Chapter 7 Data Link Layer CIS 81 Networking Fundamentals Rick Graziani Cabrillo College graziani@cabrillo.edu Last Updated: 4/27/2008

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Data Link Layer Ethernet, PPP, ISDN, Frame Relay The Data Link layer provides a means for exchanging data over a common local media.

Generic Data Link Header Application Header + data

Node to node, not host to host The Data Link layer provides a means for exchanging data over a common local media.

Focus on Data Link Layer IP IP

Reminder of encapsulation/decapsulation Data Link Header IP Header TCP Header HTTP Header Data Link Trailer Data Data Link Header Data Link Header Data Link Trailer Data Link Trailer IP Packet IP Packet Data Link Header Data Link Header Data Link Trailer Data Link Trailer IP Packet IP Packet Data Link Header Data Link Header Data Link Trailer Data Link Trailer IP Packet IP Packet Data Link Header IP Header TCP Header HTTP Header Data Link Trailer Data

Data Link Frame 1. 2. The Data Link layer provides a means for exchanging data over a common local media. The Data Link layer performs two basic services: 1. Allows the upper layers to access the media using techniques such as framing 2. Controls how data is placed onto the media and is received from the media.

Data Link Frame The Data Link layer prepares a packet for transport across the local media by encapsulating it with a header and a trailer to create a frame. The Data Link layer frame includes: Data – Layer 3 (IP Packet or other Layer 3 information) Header - Control information, such addressing Trailer - Control information, such as error detection

Data Link Frame Typical field types may include: Start and stop indicator fields Addressing fields Type field - The type of PDU contained in the frame Quality - control fields Data field -The frame payload (Network layer packet) Not all protocols include all of these fields.

Data Link Layer Data Link layer Connects the Network Layer with the Physical Layer Network Layer and above is software (IP, TCP, HTTP, etc.) Physical layer is implemented in hardware (converting bits to a transmission signal) Data Link layer is implemented in both: Software Hardware Data Link Layer prepares Network Layer packets for transmission across some form of media, be it copper, fiber, or the atmosphere.

Data Link Sublayers Data Link layer has two sublayers (sometimes): Logical Link Control (LLC) – Software processes that provide services to the Network layer protocols. Frame information that identifies the Network layer protocol. Multiple Layer 3 protocols, (IP and IPX) can use the same network interface and media. Media Access Control (MAC) - Media access processes performed by the hardware. Provides Data Link layer addressing and framing of the data according to the protocol in use.

Data Link Standards TCP/IP standards are defined by Internet Engineering Task Force (IETF). Data Link layer protocols are defined by: ISO - International Organization for Standardization IEEE - Institute of Electrical and Electronics Engineers ITU - International Telecommunication Union ANSI - American National Standards Institute

Media Access Control Media Access Control - Regulates the placement of data frames onto the media. The method of media access control used depends on: Media sharing Do more than two nodes share the media? If so, how? (Switches, hubs, etc.)

Serial vs multi-access Point-to-Point Point-to-Point networks Only two nodes /30 subnets are common Protocols: PPP, HDLC, Frame Relay Multi-access networks (LANs) Multiple nodes Subnets mask range depends upon the number of hosts (nodes) Protocols: Ethernet, 802.11 (wireless), Frame Relay Multipoint

Duplex Transmissions Simplex Transmission: One way and one way only. One way street Half-duplex Transmission: Either way, but only one way at a time. Two way street, but only one way at a time (land slide). Ethernet hubs use half-duplex Full-duplex Transmission: Both ways at the same time. Two way street Ethernet switches use full-duplex Most serial links are full-duplex

Physical Topology Hub The physical topology is an arrangement of the nodes and the physical connections between them. Switch Serial Connections

Logical Topology A logical topology - The way a network transfers frames from one node to the next. Defined by Data Link layer protocols. Media Access Control used. Type of network framing

Point-to-Point topology 11111111 A point-to-point topology connects two nodes directly together. The media access control protocol can be very simple. Frames from one devices are for the device at the other end. Point-to-point topologies, with just two interconnected nodes, do not require special addressing.

Logical Point-to-Point Networks Point-to-point networks may include intermediate devices. No affect on logical topology. The logical connection (in some cases) may be a virtual circuit. A virtual circuit is a logical connection created within a network between two network devices. The two nodes exchange the frames with each other. Data Link Destination address is the device at the other end of the virtual circuit.

Multi-access Topology A logical multi-access topology - Enables a number of nodes to communicate by using the same shared media. “Data from only one node can be placed on the medium at any one time.” (This is only true when using CSMA/CD (hubs), NOT true with switches or wireless) Every node “may” see all the frames that are on the medium. Data Link Destination Address denote which device the frame is for.

Multi-access Addressing 2222 4444 6666 3333 5555 6666 2222 Multi-access networks require an address to specifically identify the destination.

Media Access Control The media access control methods used by logical multi-access topologies are typically: CSMA/CD - Hubs CSMA/CA - Wireless Token passing – Token Ring Later

Ring Topology Token Passing media access control Each node in turn receives a frame. If data link destination address is not for that device, passes frame to next node.

Data Link Frame No one frame structure meets the needs of all data transportation across all types of media. Depending on the: Environment Amount of control information needed Topology

Data Link Frame Fields Data Link frame header fields may include: Start Frame field - Indicates the beginning of the frame Source and Destination address fields - Indicates the source and destination nodes on the media Priority/Quality of Service field - Indicates a particular type of communication service for processing Type field - Indicates the upper layer service contained in the frame Logical connection control field - Used to establish a logical connection between nodes Physical link control field - Used to establish the media link Flow control field - Used to start and stop traffic over the media Congestion control field - Indicates congestion in the media

Framing- The Trailer The signals on the media could be subject to: Interference Distortion Loss This would change the bit values that those signals represent. The trailer is used to determine if the frame arrived without error. Error detection. The Frame Check Sequence (FCS) field is used to determine if errors occurred in the transmission and reception of the frame.

Cyclic Redundancy Check Cyclic redundancy check (CRC) is commonly used. Sending node includes a logical summary of the bits in the frame. Receiving node calculates its own logical summary, or CRC. Compares the two CRC values. Equal – Accepts the frame Different – Discards the frame

Bandwidth LAN typically uses a high bandwidth technology Supporting large numbers of hosts WAN High bandwidth technology is usually not cost-effective across large geographic areas (cities or multiple cities, for example). The cost typically results in lower bandwidth capacity. Note: This is relative and the need for high bandwidth on WANs is increasing due to video, voice, and other applications.

Ethernet Protocol for LANs Ethernet is a family of networking technologies that are defined in the IEEE 802.2 and 802.3 standards. Uses 48 bit addressing (Ethernet MAC addresses) for Source and Destination More next week!

Point-to-Point Protocol for WANs Point-to-Point Protocol (PPP) is a protocol used to deliver frames between two nodes. PPP can be used on various physical media, including: Twisted pair Fiber optic lines Satellite transmission

Wireless Protocol for LANs 802.11 is an extension of the IEEE 802 standards. It uses the same 48-bit addressing scheme as other 802 LANs. Contention-based system using a Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA)

Chapter 7 Data Link Layer CIS 81 Networking Fundamentals Rick Graziani Cabrillo College graziani@cabrillo.edu Last Updated: 4/27/2008