Chabot CollegeChabot College

ELEC 99.09ELEC 99.09VLAN

Data Link SublayersData Link Sublayers

LLC (Logical Link Control)LLC (Logical Link Control)

MAC (Media Access Control)MAC (Media Access Control)

Ethernet II (DIX Ethernet)Ethernet II (DIX Ethernet)

• 1972 Xerox PARC begins work

• 1982 Ethernet v.2 spec released

Preamble DA SA Type Data FCS64 bits 48 bits 48 bits 16 bits variable 32 bits

Ethernet IIEthernet II

802.3_Raw (Novell)802.3_Raw (Novell)

Preamble DA SA Length Data FCS64 bits 48 bits 48 bits 16 bits variable 32 bits

Ethernet_802.3Ethernet_802.3

Preamble DA SA Type Data FCS64 bits 48 bits 48 bits 16 bits variable 32 bits

Ethernet IIEthernet II

IEEE 802.3 (with 802.2 LLC)IEEE 802.3 (with 802.2 LLC)

Preamble DA SA Length DSAP SSAP64 bits 48 bits 48 bits 16 bits 8 bits8 bits

Data FCSvariable 32 bits

Ethernet_802.2 (an 802.3 frame with LLC)Ethernet_802.2 (an 802.3 frame with LLC)

The 802.2 Protocol (LLC): Service Access Points serve the same purpose as a Protocol Type field.

Ethernet Frame TypesEthernet Frame Types

• Ethernet II

• Ethernet 802.3 (with 802.2 LLC)

• Ethernet 802.3 RAW (Novell only)

• Ethernet SNAP

These can coexist on the same LAN, but in order to directly communicate, nodes must use the same frame type.

Ethernet CharacteristicsEthernet Characteristics

• Media Access Method: CSMA/CD

• Broadcast

• One station transmits at a time

• Best-effort delivery

Ethernet DrawbacksEthernet Drawbacks

Ethernet DrawbacksEthernet Drawbacks

Performance negatively affected by: The data frame broadcast delivery nature of Ethernet/802.3

LANs

CSMA/CD access methods allow only one station to transmit at a time.

Network congestion due to increased bandwidth demands from multimedia applications such as video and the Internet.

Normal latency (propagation delay) of frames as they travel across the LAN layer 1 media and pass through layer 1, 2 and 3 networking devices.

Extending the distances of the Ethernet/802.3 LANs using Layer

1 repeaters.

Standard EthernetStandard Ethernet

• Can either send OR receive.

The Need for SpeedThe Need for Speed

The Need for SpeedThe Need for Speed

Balance is KeyBalance is Key

Propagation DelayPropagation Delay• Propagation delay (latency) is the time a

frame or packet of data takes to travel from the source station or node to its final destination on the network.

What causes latency?What causes latency?

• Since Ethernet LANs use CSMA/CDCSMA/CD to provide best effort delivery there must be a certain amount of latency in the system to detect collisions and negotiate transmission rights on the network.

• The greater the number of devices the greater the latency or propagation delay.

Ethernet Transmission TimesEthernet Transmission Times

• The time it takes for the Data Link layer to “hand off” to the cable (Physical Layer).

Extending the MediaExtending the Media

Improving LAN performanceImproving LAN performance

Performance can be improved by:

• Segmenting the network using Bridges, Routers and LAN Switches

• Moving to full duplex transmitting

• Upgrading to the Fast Ethernet Standard 802.3u

• Upgrading to the Gig Ethernet Standard 802.3z and 802.3ab

Why Segment LANs?Why Segment LANs?

Segmenting and CollisionsSegmenting and Collisions

Segmenting with BridgesSegmenting with Bridges

Bridges Add LatencyBridges Add Latency

Increases propagation delay by up to 30%

Segmentation With RoutersSegmentation With Routers

20-40% loss of throughput

Segmentation with SwitchesSegmentation with Switches

How Switches WorkHow Switches Work

• A switch can create a network that behaves like it only has two nodes - the sender and the receiver.

• These two nodes share the 10 Mbps bandwidth between them, available bandwidth can reach closer to 100%.

How Switches WorkHow Switches Work

• Switches are high speed multi-port bridges with one port for each node or segment of the LAN.

• A switch segments a LAN into microsegmentsmicrosegments creating collision free domains from one larger collision domain.

MicrosegmentationMicrosegmentation

Switch LatencySwitch Latency

• Switches add latency, but they can overcome this by forwarding frames before they are completely received.

Full-Duplex EthernetFull-Duplex Ethernet• Allows the transmission of a packet and the reception of a

different packet at the same time.

• Requires two pairstwo pairs of wires and a switched connectionswitched connection between each node.

• Point-to-point connection, nearly collision free.

• No negotiations for bandwidth.

Full-Duplex EthernetFull-Duplex Ethernet

Offers 100% bandwidth in both directions (potential 20 Mbps, 200 Mbps, etc).

Switches and BroadcastsSwitches and Broadcasts

Switches Live at Layer 2Switches Live at Layer 2

Switches Create Virtual CircuitsSwitches Create Virtual Circuits

Switches Learn the NetworkSwitches Learn the Network

Content Addressable MemoryContent Addressable Memory• An Ethernet switch can learn the address of each device

on the network by reading the source address of each packet transmitted and

noting the port where the frame was heard

• The switch then adds this information to its forwarding database. Addresses are learned dynamically. This means that as new addresses are read they are learned and stored in content addressable memory content addressable memory (CAM).(CAM).When a source is read that is not found in the CAM it is learned/stored for future use.

Aging OutAging Out

Each time an address is stored it is time stampedtime stamped. This allows for addresses to be stored for a set period of time. Each time an address is referenced or found in the CAM, it receives a new time stamp. Addresses that are not referenced during set period of time are removed from the list. By removing aged or old addresses the CAM maintains an accurate and functional forwarding database.

Benefits of SwitchingBenefits of Switching

• Improved manageability.

• Easy to migrate from shared media.

Symmetric SwitchingSymmetric Switching

Asymmtric SwitchingAsymmtric Switching

Memory BufferingMemory Buffering

Two ways to buffer incoming and outgoing frames:

• port-based memory buffering

• shared memory buffering

Port-Based Memory BufferingPort-Based Memory Buffering

• Packets are stored in queuesqueues that are linked to specific incoming ports. A packet is transmitted to the outgoing port only when all the packets ahead of it in the queue have been successfully transmitted.

Port-Based DrawbacksPort-Based Drawbacks

• It is possible for a single packet to delay the transmission of all the packets in memory because of a busy destination port. This delay occurs even if the other packets can be transmitted to open destination ports.

Shared Memory BufferingShared Memory Buffering

• Deposits all packets into a common memory buffer that is shared by all the ports on the switch. The amount of memory allocated to a port is determined by how much is required by each port. This is called dynamic allocation of buffer memory.

Two Switching MethodsTwo Switching Methods

Cut-through v. Store & ForwardCut-through v. Store & Forward

Spanning-Tree ProtocolSpanning-Tree Protocol

• allows redundant switched/bridged paths without suffering the effects of loops in the network.

STP StatesSTP States

Virtual Local-Area NetworksVirtual Local-Area Networks