-lets upper layers access the media using techniques like framing -controls how data is placed on the media and received from the media using techniques like media access control & error detection
Media Access Control
(desc'd by the data link layer protocols) define processes by which network devices can access the network media & transmit frames in diverse network environments
Error Detection in the Data Link Layer
Happens in the frame trailer. There is the Frame Check Sequence (FCS) field, which contains the Cyclical Redundancy Check (CRC) which is a math summarization of the frame. Can be checked at the receiving end.
Data Link Layer PDU
Layer 2 notation for network devices connected to a common medium
part of a computer network where the devices communicate using the same physical medium. Network segments are extended by hubs or repeaters. AKA physical network
Control Info may tell:
-which nodes are in comm -when comm b/t individual nodes begins and when it ends -which errors occurred while the nodes comm'd -which nodes will comm next
Data Link layer frame includes:
Header - control info, IE addressing, @beginning Data - The Packet from the Network Layer Trailer - Contains control info @ end
Data Link Layer Typical Field Types:
-start, stop indicator fields - beginning & end -naming/address fields -type field - type of PDU in the frame -control - flow control services -a data field - frame payload
Data Link Layer Sublayers
-Logical Link Control (LLC)(upper) -Media Access Control (lower)
Logical Link Control Sublayer
Upper sub-layer of the Data Link Layer. Puts info in the frame that ID's which Network Layer Protocol is being used, allows multiple Layer 3 Protocols (IE IP & IPX) to use the same network interface & media
Media Access Control Sublayer
provides Data Link Layer addressing & delimiting of data according to the physical signaling requirements of the medium & the type of Data Link Layer Protocol in use. (converts to bits to go over physical medium)
Engineering Organizations that define open standards and protocols that apply to the data link layer:
-ISO = International Organization for Standardization -IEEE = Institute of Electrical & Electronics Engineers -ANSI = American National Standards Institute -ITU = International Telecommunications Union
Media Access Control
Regulating the placement of data frames onto the media
The method of media access control used depends on:
-Media sharing - if & how the nodes share the media -Topology - How the connection between the nodes appears to the Data Link Layer
If & how the nodes share the media
How the connection between the nodes appears to the data link layer. (Kind of like a map of the network)
Two basic media access control methods for shared media:
-Controlled - Each nodes has it's own time to use the medium -Contention-based - All nodes compete for the use of the medium
Features of Controlled Access for Shared Media
network devices take turns in sequence, well ordered, provides predictable throughput, but inefficient b/c a devices has to wait it's turn
Used in multiple access topologies where there are rules that govern who can transmit. An example is Token Ring topologies where only station w/ token can transmit, other stations must wait
The amount of digital data/time unit that is delivered from one node to another. Takes into account latency variables.
Features of Contention-Based Access for Shared Media
allow any device to try to access the medium whenever it has data to send, uses carrier sense multiple access to try and avoid data collision, less overhead than controlled access, contention-based doesn't scale well b/c more data collision
Carrier Sense Multiple Access (CSMA)
A protocol where a node listens before it tries sending. It carrier wave is sense, node waits for transmission to finish.
Electromagnetic wave or alternating current of a single frequency, suitable for modulation by another, data bearing signal.
When 2 stations transmit at the same time on a multiple access topology & the frames of both stations collide w/ each other altering the original frame structure and creating fragments. (They both get messed up)
Carrier Sense Multiple Access/Collision Detection (CSMA/CD)
The device monitors the media for data signal, if none transmits data. If signals show two devices transmitted (collision), all devices stop and try again.
Carrier Sense Multiple Access/Collision Avoidance (CMSA/CA)
A flavor of CSMA w/ Collision Avoidance. Sends a notification of it's intent to use the media. Then sends data.
When media interconnects just two nodes. They do not have to share the media or determine if a frame is destined for that node.
Both devices can transmit & receive on the media at the same time. Data Link layer assumes the media is available to transmission for both nodes all the time. No media arbitration necessary in data link layer b/c no collisions
Both devices can transmit & receive on the media but not at the same time. Uses estb'd arbitration rules to resolve conflicts
Refers to the configuration of cables, computers, & other peripherals. The representation of how the media is used to interconnect the devices is the physical topology. AKA cabled topology.
The way a network transfers froms from one node to another. Includes virtual connections between the nodes of a network independent of physical layout. Defined by Data Link layer protocols.
Logical + Physical Topologies typically used in Networks are:
-Point-to-Point -Multi-Access -Ring
Two nodes directly connected. MAC protocols can be very simple. All frames on media can only travel to/from the two nodes. Could have more complicated MAC processes, but would add overhead.
Logical connection between 2 network devices. 2 nodes on either end exchange frames, even if they go thru more intermediary devices.
Logical Point-to-Point Networks
Two devices being in a logical point-to-point network b/c of a virtual circuit, which may go thru many intermediary devices.
Deterministic network method where a "token" is passed between nodes that authorizes the node to communicate
A number of nodes communicate by using the same shared media. Data from one node on the media at a time. All nodes read data but only receiver processes. Needs MAC methods (IE CSMA/CD, CSMA/CA, or Token Passing.)
Each node receives a frame in turn, passes it if it's not addressed to it.
Each Type of Data Link Layer Frame has 3 Basic Parts:
Indicate the source & destination nodes on the media
Frame Header Fields: Priority/Quality of Service Field
Indicates a particular type of communication service for processing
Frame Header Fields: Type Field
Indicates the upper layer service contained in the frame
Frame Header Fields: Logical Connection Control Field
Used to establish a logical connection between nodes
Frame Header Fields: Physical Link Control Field
Used to establish the media link
Frame Header Fields: Flow Control Field
Used to start & stop traffic over the media
Frame Header Fields: Congestion Control Field
Indicates congestion over the media
Trailer is used to determine if frame arrived w/o error. Accomplished by putting logical/mathematical summary of the bits in the frame in the trailer.
Frame Check Sequence (FCS)
Used to see if errors occurred in the transmission & reception of the frame. Could be lost b/c interference, distortion, or loss. FCS finds most errors.
Cyclic Redundancy Check (CRC)
CRC = a value calculated based on the contents of the frame and placed in the Frame Check Sequence (FCS) field. Then recalculated @ the destination and checked against 1st value.
Layer 2 Protocols we'll talk about
-Ethernet -Point-to-Point Protocol (PPP) -High-Level Data Link Control (HDLC) -Frame Relay -Asynchronous Transfer Mode (ATM)
Important & Most Widely Used Protocol. (802.2 & 802.3). Uses CSMA/CD. Ethernet II is the Ethernet frame format used in TCP/IP network
Point-to-Point Protocol (PPP)
used mostly for WANs, layered architecture. Can encapsulate multiple protocols, each w/ it's own session. Can use different types of media.
Wireless Protocols for LANs
802.11 (Called Wifi). Like other 802 protocols but has additional control b/c wireless is susceptible to interference. Collision Avoidance w/ random backoff. Incl: Authentication, Association, & Encryption.