cim 2465 fundamentals of ethernet1 fundamentals of ethernet (topic 3) textbook: networking basics,...

CIM 2465 Fundamentals of Ethernet 1

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


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


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


Naming of Ethernet Types

• Ethernet standards differ in two main respects:– Speed– Type of cabling supported

• IEEE 802.3• 10BASE-T, 100BASET etc.

– Speed– Baseband Transmission– Cabling

• Type of cabling, T stands for twisted pair


IEEE Standards and the OSI Model

• 802.3 standards define different types of Ethernet concern– Physical transmission details (cables, connectors, enc

oding, speeds)– Media access issues (e.g. CSMA/CD)– Errors handling (e.g. Frame Check Sequence in 802.3

trailer)– MAC addresses (format, location in the 802.3 header)– Ability of NICs to synchronize to the incoming signal b

y providing a Preamble and Start Frame Delimiter (SFD)



• 802.3 standards define many physical layer details, as well as the lower half of the data link layer, called Media Access Control (MAC)



• 802.3 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)


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

46-1500

FCS

4


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


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


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 24 bits

Example 00 60 2F 3A 07 BC

cisco Particular device


Ethernet Operation• The 802.3 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


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 frame3. The device listens for collisions while it is transmitting4. If no collisions occurred, the process is ok. If a collision

does occur, the following steps are taken5. 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


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


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


Token Passing in a Token Ring

• Free token is required to get the right to send data


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)


10Mbps Ethernet

• 10BASE5– Thicknet, Thick coaxial– 500m– IEEE 802.3

• 10BASE2– Thinnet, Thin coaxial– 185m– IEEE 802.3a

• 10BASE-T– UTP twisted pair– 100m– IEEE 802.3i


10BASE-T Wiring• Cat3, Cat5, Cat 5e UTP


10BASE-T Design: Using Hubs, CSMA/CD

• Key design rule (10BASE5, 10BASE2, and 10BASE-T) is called the 5-4-3 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


5-4-3 Rule


Good Design Practice


10BASE-T Design: Using Switches

• Use of switches, instead of hubs, removes the 5-4-3 rule’s design restrictions

• The 5-4-3 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 5-4-3 rule applies to each collision domain


100Mbps Ethernet (Fast Ethernet)

• 100BASE-T (support Cat3), 100BASE-TX• Designing Ethernets with Two Speeds (autonegotiation)


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)


Ethernet Switching

• Related to how a bridge or switch makes a decision of how and where to forward Ethernet frames


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 802.3. Usually are PCs running software that performed the bridging.

• Switches do similar things, but usually in hardware, thus much faster than bridges


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)


Bridge Filtering Decision

• Fig 8-1


Bridge Forwarding Decision

• Fig 8-2


Learning CAM Table Entries

• Switches (and bridges) must dynamically learn the entries in the CAM


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)


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 0100.5E or 0100.5F, such frames should be delivered to multiple devices on the LAN, but not necessarily to all devices


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 wors

ening LAN performance• Large collision domains should not be used


Solution

• Creating many small collision domains


Advantages of having Small Collision Domains

• 5-4-3 rule applies to smaller part

• Reduce the probability of overutilization (fewer devices)

• Each collision domain gets its own separated switched bandwidth


How Switches and Bridges Prevent Collisions

• Buffering


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)


Data Flow with Layer 1, 2, and 3 Devices

• Fig 8-21

cim 2465 fundamentals of ethernet1 fundamentals of ethernet (topic 3) textbook: networking basics,...

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