OSI Layer 2 Protocols 

• Ethernet / IEEE 802.3

• Token Ring / IEEE 802.5

• FDDI Fiber Distributed Data Interface

• PPP Point-to-Point Protocol

• X.25

• Frame Relay

• ISDN - Integrated Services Digital Network

Ethernet

• CSMA /CD - Carrier Sense Multiple Access / Collision Detection local area network
• A broadcast network - all frames that cross a segment can be heard by all machines on the segment
• All machines have equal access, they wait for a quiet time to transmit, collisions occur when two machines transmit at the same time.  When a collision occurs both the machines apply a back off time that prevents them from colliding again
• Services both the Physical and Data Link layers

IEEE 802.3

• Also a broadcast network
• More concerned with the Physical Layer and how it talks to the Data Link Layer

Token Ring / IEEE 802.5

• Specifies baseband signaling, token passing, and data rates
• Equal time is allotted to each station. achieved by passing a token around the ring

Tokens

• A small frame, a station cannot transmit data unless in possession of the token
• The Token is passed around the ring, each station waits for the token, if the station doesn't have anything to transmit, it passes the token along.  If it does have something to transmit, it change the frame to a start of frame identifier and appends data to the frame.  When the token is changed, no other station on the ring can use the token to transmit data.  This eliminates collisions.  The altered frame is passed along the ring until it reaches the destination address.  The destination station copies the data and the original frame continues around the token until it reaches the source station.  At the source, the token is removed and a new token is sent out onto the network.  This way each station has an equal opportunity to transmit data.
• Fault Recovery - an Active Monitor is designated by the workstations on the ring
• Active Monitor - responsible for token monitoring, token generation when token disappears, purging recycled frames, and other ring maintenance

FDDI

• Physical media is Fiber Optic, dual ringed, operates at 100Mbps Bandwidth
• Uses tokens to control media access, uses a ring topology
• One ring is Primary ring flows clockwise
• Other ring is secondary ring flows counterclockwise

Specifications

1. Physical Layer Medium Independent (PMD) - defines physical transmission characteristics - light levels, allowable bit errors, decibel loss, actual physical equipment
2. Physical Layer Protocol (PHY) - encodes and decodes data, before and after transmission over the physical medium
3. MAC - bridges FDDI with layer 2 of OSI model, responsible for how FDDI runs as layer 2 technology, the physical device, and the way data is presented to the physical layer
4. Station Management (SM) - responsible for station and ring configuration, fault recovery and other management duties

PPP

• Used to transfer data over serial point-to-point links, accomplishes this by using a layer 2 serial encapsulation HDLC ( High Level Data Link Compression)
• Uses LCP (Link Control Protocol) to manage the serial connection
• Network Control Protocols are used to allow PPP to use other protocols from layer 3, thus enables PPP to assign IP addresses dynamically