CIM 2465 Fundamentals of Ethernet1 Fundamentals of Ethernet (Topic 3) Textbook: Networking Basics, CCNA 1 Companion Guide, Cisco Press Cisco Networking Academy Program, CCNA 1 and 2, Companion Guide, Cisco Press, Latest Edition

CIM 2465 Fundamentals of Ethernet2 Introduction (1) Created at Xerox at 1970s Took over by IEEE at 1980s IEEE 802 committee (standards for LANs) –802.1 LAN management and control functions –802.2 Functions common to several LAN types –802.3 Ethernet LANs –802.4 Token Bus LANs –802.5 Token Ring LANs

CIM 2465 Fundamentals of Ethernet3 Introduction (2) Ethernet has been very successful –Relatively simple –Adding a new type of Ethernet is easy, many people already understand other types of Ethernet –Reliable, with well-tested components and protocols –Inexpensive

CIM 2465 Fundamentals of Ethernet4 Naming of Ethernet Types Ethernet standards differ in two main respects: –Speed –Type of cabling supported IEEE BASE-T, 100BASET etc. –Speed –Baseband Transmission –Cabling Type of cabling, T stands for twisted pair

CIM 2465 Fundamentals of Ethernet5 IEEE Standards and the OSI Model standards define different types of Ethernet concern –Physical transmission details (cables, connectors, encoding, speeds) –Media access issues (e.g. CSMA/CD) –Errors handling (e.g. Frame Check Sequence in trailer) –MAC addresses (format, location in the header) –Ability of NICs to synchronize to the incoming signal by providing a Preamble and Start Frame Delimiter (SFD)

CIM 2465 Fundamentals of Ethernet6 IEEE Standards and the OSI Model standards define many physical layer details, as well as the lower half of the data link layer, called Media Access Control (MAC)

CIM 2465 Fundamentals of Ethernet7 IEEE Standards and the OSI Model includes the MAC protocol to perform several functions that physical layer (layer 1) standards cannot –Communicates with higher layers –Defines physical addressing in the form of MAC addresses –Defines the structure (and meaning) of data frame –Manages the process of which device can send at what time (e.g. CSMA/CD)

CIM 2465 Fundamentals of Ethernet8 Ethernet Frame (1) Framing refers –The process of encapsulating data inside a header and possibly a trailer –The meaning defined to the bits inside those headers and trailers Ethernet frame structure Preamble 7 SFD 1 Destination 6 Source 6 Length/Type 2 Data FCS 4

CIM 2465 Fundamentals of Ethernet9 Ethernet Frame (2) Preamble: for synchronization Start Frame Delimiter (SFD) Destination MAC Address (48 bits) Source MAC Address (48 bits) Length/Type: data length or type of protocol Data Frame Check Sequence (FCS): allows the receiving NIC to decide if the frame had an error

CIM 2465 Fundamentals of Ethernet10 Encapsulating Packets Inside Ethernet Frames 1.IP software gives the IP packet to the Ethernet software 2.Ethernet software encapsulates the IP packet between an Ethernet header and trailer 3.Ethernet NIC physically sent the bits over an Ethernet LAN

CIM 2465 Fundamentals of Ethernet11 Format of MAC Address MAC addresses are 6-byte (48 bits) hexadecimal numbers that are used to identify NICs and other Ethernet interfaces that connect to a LAN Organizational Unique Identifier (OUI) Vendor Assigned (NIC Cards, Interfaces) Size (bits)24 bits Example F3A 07 BC ciscoParticular device

CIM 2465 Fundamentals of Ethernet12 Ethernet Operation The Media Access Control (MAC) standards defines the rules how Ethernet devices access the LAN media Ethernet use Carrier Sense Multiple Access with Collision Detection (CSMA/CD), the general rules are: –Wait until the LAN is unused, and then send the frame. However, listen to detect whether the frame being sent collided with another frame. If no collisions occurred, the frame must have made it across the LAN. If a collision did occur, wait, and try to send the frame again. Note that the performance degrades as the number of collisions increases

CIM 2465 Fundamentals of Ethernet13 CSMA/CD Algorithm 1.A device listens until the LAN is silent, that is no device is currently sending data 2.The device sends its frame 3.The device listens for collisions while it is transmitting 4.If no collisions occurred, the process is ok. If a collision does occur, the following steps are taken 5.All devices whose transmitted frames collided send a jamming signal, ensure that all devices notice that a collision occurred 6.All devices whose frame collided re-send the frame after an independent random time

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CIM 2465 Fundamentals of Ethernet15 Preventing Collisions with Switch Buffering Switches prevent collisions by buffering frames, it holds the frames in memory (buffering) until it’s OK to send Each switch port is a Collision Domain A Collision domain refers to the set of Ethernet devices whose frames could possibly collide A frame in one Collision Domain does NOT cause collisions in another domain

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CIM 2465 Fundamentals of Ethernet18 Deterministic and Nondeterministic Media Access CSMA/CD is nondeterministic (probabilistic). The number of times a single device is allowed to use the LAN, or the amount of LAN bandwidth a device can use, CANNOT be determined ahead of time Deterministic media access, means that we can accurately predict the bandwidth allowed to the devices on the LAN –E.g. Token passing mechanism

CIM 2465 Fundamentals of Ethernet19 Token Passing in a Token Ring Free token is required to get the right to send data

CIM 2465 Fundamentals of Ethernet20 Ethernet Technologies Most common –10 Mbps 10BASE2, 10BASE5, 10BASE-T –100 Mbps 100BASE-T, 100BASE-TX 100BASE-FX (optic) –1 Gbps Next major evolution 1000BASE-T 1000BASE-LX and 1000BASE-SX (optic)

CIM 2465 Fundamentals of Ethernet21 10Mbps Ethernet 10BASE5 –Thicknet, Thick coaxial –500m –IEEE BASE2 –Thinnet, Thin coaxial –185m –IEEE 802.3a 10BASE-T –UTP twisted pair –100m –IEEE 802.3i

CIM 2465 Fundamentals of Ethernet22 10BASE-T Wiring Cat3, Cat5, Cat 5e UTP

CIM 2465 Fundamentals of Ethernet23 10BASE-T Design: Using Hubs, CSMA/CD Key design rule (10BASE5, 10BASE2, and 10BASE-T) is called the rule Between an two devices on a LAN, there can be at most: –Five cable segments –Four hubs –Three LAN segments with devices attached to them This rule makes sure that CSMA/CD works correctly by ensuring that a collision can be heard in a reasonable amount of time

CIM 2465 Fundamentals of Ethernet Rule

CIM 2465 Fundamentals of Ethernet25 Good Design Practice

CIM 2465 Fundamentals of Ethernet26 10BASE-T Design: Using Switches Use of switches, instead of hubs, removes the rule’s design restrictions The rule relates to round-trip time in long collision domains For switches (or bridges), each interface is a separate collision domain, thus collision domains are small The rule applies to each collision domain

CIM 2465 Fundamentals of Ethernet27 100Mbps Ethernet (Fast Ethernet) 100BASE-T (support Cat3), 100BASE-TX Designing Ethernets with Two Speeds (autonegotiation)

CIM 2465 Fundamentals of Ethernet28 Gigabit Ethernet and Beyond Gigabit Ethernet was the next major evolution Fiber based –1000BASE-X (1000BASE-SX, 1000BASE-LX) Copper UTP-based –1000BASE-T Fiber based 10 G Ethernet –10GBASE-X, IEEE 802.3ae (2002)

CIM 2465 Fundamentals of Ethernet29 Ethernet Switching Related to how a bridge or switch makes a decision of how and where to forward Ethernet frames

CIM 2465 Fundamentals of Ethernet30 Layer 2 Bridging and Switching Operations Bridges forward Ethernet frames based on the destination MAC address, requiring that bridges be aware of Ethernet framing and MAC addresses defined in IEEE Usually are PCs running software that performed the bridging. Switches do similar things, but usually in hardware, thus much faster than bridges

CIM 2465 Fundamentals of Ethernet31 The Forward and Filtering Decision Examine the incoming signal, find the destination MAC address Examine a table that lists MAC addresses and corresponding bridge/switch interfaces, which identifies the interface to which the frame should be forwarded If the frame came in a different interface from the one listed in the table, forward the frame, using a clean regenerated signal (forwarding) If the frame came in the same interface as the one listed in the table, discard the frame (filtering)

CIM 2465 Fundamentals of Ethernet32 Bridge Filtering Decision Fig 8-1

CIM 2465 Fundamentals of Ethernet33 Bridge Forwarding Decision Fig 8-2

CIM 2465 Fundamentals of Ethernet34 Learning CAM Table Entries Switches (and bridges) must dynamically learn the entries in the CAM

CIM 2465 Fundamentals of Ethernet35 Learning CAM Table Entries When a frame for which a switch does not have the frame’s destination MAC address in the CAM, the switch floods the frame (forward to all ports except the incoming one)

CIM 2465 Fundamentals of Ethernet36 Forwarding Broadcasts and Multicasts Broadcast frames –Frames sent to a destination MAC address of FFFF.FFFF.FFFF, such frames should be delivered to all devices on the same LAN Multicast frames –Frames sent to one of a range of multicast MAC addresses (e.g. begin with E or F, such frames should be delivered to multiple devices on the LAN, but not necessarily to all devices

CIM 2465 Fundamentals of Ethernet37 LAN Design: Collision Domains and Broadcast Domains Collision domain –A set of LAN interfaces for which a frame sent out any two of these interfaces, at the same time, would cause a collision –The bandwidth in a single collision domain must be shared Disadvantages of having large collision domains –Shared bandwidth: amount of bandwidth available to each end-user device is smaller –Higher utilization: resulting in varying degrees or worsening LAN performance Large collision domains should not be used

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CIM 2465 Fundamentals of Ethernet40 Solution Creating many small collision domains

CIM 2465 Fundamentals of Ethernet41 Advantages of having Small Collision Domains rule applies to smaller part Reduce the probability of overutilization (fewer devices) Each collision domain gets its own separated switched bandwidth

CIM 2465 Fundamentals of Ethernet42 How Switches and Bridges Prevent Collisions Buffering

CIM 2465 Fundamentals of Ethernet43 Layer 2 Broadcast Domains Broadcast domain –The set of LAN interfaces for which a broadcast frame sent by any one device will be forwarded to all the other interfaces in that same broadcast domain More Broadcasts, Less CPU Capacity for End-user work (i.e. negative impact to performance)

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CIM 2465 Fundamentals of Ethernet45 Data Flow with Layer 1, 2, and 3 Devices Fig 8-21