Download Why 5-4-3?

Ethernet
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Ethernet is now the predominant LAN
technology in the world.
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History of Ethernet
• Derived from Aloha Net (U. of Hawaii)
• Xerox Corporation's Palo Alto Research Center
(PARC) developed Ethernet in the 1970s
• IEEE 802.3 was based on Ethernet & released in
1980
• Digital, Intel & Xerox jointly developed and
released an Ethernet 2.0, that was substantially
compatible with IEEE 802.3.
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Scope of Ethernet (within OSI)
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Two sub-layers of Ethernet
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LLC
LLC is implemented in software, and its
implementation is independent of the physical
equipment.
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MAC
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Logical Topology (of Ethernet)
• The underlying logical topology of Ethernet
is a multi-access bus.
• This means that all the nodes (devices) in
that network segment share the medium.
– This further means that all the nodes in that
segment receive all the frames transmitted
by any node on that segment.
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Reasons for Ethernet Success
•
•
•
•
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Simplicity and ease of maintenance
Ability to incorporate new technologies
Reliability
Low cost of installation and upgrade
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Historic Ethernet
10BASE5
(Thicknet)
10BASE2
(Thinnet)
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Migration from Hub-based to Switchbased Ethernet (collision management)
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Current Ethernet Development
• Moving to G-bits & beyond
• Ethernet Beyond the LAN
– Can now be applied across a city in what is
known as a Metropolitan Area Network (MAN).
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So, there are two Ethernet standards …
Ethernet Frame
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Ethernet 與 802.3 之 “封包” 差異
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FCS : Frame Check Sequence
frame will be dropped if
FCS is incorrect
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Ethernet Frame Size
• Originally between 64 bytes and 1518 bytes.
– includes all bytes from the Destination MAC Address
field through the Frame Check Sequence (FCS) field.
– The Preamble and Start Frame Delimiter fields are not
included
• The IEEE 802.3ac standard, released in 1998,
extended the maximum allowable frame size to
1522 bytes.
– to accommodate a technology called Virtual Local Area
Network (VLAN). (will be presented in a later course)
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Ethernet Frame Size
• If the size of a transmitted frame is less than
the minimum or greater than the maximum,
the receiving device drops the frame.
– Dropped frames are likely to be the result of
collisions or other unwanted signals and are
therefore considered invalid.
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Addressing in Ethernet
MAC Address burned in ROM on NIC card &
will be copied into RAM when start-up.
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MAC address structure
In DOS command window, type “ipconfig/all” to view MAC address
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MAC vs IP Address
• The Network layer address enables the
packet to be forwarded toward its
destination.
• The Data Link layer address enables the
packet to be carried by the local media
across each segment.
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Ethernet Uni-cast & Broadcast
Uni-cast
Broadcast
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Ethernet Multicast
A special value that begins with 01-00-5E in hexadecimal.
The value ends by converting the lower 23 bits of the IP
multicast group address into the remaining 6 hexadecimal
characters of the Ethernet address. The remaining bit in the
MAC address is always a "0".
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Media Access Control in Ethernet
(CSMA/CD)
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Collision 以後?
Jam signal:
maybe a 32-bit
repeating one,
zero, one, zero
pattern)
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Collision 以後?  Backoff Timing
Each computer has different backoff time
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HUB extends Collision Domain
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Ethernet Delay (Latency)
More latency, more likely the collision
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Ethernet 的 Timing 限制
• 想像一個極端的例子 …
– A 電腦送出一個 Ethernet 所能允許的最小封包, 這個
封包在旅行了 Ethernet 所能允許的最遠距離後, 剛好
與遠端的 B 電腦送出的封包發生碰撞, 然後, 在這發
生碰撞後的封包傳回原發送封包之 A 電腦前, 該 A
電腦的封包已經傳完 …..
– OOPS, 所以, A 電腦以為它剛送出的封包已經成功
傳送, 但, 實際上不然!
• Houston, we’ve got a problem!
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Ethernet 的 Timing 限制
• 因此, Ethernet 在 …
– 傳輸速度 (rate) : R
– 最大傳輸距離間來回之傳輸延遲 (delay
time) : T
– 最小封包大小 (size) : S
間需滿足以下條件
T<S/R
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Ethernet 的 Timing 限制
• 舉例而言, 在 UTP 線上之傳輸 “速度” 約為
20.3 cm per nanosecond
– 一個直徑 100 m 的 LAN 之來回 delay 為 2 x 100 (m)
x 100 (cm) / 20.3 = 985 (ns)
– 一個直徑 200 m 的 LAN 之來回 delay 為 2 x 200 (m)
x 100 (cm) / 20.3 = 1970 (ns)
– 一個直徑 400 m 的 LAN 之來回 delay 為 2 x 400 (m)
x 100 (cm) / 20.3 = 3940 (ns)
• 別忘了, Repeaters (Hubs) 及電腦本身也會有
delay ㄛ!
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Bit Time vs Slot Time
• Bit-time:Time to transmit one bit
• Slot-time: 最大傳輸距離間來回之傳輸延遲
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Slot Time
• 電腦傳送完成最小 size 封包 (64-byte) 所需時間
 should be ≧ max. two-way latency
= 64 bytes
> 0.985 μs (100 m)
= 64 bytes
> 0.985 μs (100 m)
= 512 bytes
Why not use 512 bit time?
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> 0.985 μs (100 m)
Operates at full-duplex only, no
CSMA/CD is required
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How timing affects Ethernet Topology
10Base2 Ethernet Installation specification
(5-4-3 rules)
Why 5-4-3?
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Inter-frame Spacing
• The minimum spacing between two noncolliding frames is also called the interframe spacing
Inter-frame Spacing allows:
1. media time to stabilize after the transmission of
the previous frame
2. devices time to process the frame and prepare for
the next frame
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Inter-frame Spacing
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Ethernet Physical Layer
• The differences between standard Ethernet,
Fast Ethernet, Gigabit Ethernet, and 10
Gigabit Ethernet occur at the Physical
layer, often referred to as the Ethernet PHY.
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Types of Ethernet
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10Gbps
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1000BASE-T Ethernet
• 1000BASE-T Ethernet provides full-duplex
transmission using all four pairs in Category 5 or
later UTP cable.
• Gigabit Ethernet over copper wire enables an
increase from 100 Mbps per wire pair to 125
Mbps per wire pair
– 500 Mbps for the four pairs.
– Each wire pair signals in full duplex, doubling the 500
Mbps to 1000 Mbps.
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1000BASE-T Ethernet
• Wire pairs are no longer separated into a
pair for transmitting and a pair for receiving
• Any wire pair can be used for transmitting
or receiving at the same time if necessary.
– This means that there are permanent collisions
on the wire.
– Hybrid circuits at the ends of each wire pair can
separate out transmission signals from receive
signals.
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1000BASE-T Ethernet
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1000BASE-SX and 1000BASE-LX
Ethernet
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1000BASE-SX and 1000BASE-LX
• Advantages over UTP
– noise immunity
– small physical size
– increased unrepeated distances and bandwidth.
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10 Gbps Ethernet
• IEEE 802.3ae standard was adapted to include 10
Gbps, full-duplex transmission over fiber-optic
cable.
• The 802.3ae standard and the 802.3 standards for
the original Ethernet are very similar.
• 10-Gigabit Ethernet (10GbE) is evolving for use
not only in LANs, but also for use in WANs and
MANs.
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10Gbps vs other varieties of Ethernet
• Frame format is the same, allowing interoperability
between all varieties Ethernet, with no reframing or
protocol conversions necessary.
• Bit time is now 0.1 ns. All other time variables scale
accordingly.
• Because only full-duplex fiber connections are used, there
is no media contention and CSMA/CD is not necessary.
• The IEEE 802.3 sub-layers within OSI Layers 1 and 2 are
mostly preserved, with a few additions to accommodate 40
km fiber links and interoperability with other fiber
technologies.
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With 10Gbps Ethernet …
• Flexible, efficient, reliable, relatively low
cost end-to-end Ethernet networks become
possible.
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Future Ethernet Speeds
• 1-Gigabit Ethernet is now widely available
and 10-Gigabit products are becoming more
available
• IEEE and the 10-Gigabit Ethernet Alliance
are working on 40-, 100-, or even 160-Gbps
standards.
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HUB-based Ethernet
Lack of scalability
Increased latency
Increased latency
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Switch-based Ethernet
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Switch-based Ethernet
Dedicated bandwidth
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Switch-based Ethernet
Collision Free
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Switch-based Ethernet
Full Duplex
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Switch – selective forwarding
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Switch
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Switch Operation
• To accomplish their purpose, Ethernet LAN
switches use five basic operations:
–
–
–
–
–
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Learning
Aging
Flooding
Selective Forwarding
Filtering
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Learning
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Flooding
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Learning again
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Selective Forwarding
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Aging
• The entries in the MAC table acquired by
the Learning process are time stamped.
– is used as a means for removing old entries in
the MAC table.
– the entry in the table will be refreshed when the
switch next receives a frame from that node on
the same port.
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ARP
• In order for devices to communicate, the
sending devices need both the IP addresses,
and the MAC addresses of the destination
devices
• When they try to communicate with devices
whose IP addresses they know, they must
determine the MAC addresses
• WHY?
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With ARP protocol …
• Two basic functions are provided
– Resolving IPv4 addresses to MAC
addresses
– Maintaining a cache of mappings
• ARP table (cache)
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ARP Procedure - 1
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ARP Procedure - 2
Broadcast
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ARP Procedure - 3
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ARP Procedure - 4
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ARP Procedure - 5
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ARP Table
• These dynamic entries in the ARP table are
time-stamped
– If a device does not receive a frame from a
particular device by the time the timestamp
expires, the entry for this device is removed
from the ARP table.
• Static map entries can be entered in an ARP
table, but this is rarely done.
– must be manually removed.
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What if devices can’t find
corresponding entry in ARP table
• The device initiates a process called an ARP
request, that enables it to discover the
destination MAC address
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ARP Procedure
• broadcast an ARP request (use broadcast MAC
address: FF-FF-FF-FF-FF-FF). The request
contains frame header (MAC header and an IP
header), and the ARP message.
• The device with the IP address matches the one in
ARP request responds by sending the source its
MAC address - ARP reply
(continue on next slide)
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ARP Procedure (continued)
• the originating device receives the ARP
reply, it extracts the MAC address from the
MAC header, and updates its ARP table.
• The originating device can then properly
address its data with both, a destination
MAC address, and a destination IP address
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What if ARP procedure returns no
MAC address?
• If no device responds to the ARP request,
the packet is dropped because a frame
cannot be created.
– This encapsulation failure is reported to the
upper layers of the device.
– If the device is an intermediary device, like a
router, the upper layers may choose to respond
to the source host with an error in an ICMPv4
packet.
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What if the destination device is
in another network?
• The source node needs to deliver the frame
to the router interface that is the gateway or
next hop used to reach that destination.
– The source node will use the MAC address of
the gateway as the destination address for
frames containing an IPv4 packet addressed to
hosts on other networks.
• Same ARP procedure is repeated again, but
with default gateway as its destination
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What if the destination device is in
another network?
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Removing ARP Entry
• For each device, an ARP cache timer removes
ARP entries that have not been used for a
specified period of time.
– The times differ depending on the device and its
operating system.
• For example, some Windows operating systems store ARP
cache entries for 2 minutes. If the entry is used again during
that time, the ARP timer for that entry is extended to 10
minutes.
• Commands may also be used to manually remove
all or some of the entries in the ARP table.
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Issues with ARP Protocol
• Broadcast
• Security
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Issues with ARP Protocol
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