Chabot College
ELEC 99.09
VLAN
Data Link Sublayers
LLC (Logical Link Control)
MAC (Media Access Control)
Ethernet II (DIX Ethernet)
Ethernet II
64 bits
Preamble
48 bits
DA
48 bits
SA
16 bits
Type
variable
Data
• 1972 Xerox PARC begins work
• 1982 Ethernet v.2 spec released
32 bits
FCS
802.3_Raw (Novell)
Ethernet_802.3
64 bits
Preamble
48 bits
DA
48 bits
SA
Ethernet II
64 bits
Preamble
48 bits
DA
48 bits
SA
16 bits
Length
16 bits
Type
variable
Data
variable
Data
32 bits
FCS
32 bits
FCS
IEEE 802.3 (with 802.2 LLC)
Ethernet_802.2 (an 802.3 frame with LLC)
64 bits
Preamble
variable
Data
48 bits
DA
48 bits
SA
16 bits
Length
8 bits
DSAP
8 bits
SSAP
32 bits
FCS
The 802.2 Protocol (LLC): Service
Access Points serve the same
purpose as a Protocol Type field.
Ethernet 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 Characteristics
•
•
•
•
Media Access Method: CSMA/CD
Broadcast
One station transmits at a time
Best-effort delivery
Ethernet Drawbacks
Ethernet 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 Ethernet
• Can either send OR receive.
The Need for Speed
The Need for Speed
Balance is Key
Propagation 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?
• Since Ethernet LANs use CSMA/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 Times
• The time it takes for the Data Link layer to
“hand off” to the cable (Physical Layer).
Extending the Media
Improving 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?
Segmenting and Collisions
Segmenting with Bridges
Bridges Add Latency
Increases propagation delay by up to 30%
Segmentation With Routers
20-40% loss of throughput
Segmentation with Switches
How 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 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
microsegments creating collision free
domains from one larger collision domain.
Microsegmentation
Switch Latency
• Switches add latency, but they can
overcome this by forwarding frames
before they are completely received.
Full-Duplex Ethernet
• Allows the transmission of a packet and the reception of a
different packet at the same time.
• Requires two pairs of wires and a switched connection
between each node.
• Point-to-point connection, nearly collision free.
• No negotiations for bandwidth.
Full-Duplex Ethernet
Offers 100% bandwidth in both directions
(potential 20 Mbps, 200 Mbps, etc).
Switches and Broadcasts
Switches Live at Layer 2
Switches Create Virtual Circuits
Switches Learn the Network
Content 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 (CAM).When a source is
read that is not found in the CAM it is
learned/stored for future use.
Aging Out
Each time an address is stored it is time 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 Switching
• Improved manageability.
• Easy to migrate from shared media.
Symmetric Switching
Asymmtric Switching
Memory Buffering
Two ways to buffer incoming and outgoing
frames:
• port-based memory buffering
• shared memory buffering
Port-Based Memory Buffering
• Packets are stored in queues 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 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 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 Methods
Cut-through v. Store & Forward
Spanning-Tree Protocol
• allows redundant switched/bridged paths without
suffering the effects of loops in the network.
STP States
Virtual Local-Area Networks