Networking 101

Copyright Priscilla Oppenheimer

Study Topics 3

Data Link Layer

Ethernet Fundamentals

Design goals and reasons for Ethernet's success

Simplicity
Low cost
Compatibility
Fairness
Low delay
High speed
Ease of maintenance
Capability to incorporate new technologies
Reliability

Ethernet history

Ethernet was created by Digital Equipment Corporation, Intel, and Xerox
The early versions of Ethernet are sometimes called DIX Ethernet
DIX Ethernet II (Ethernet Version 2) is still widely used
Ethernet was standardized by the Institute of Electrical and Electronics Engineers (IEEE) 802.3 committee
In general, you can use the terms Ethernet and 802.3 interchangeably, although there are a few differences

IEEE 802.3 naming rules

Speed
Baseband or broadband
Final letters or numbers that refer to technology or distance

For example, 10BaseT is 10 Mbps, uses baseband signaling, and supports twisted-pair cabling. 10Base5 is 10 Mbps, uses baseband signaling, and can reach 500 meters. Other examples are shown in the following table. (The table is FYI. There's no need to memorize this!)

IEEE 802.3 Standards

Standard	Specification	Maximum Distance in Meters
10Base5	1 thick coaxial cable (ThickNet)	500
10Base2	1 thin coaxial cable, also known as CheaperNet	185
10Broad36	3 channels (each direction) of a private CATV system	3600
10BaseT	2 pairs Category-3 or better Unshielded Twisted Pair (UTP)	100
10BaseF	Generic name for 10-Mbps fiber-optic standards	N/A
10BaseFL	2 multimode optical fibers with asynchronous active hub	2000
10BaseFB	2 multimode optical fibers with synchronous active hub	2000
10BaseFP	2 multimode optical fibers with passive hub	1000
100BaseT	Generic name for 100-Mbps standards	N/A
100BaseX	Generic name for 100BaseT standards using 4B/5B encoding	N/A
100BaseTX	2 pairs Category-5 UTP, uses 4B/5B encoding	100
100BaseFX	2 multimode optical fibers, uses 4B/5B encoding	412 (half duplex) 2000 (full duplex)
100BaseT4	4 pairs Category-3 (or better) UTP, uses 8B/6T encoding	100
100BaseT2	2 pairs Category-3 (or better) UTP, uses PAM 5 encoding	100
1000BaseX	Generic name for 1000-Mbps standards using 8B/10B encoding	N/A
1000BaseCX	2 pairs 150-ohm shielded twisted-pair (STP), uses 8B/10B encoding	25
1000BaseSX	2 multimode optical fibers using shortwave laser optics, uses 8B/10B encoding	550
1000BaseLX	2 multimode or single-mode optical fibers using longwave laser optics, uses 8B/10B encoding	550 multimode, 5000 single-mode
1000BaseT	4 pairs Category-5 UTP, uses PAM 5 encoding	100
10GBaseLX4	2 multimode or single-mode optical fibers using longwave laser optics and wide wavelength division multiplexing	300 multimode, 10,000 single-mode
10GBaseE	2 single-mode optical fibers	40,000! (if attenuation is low enough)

Baseband Vs. Broadband Signaling

Baseband

Entire bandwidth of the transmission medium is used to transmit a single digital signal
The signal is transferred directly onto the transmission medium without modulation

Broadband

Carries multiple voice, video or data channels simultaneously
Each channel occupies (is modulated to) a different frequency on the transmission medium
Data signal not placed directly on medium
An analog carrier signal is modulated by the data signal

IEEE and the OSI Model

IEEE subdivides the OSI Data Link layer into the Media Access Control (MAC) sublayer and the Logical Link Control (LLC) sublayer. LLC is also known as IEEE 802.2.

The MAC sublayer deals with:

Rules for accessing a medium
Framing
MAC addresses (aka hardware, NIC, Layer 2, Data Link, Ethernet, burned-in addresses)
Topology
Line discipline (shared media versus point-to-point link)

The LLC (802.2) sublayer:

Allows multiple Layer 3 protocols to use the medium
Remains independent of the physical equipment
Adds Service Access Point (SAP) addressing

MAC addresses

48 bits
Expressed as 12 hexadecimal digits
The first 3 bytes (first 6 hex digits) are a vendor code, also known as the Organizationally Unique Identifier (OUI)
The remaining 3 bytes are a serial number that uniquely identifies the NIC
Example 00:00:0C:12:34:56 (00:00:0C is one of Cisco's vendor codes.)

802.3 Frame Structure

Preamble	Start Frame Delimiter	Dest. MAC Address	Source MAC Address	Length or Type	Data (plus pad if necessary)	Frame Check Sequence
7 bytes	1 byte	6 bytes	6 bytes	2 bytes	46 to 1500 bytes	4 bytes

Ethernet II (DIX) Frame Structure

Same as 802.3 except it:

Considers Start Frame Delimiter part of an 8-byte Preamble field
Always uses Type rather than Length field

Ethernet Operation

CSMA/CD: Carrier Sense (CS) - listen before sending. Multiple Access (MA) - it's a shared network. Collision Detection (CD) - listen while sending for a collision. If another station is also sending, back off and wait a random amount of time before trying again.

Transmission Modes

Simplex: unidirectional; traffic can only travel in one direction
Half duplex: traffic can travel in both directions, but not in both directions at the same time
Full duplex: traffic can travel in both directions at the same time

Collision Domains Versus Broadcast Domains

Collision Domain

The connected physical network where collisions can occur
Should be divided up via LAN segmentation methods
Switches and routers divide up collision domains
Repeaters and hubs do not divide up collision domains

Broadcast Domain

The part of the network where devices can hear each other's broadcasts (frames to FF:FF:FF:FF:FF:FF)
Grouping of collision domains
Routers divide up broadcast domains
Switches and hubs do not divide up broadcast domains

Ethernet switching

Basic jobs of a Layer 2-forwarding device (Bridge or Switch)

Forward frames transparently
Learn which port to use for each MAC address
Flood frames when the destination unicast address hasn't been learned yet
Filter frames from going out ports that don't include the destination address
Flood broadcasts and multicasts

Switch modes

Store and forward: Fully receive all bits in frame before forwarding
Cut-through: Forward as soon as destination address is known
Fragment free: Wait for 64 bytes before forwarding

(Almost all modern switches are store-and-forward)

Spanning Tree Protocol (STP)

Prunes a mesh (mess?) topology of redundant switches into a logical tree
Avoids loops
Blocks ports so that duplicate frames are avoided
Sends Bridge Protocol Data Units (BPDUs)
Adjusts to changes by unblocking ports when other ports fail

Back to Networking 101