IST 220 CHAPTER 5: ETHERNET (802.3) SWITCHED LANs

Multi-Switch Ethernet Operation

1. A packet from A1... to E5... must pass through switches 1,2, and 3. 2. Switch 1 sees that it should send the frame to E5 out Port 5 to Switch 2 3. Switch 2 sees that it should send the frame to E5 out Port 7. 4. Switch 3 sees that it should send the frame to E5 out Port 6.

Modal Dispersion in Multimode Fiber

1. Perfect Internal Reflection; No Radiative Attenuation. 2. Light can only enter the core at certain angles (modes). This creates arrival time delays called modal dispersion. At some distance, successive signals overlap, become unreadable.

Transmission Media

4-pair UTP -8 copper wires organized as four pairs whose two wires are twisted around each other -Carries signals as voltage, etc. changes Optical Fiber -Carries signal pulses through glass DIFFERENCE IS DISTANCE, NOT SPEED, EXCEPT ATVERY HIGH SPEEDS.

Initial User Authentication with 802.1X

802.1X Authentication: Supplicant must be verified by an authentication server before being allowed to use the workgroup switch to which it is attached. Attacker cannot walk up to any Ethernet wall jack, plug in, and bypass the firewall.

Types of Ethernet Switches and Transmission Links

Access links connect hosts to the network. Normally 4-Pair unshielded twisted pair cable. Workgroup switches connect hosts to the network. Core switches connect switches to other switches (the collection of core switches is the "Core) Trunk links connect switches to other switches. Mostly optical fiber.

Switch-to-Switch Security: 802.1AE

Adversary Impersonates a Switch, Sends the Target Switch a Malicious Command Switches send supervisory frames to one another. Without 802.1AE, an adversary can impersonate a switch and send malicious commands.

Arp Cache Poisoning

Before the attack, the victim's ARP cache correctly tells him that the Router 1.2.3.4 has the data link layer address A1-CE-... The victim sends packets in frames to that data link address. Attacker sends the victim an ARP update. This changes the victim's router DLL address to the DLL address of the attacker Subsequent packets to the router will go in frames addressed to the attacker, who can read them before sending them on to the router.

The Undesirability of Attaining the Required Distance with an Intermediate Switch

Better: Use a more expensive physical medium. Worse: Use an intermediate switch/ Media are Cheap, Switches are Expensive.

Ethernet Signaling Standards

Developed in response to growing access point speeds. Category (Cat) is a designation of UTP Quality. OM is a designation of multimode optical fiber Quality SR4 and SR10 use parallel transmission with 4 versus 10 standards in each direction. OM is a designation of multimode optical fiber Quality.

Radiative Attenuation in 4-Pair UTP

Each pair radiates radio signals, dissipating the signal. This causes attenuation, which increases with propagation distance.

Hierarchical Ethernet Topology

Ethernet requires a hierarchical switch organization. Each host and switch has a single parent. In a hierarchy, there is only one possible path between any two hosts. The path for Client A to Server X is shown.

Failures and Backup Links

Filed linked = Failures divide the network into two parts RSTP Permits Backup links that violate the hierarchy. RSTP disables backup links until they are needed

Virtual LANs (VLANs)

Host A1 on VLAN 3 can only communicate with Hosts B2 and E5, which are on the same VLAN. VLANs segregate traffic, creating security that can be broken but that is quite good.

Binary and Digital Signaling

In binary transmission, there are two states (1 or 0). One bit is send in each clock cycle. In digital transmission, there are a few states (2, 4, 8, 16, ...). More than one bit is send in each clock cycle. Binary( two states[ two is few])is a special case of digital(few states).

Light Transmission Metrics

Light travels in waves. A wave's amplitude is its power. Optical fiber transmission is described in terms of wavelength. Wavelength is the physical distance between comparable points on adjacent cycles. Wavelengths for optical fiber are measured in nanometers (nm). LAN fiber uses 850 nm light almost exclusively because it's cheap and gets the job done. Wide area networks use 1,310 and 1,550 nm light to support longer distances.

Ethernet Standards for Signaling and Frame Forwarding

One data link layer standard Many physical layer standards

Local Area Networks (LANs)

Operate on a customer premises The property owned by the person or organization that uses the network Companies can use whatever technology standards they wish

Closeup of the Wire Pairs

Pair colors are: orange/white-orange green/white-green blue/white-blue brown/white-brown

Switched Ethernet Network: Physical and Data Links

Physical Link: Path Between Adjacent Devices Data Link: Path of Frame Through a Single Network

Ethernet Origins

Requires standards at Layers 1 (wires and signals) and 2 (frames and switches) -OSI standards dominate at these layers Ethernet standards are created by the IEEE 802.3 Working Group of the IEEE Standards Association's 802 LAN/MAN standards committee -Called 802.3 standards -Submitted to ISO, which ratifies them as OSI standards Other 802 Working Group (WG) -The 802.11 WG creates Wi-Fi standards -The 802.1 WG creates security standards

Parallel Transmission in Ethernet

Serial Transmission N bits per clock cycle Parallel Transmission 4*N bits per clock cycle Parallel transmission carries more bits per clock cycle than serial transmission

Secondary Fields in an Ethernet II Frame

Tag Protocol (Optional) (2 Octets) Tag Control Information (2 Octets) Indicates that the frame is tagged, specifies priority and VLAN Membership. PAD- Added for Frames < 64 Octets Long

Single-Mode Fiber

There is no modal dispersion. There is only he absorptive attenuation of the glass core. This is very small, so distance limits are very large.

Link Aggregation (Bonding)

Two links provide 2 Gbps of trunk capacity between the switches.

Distance for Physical links Versus Data Links

UTP has max transmission distance of 100m. Longer distance will degrade the signal too much to read correctly. We have been looking at propagation distances for physical links. What about data links? Each switch along the way REGENERATES the signal. It does not simply amplify the distorted received signal. The signal can now travel over the next physical link.

Link Aggregation Increases Speed, Not Distance

Using two cords to increase distance. This is NOT link aggregation.

Bob Metcalfe

Working on his PhD at MIT Hit a snag, but the Xerox Palo Alto Research Center still hired him Before went there, visited the University of Hawaii's ALOHANET packet radio project Developed a way to make it more efficient Applied it to a cabled network, called it Ethernet as a joke Eventually commercialized it