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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

• Simplicity and ease of maintenance

• Ability to incorporate new technologies

• Reliability

• Low cost of installation and upgrade

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Historic Ethernet10BASE5(Thicknet)

10BASE2 (Thinnet)

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Migration from Hub-based to Switch-based 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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Ethernet FrameSo, there are two Ethernet standards …

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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, zeropattern)

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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

= 64 bytes

= 512 bytes

> 0.985 μs (100 m)

> 0.985 μs (100 m)

> 0.985 μs (100 m)

Operates at full-duplex only, no CSMA/CD is requiredWhy not use 512 bit time?

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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 non-

colliding frames is also called the inter-frame 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

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 scalabilityIncreased latencyIncreased latency

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Switch-based Ethernet

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Switch-based EthernetDedicated bandwidth

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Switch-based EthernetCollision Free

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Switch-based EthernetFull 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: – 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