Token Ring MAC Frame
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Priority Scheme
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Token Ring Extensions
• Early Token Release (ETR):
– If the bit length of the ring is less than that of a frame:
• the transmitter release a token as soon as the frame is sent.
– Otherwise:
• Must wait for the frame header to return before issuing a
token.
• Some of the potential capacity is unused.
– ETR allows the transmitter to release a token as soon as the
frame is sent.
– Priority mechanism is partially disabled.
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Token Ring Extensions
• Dedicated Token Ring (DTR):
– Token passing algorithm in star topology using a central hub
or concentrator.
– Central hub functions as a switch so that the connection is
full-duplex point-to-point link.
– Concentrator acts as frame level relay rather than a bit-level
repeater.
– Dedicated link with immediate access.
– No token passing.
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IEEE 802.5 Physical Layer
Data Rate
(Mbps)
4
16
100
Medium
UTP, STP,
Fiber
Differential
Manchester
UTP, STP,
Fiber
Differential
Manchester
UTP, STP,
Fiber
MLT-3 or
4B5B/NRZI
4550
18,200
18,200
TP or DTR
TP or DTR
DTR
Signaling
Max Frame
Size (octets)
Access
Control
TP = Token Passing access control: DTR = Dedicated Token Ring
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Fiber Distributed Data
Interface (FDDI)
• Token Ring scheme similar to IEEE 802.5 specifications.
• Designed for LAN and MAN applications at higher data rate
(100Mbps).
• Key differences in FDDI MAC protocol from IEEE 802.5 are:
– Station seizes token by aborting token transmission.
– Once token captured, one or more data frames are transmitted.
– New token released as soon as transmission finished (early
token release in 802.5).
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FDDI MAC Frame Format
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FDDI Operation
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FDDI Physical Layer
Medium
Optical Fiber
Twisted Pair
Data rate
100 Mbps
100 Mbps
Signaling
4B/5B/NRZI
MLT-3
Max repeaters
100
100
Between repeaters
2km
100m
• Two rings run on fibers or twisted pair.
- Primary ring
- Secondary ring
• A station can be configured as a Single Attach Station (SAS) or
Dual Attach Station (DAS).
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FDDI: Topology
FDDI
SAS
primary ring
SAS
(bypass mode)
wiring
concentrator
DAS
(bypass mode)
DAS
secondary ring
primary ring
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FDDI Fault Tolerance
Normal operation:
Link failure:
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ATM LANs
• Asynchronous Transfer Mode (ATM) uses virtual paths and
virtual channels.
• Multiple classes of services are easily accommodated either in
preconfigured fashion or switched connections.
• Easily scalable by adding more ATM switches or devices.
• Seamless integration of LANs and WANs.
• Possible types of ATM LANs are:
– Gateway to ATM WAN:
• ATM switch acts as a router and traffic concentrator.
– Backbone ATM switch:
• Single ATM switch or local network of ATM switches.
– Workgroup ATM.
• End systems connected directly to ATM switch.
• In practice, mixed system.
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Example: ATM LAN
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Example: ATM LAN HUB
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ATM LAN: Compatibility
• Interaction between end system on ATM and end
system on legacy LAN.
• Interaction between stations on legacy LANs of same
type.
• Interaction between stations on legacy LANs of
different types.
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Backbone Networks:
Shared versus Switched
• A shared LAN is usually low in cost, but low in
performance. It is suitable for connecting a small
number of LANs that do not use the network
intensively.
• A switch is usually high in cost, but high in
performance. It is suitable for building a large
backbone dealing with a large number of users.
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