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Ethernet 2017/5/25 1 Ethernet is now the predominant LAN technology in the world. 2017/5/25 2 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. 2017/5/25 3 Scope of Ethernet (within OSI) 2017/5/25 4 Two sub-layers of Ethernet 2017/5/25 5 LLC LLC is implemented in software, and its implementation is independent of the physical equipment. 2017/5/25 6 MAC 2017/5/25 7 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. 2017/5/25 8 Reasons for Ethernet Success • • • • 2017/5/25 Simplicity and ease of maintenance Ability to incorporate new technologies Reliability Low cost of installation and upgrade 9 Historic Ethernet 10BASE5 (Thicknet) 10BASE2 (Thinnet) 2017/5/25 10 Migration from Hub-based to Switchbased Ethernet (collision management) 2017/5/25 11 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). 2017/5/25 12 So, there are two Ethernet standards … Ethernet Frame 2017/5/25 13 Ethernet 與 802.3 之 “封包” 差異 2017/5/25 14 FCS : Frame Check Sequence frame will be dropped if FCS is incorrect 2017/5/25 15 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) 2017/5/25 16 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. 2017/5/25 17 Addressing in Ethernet MAC Address burned in ROM on NIC card & will be copied into RAM when start-up. 2017/5/25 18 MAC address structure In DOS command window, type “ipconfig/all” to view MAC address 2017/5/25 19 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. 2017/5/25 20 Ethernet Uni-cast & Broadcast Uni-cast Broadcast 2017/5/25 21 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". 2017/5/25 22 Media Access Control in Ethernet (CSMA/CD) 2017/5/25 23 Collision 以後? Jam signal: maybe a 32-bit repeating one, zero, one, zero pattern) 2017/5/25 24 Collision 以後? Backoff Timing Each computer has different backoff time 2017/5/25 25 HUB extends Collision Domain 2017/5/25 26 Ethernet Delay (Latency) More latency, more likely the collision 2017/5/25 27 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! 2017/5/25 28 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 2017/5/25 29 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! 2017/5/25 30 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 2017/5/25 31 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? 2017/5/25 > 0.985 μs (100 m) Operates at full-duplex only, no CSMA/CD is required 32 How timing affects Ethernet Topology 10Base2 Ethernet Installation specification (5-4-3 rules) Why 5-4-3? 2017/5/25 33 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 2017/5/25 34 Inter-frame Spacing 2017/5/25 35 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. 2017/5/25 36 Types of Ethernet 2017/5/25 10Gbps 37 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. 2017/5/25 38 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. 2017/5/25 39 1000BASE-T Ethernet 2017/5/25 40 1000BASE-SX and 1000BASE-LX Ethernet 2017/5/25 41 1000BASE-SX and 1000BASE-LX • Advantages over UTP – noise immunity – small physical size – increased unrepeated distances and bandwidth. 2017/5/25 42 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. 2017/5/25 43 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. 2017/5/25 44 2017/5/25 45 With 10Gbps Ethernet … • Flexible, efficient, reliable, relatively low cost end-to-end Ethernet networks become possible. 2017/5/25 46 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. 2017/5/25 47 HUB-based Ethernet Lack of scalability Increased latency Increased latency 2017/5/25 48 Switch-based Ethernet 2017/5/25 49 Switch-based Ethernet Dedicated bandwidth 2017/5/25 50 Switch-based Ethernet Collision Free 2017/5/25 51 Switch-based Ethernet Full Duplex 2017/5/25 52 Switch – selective forwarding 2017/5/25 53 Switch 2017/5/25 54 Switch Operation • To accomplish their purpose, Ethernet LAN switches use five basic operations: – – – – – 2017/5/25 Learning Aging Flooding Selective Forwarding Filtering 55 Learning 2017/5/25 56 Flooding 2017/5/25 57 Learning again 2017/5/25 58 Selective Forwarding 2017/5/25 59 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. 2017/5/25 60 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? 2017/5/25 61 With ARP protocol … • Two basic functions are provided – Resolving IPv4 addresses to MAC addresses – Maintaining a cache of mappings • ARP table (cache) 2017/5/25 62 ARP Procedure - 1 2017/5/25 63 ARP Procedure - 2 Broadcast 2017/5/25 64 ARP Procedure - 3 2017/5/25 65 ARP Procedure - 4 2017/5/25 66 ARP Procedure - 5 2017/5/25 67 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. 2017/5/25 68 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 2017/5/25 69 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) 2017/5/25 70 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 2017/5/25 71 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. 2017/5/25 72 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 2017/5/25 73 What if the destination device is in another network? 2017/5/25 74 2017/5/25 75 2017/5/25 76 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. 2017/5/25 77 Issues with ARP Protocol • Broadcast • Security 2017/5/25 78 Issues with ARP Protocol 2017/5/25 79