Chapter 10: Ethernet and Fibre Cable
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Increase in High-Speed LANs
• Extraordinary growth in speed, power, and storage capacity of PCs
• Increasing use of LANs as computing platforms
• Examples– Server farms– Workgroups with “power” requirements– High-speed backbones
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Increase in High-Speed LANs
• Fast Ethernet and Gigabit Ethernet
• Fibre Channel
• High-speed Wireless LANs
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Characteristics of Some High-Speed LANS
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Trends Influencing Emergenceof High-Speed LANs
• Explosive growth of speed and computing power of PCs
• Recognition by MIS organizations of the value and importance of networked computing– Centralized server farms– Power workgroups– High-speed local backbone
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Traditional Ethernet
• Ethernet and CSMA/CD (IEEE 802.3)• Carrier sense multiple access with collision
detection• Four step procedure
– If medium is idle, transmit
– If medium is busy, listen until idle and then transmit
– If collision is detected, cease transmitting
– After a collision, wait a random amount of time before retransmitting
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Ethernet MAC Frame Format
• Preamble: 7-octet pattern of 0s &1s used to establish bit synchronization.
• Start Frame Delimiter (SFD): Indicates actual start of frame.• Destination Address (DA): Specifies the station(s) for which the
frame is intended• Source Address (SA): Specifies the station that sent the frame.• Length: Length of LLC data field in octets. • LLC Data: Data unit supplied by LLC.• Pad: Octets added to ensure that the frame is long enough for proper
CD operation.• Frame Check Sequence (FCS): A 32-bit CRC, based on all fields
except preamble, SFD, and FCS.
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Ethernet MAC Frame
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802.3 Medium Notation
• Notation format:<data rate in Mbps><signaling method><maximum segment length in hundreds of meters>
• e.g 10Base5 provides 10Mbps baseband, up to 500 meters
• T and F are used in place of segment length for twisted pair and fiber
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802.3 10BaseX Media Options
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Bridges
• Allow connections between LANs and to WANs• Used between networks using identical
physical and link layer protocols• Provide a number of advantages
– Reliability: Creates self-contained units
– Performance: Less contention
– Security: Not all data broadcast to all users
– Geography: Allows long-distance links
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Bridge Functions
• Read all frames from each network
• Accept frames from sender on one network that are addressed to a receiver on the other network
• Retransmit frames from sender using MAC protocol for receiver
• Must have some routing information stored in order to know which frames to pass
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Bridge Operation
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Key Aspects of Bridge Function
• Makes no modification to content or format of frames it receives; simply copies from one LAN and repeats with exactly the same bit pattern as the other LAN.
• Should contain enough buffer space to meet peak demands.
• Must contain addressing and routing intelligence. • May connect more than two LANs.
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Hubs
• Alternative to bus topology• Each station is connected to the hub by two lines
(transmit and receive)• When a single station transmits, the hub repeats
the signal on the outgoing line to each station.• Physically a star; logically a bus.• Hubs can be cascaded in a hierarchical
configuration.
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Two-Level Hub Topology
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Layer 2 Switches
• Also called a “switching hub”• Has replaced hub in popularity, particularly for
high-speed LANs• Provides greater performance than a hub• Incoming frame from a particular station is
switched to the appropriate output line to be delivered to the intended destination
• At the same time, other unused lines can be used for switching other traffic
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Ethernet Hubs and Switches
• Shared medium hubs
• Switched LAN hubs
x
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Advantages of Switched Hubs
• No modifications needed to workstations when replacing shared-medium hub
• Each device has a dedicated capacity equivalent to entire LAN
• Easy to attach additional devices to the network
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Types of Switched Hubs
• Store and forward switch– Accepts a frame on input line– Buffers it briefly– Routes it to appropriate output line
• Cut-through switch– Begins repeating the frame as soon as it
recognizes the destination MAC address– Higher throughput, increased chance of error
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Differences Between Switched Hubs and Bridges
• Bridge frame handling is done in software. A layer 2 switch performs the address recognition and frame forwarding functions in hardware.
• Bridges typically only analyze and forward one frame at a time; a layer 2 switch can handle multiple frames at a time.
• Bridges uses store-and-forward operation; layer 2 switches use cut-through instead of store-and-forward operation
• New installations typically include layer 2 switches with bridge functionality rather than bridges.
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Problems With Layer 2 Switches
• Broadcast overload • Lack of multiple links• Can be solved with subnetworks connected by
routers• However, high-speed LANs layer 2 switches
process millions of packets per second whereas a software-based router may only be able to handle well under a million packets per second
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Layer 3 Switches
• Implement the packet-forwarding logic of the router in hardware.
• Packet-by-packet switch operates like a traditional router– Forwarding logic is in hardware– Achieves an order of magnitude increase in performance
compared to software-based routers
• Flow-based switch identifies flows of IP packets that have the same source and destination– Once flow is identified, a predefined route can be established to
speed up the forwarding process– Again, huge performance increases over a pure software-based
router are achieved
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Why Use Ethernet for High-Speed Networks?
• Negative– CSMA/CD is not an ideal choice for high-speed LAN
design due to scaling issues, but there are reasons for retaining Ethernet protocols
• Positive– Use of switched Ethernet hubs in effect eliminates
collisions– CSMA/CD protocol is well understood; vendors have
experience building the hardware, firmware, and software
– Easy for customers to integrate with existing systems
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Fast Ethernet
• Refers to low-cost, Ethernet-compatible LANs operating at 100 Mbps
• 802.3 committee defined a number of alternatives to be used with different transmission media
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802.3 100Base-T Options
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802.3 100BaseX Media Options
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Gigabit Ethernet
• Retains CSMA/CD protocol and Ethernet format, ensuring smooth upgrade path
• Uses optical fiber over short distances
• 1-gbps switching hub provides backbone connectivity
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Gigabit Ethernet Media Options
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10-Gbps Ethernet
• Driven by increased network traffic– Increased number of network connections
– Increased connection speed of each end-station (e.g., 10 Mbps users moving to 100 Mbps, analog 56k users moving to DSL and cable modems)
– Increased deployment of bandwidth-intensive applications such as high-quality video
– Increased Web hosting and application hosting traffic
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10-Gbps Ethernet vs ATM
• No expensive, bandwidth-consuming conversion between Ethernet packets and ATM cells is required
• Combination of IP and Ethernet offers quality of service and traffic policing capabilities that approach those provided by ATM
• A wide variety of standard optical interfaces have been specified for 10-Gbps Ethernet, optimizing its operation and cost for LAN, MAN, or WAN applications
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Physical Layer Options for 10-Gbps Ethernet
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Example 100-Mbps Ethernet Backbone Strategy
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Fibre Channel
• Combine the best features of channel and protocol-based technologies– the simplicity and speed of channel communications
– the flexibility and inter-connectivity that characterize protocol-based network communications.
• More like a traditional circuit-switched or packet-switched network, in contrast to the typical shared-medium LAN
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Fibre Channel Goals
• Full-duplex links with two fibers per link
• Performance from 100 Mbps to 800 Mbps on a single link (200 Mbps to1600 Mbps per link)
• Support for distances up to 10 km
• Small connectors• High-capacity utilization
with distance insensitivity
• Greater connectivity than existing multidrop channels
• Broad availability (i.e., standard components)
• Support for multiple cost/performance levels, from small systems to supercomputers
• Ability to carry multiple existing interface command sets for existing channel and network protocols
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Fibre Channel Elements
• Nodes– The end systems
– Includes one or more N_ ports for interconnection
• Fabric– Collection of switching elements s between systems
– Each element includes multiple F_ ports
– Responsible for buffering and for routing frames between source and destination nodes
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Fibre ChannelProtocol Architecture
• FC-0 Physical Media: Includes optical fiber, coaxial cable, and shielded twisted pair, based on distance requirements
• FC-1 Transmission Protocol: Defines the signal encoding scheme
• FC-2 Framing Protocol: Defines topologies, frame format, flow/error control, and grouping of frames
• FC-3 Common Services: Includes multicasting• FC-4 Mapping: Defines the mapping of various
channel and network protocols to Fibre Channel
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Fibre Channel Media
• Media options include shielded twisted pair, video coaxial cable, and optical fiber
• Data rates range from 100 Mbps to 3.2 Gbps
• Point-to-point link distances range from 33 m to 10 km
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Fibre Channel Topologies
• Fabric/Switched Topology– includes at least 1 switch to interconnect end systems.
– may also use multiple switches for a switched network, with switches also supporting end nodes
• Point-to-point topology – only two ports, directly connected, with no intervening fabric
switches
• Arbitrated loop topology – Simple, low-cost topology for connecting up to 126 nodes in a
loop.
– Operates in a manner roughly equivalent to token ring protocols