2014/10/11 2 Ethernet is now the predominant LAN technology in the world.
2014/10/11 3 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.
2014/10/11 8 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.
2014/10/11 9 Reasons for Ethernet Success Simplicity and ease of maintenance Ability to incorporate new technologies Reliability Low cost of installation and upgrade
2014/10/11 15 FCS : Frame Check Sequence frame will be dropped if FCS is incorrect
2014/10/11 16 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)
2014/10/11 17 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.
2014/10/11 18 Addressing in Ethernet MAC Address burned in ROM on NIC card & will be copied into RAM when start-up.
2014/10/11 19 MAC address structure In DOS command window, type “ipconfig/all” to view MAC address
2014/10/11 20 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.
2014/10/11 22 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".
2014/10/11 23 Media Access Control in Ethernet (CSMA/CD)
2014/10/11 24 Collision 以後 ? Jam signal: maybe a 32-bit repeating one, zero, one, zero pattern)
2014/10/11 25 Collision 以後 ? Backoff Timing Each computer has different backoff time
2014/10/11 27 Ethernet Delay (Latency) More latency, more likely the collision
2014/10/11 28 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!
2014/10/11 29 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
2014/10/11 30 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 !
2014/10/11 31 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
2014/10/11 32 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 = 512 bytes > 0.985 μs (100 m) Operates at full-duplex only, no CSMA/CD is required Why not use 512 bit time?
2014/10/11 34 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
2014/10/11 36 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.
2014/10/11 38 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.
2014/10/11 39 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.
2014/10/11 41 1000BASE-SX and 1000BASE-LX Ethernet
2014/10/11 42 1000BASE-SX and 1000BASE-LX Advantages over UTP –noise immunity –small physical size –increased unrepeated distances and bandwidth.
2014/10/11 43 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.
2014/10/11 44 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.
2014/10/11 46 With 10Gbps Ethernet … Flexible, efficient, reliable, relatively low cost end-to-end Ethernet networks become possible.
2014/10/11 47 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.
2014/10/11 48 HUB-based Ethernet Lack of scalability Increased latency
2014/10/11 60 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.
2014/10/11 61 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?
2014/10/11 62 With ARP protocol … Two basic functions are provided –Resolving IPv4 addresses to MAC addresses –Maintaining a cache of mappings ARP table (cache)
2014/10/11 68 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.
2014/10/11 69 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
2014/10/11 70 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)
2014/10/11 71 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
2014/10/11 72 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.
2014/10/11 73 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
2014/10/11 74 What if the destination device is in another network?
2014/10/11 77 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.
2014/10/11 78 Issues with ARP Protocol Broadcast Security