Data Link Issues Relates to Lab 2. This module covers data link layer issues, such as local area networks (LANs) and point-to-point links and broadcast environments.

Data Link Layer The main tasks of the data link layer are: Transfer data from the network layer of one machine to the network layer of another machine using hop by hop transmission over single physical links - single segments. Convert the raw bit stream of the physical layer into groups of bits (“frames”) and vice versa Datagram Frame Electrical and Optical signals

Hop-by-Hop Transport Argon Neon Data Link Hop Single Segment The actual path followed by data Data Link Hop Single Segment

Data Link Layer Transfer of an IP datagram via a link protocol over a “physical” link (optical, air, coax, twisted pair….) There are many different link protocols. Each designed for a specific type of physical layer and corresponding access method. E.g.: Ethernet (802.3), Frame Relay, WiFi (802.11) Link protocols provide different services to IP depending on the type of link and access method E.g.: optical cable very reliable, does not require error control. Whereas air is unreliable - uses error control. Frame Relay over long haul network may require flow control as end device has sole access to the medium. Ethernet does not, the broadcast media access controls user flows.

Data Link Layer Services flow control pacing between adjacent sending and receiving nodes error detection errors caused by signal attenuation, noise receiver detects presence of errors: signals sender for retransmission or drops frame error correction receiver identifies and corrects bit error(s) without resorting to retransmission transmission - half-duplex and full-duplex half duplex - nodes at both ends of link can transmit, but not at same time full duplex – nodes can transmit in both directions at the same time channel access

Data Link Layer Implementation In each device: network and host Data link layer is implemented in an “adaptor” - network interface card (NIC) or on a chip. E.g.: Ethernet card Ethernet chipset Attaches into a device’s system bus combination of hardware, software, firmware NIC or chip implements: Data Link layer, AND Physical layer controller physical transmission

A LAN Data Link Layer Example: Ethernet II LLC very simple: Frame structure (next slide) connectionless: no handshaking between sending and receiving NICs unreliable: uses error detection only no recovery detected error: frame dropped receiving NIC doesnt send acks or nacks to sending NIC data in dropped frames recovered only if initial sender uses a reliable higher layer (e.g., TCP), otherwise dropped data lost MAC protocol: CSMA/CD with binary backoff when collisions occur

Ethernet II frame structure sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame preamble: 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 used to synchronize receiver and sender clock rates dest. address source data (payload) CRC preamble type

Ethernet II frame structure (more) addresses: 6 byte source/destination MAC addresses if adapter receives frame with matching destination address, or with broadcast address (e.g. ARP packet), it passes data in frame to upper layer (e.g., IP or ARP) otherwise, adapter discards frame type: indicates higher layer protocol (mostly IP but others possible, e.g., Novell IPX, AppleTalk) or ARP CRC: cyclic redundancy check at receiver error detected: frame is dropped dest. address source data (payload) CRC preamble type

Ethernet II standards: link & physical layers many different Ethernet standards common MAC protocol and LLC frame format different speeds: 2 Mbps, 10 Mbps, 100 Mbps, 1Gbps, 10G bps different physical layer media: fiber, cable application transport network link physical MAC protocol and LLC frame format copper (twisted pair) physical layer 100BASE-TX fiber physical layer 100BASE-T2 100BASE-FX 100BASE-T4 100BASE-SX 100BASE-BX

Ethernet II Frame vs IEEE 802.3 Note: all fields in bytes IEEE 802.3 has embedded 8 byte Link Control header 802.2 Data field is padded if payload is less than < 38bytes Ethernet II: FCS is CRC Data field is padded if payload is less than < 46bytes

Ethernet Star Configurations: Hubs vs. Switches An Ethernet switch is a packet switch for Ethernet frames Buffering of frames prevents collisions Each port is isolated and builds its own collision domain An Ethernet Hub does not perform buffering Collisions occur if two frames arrive at the same time (WiFi access points are virtually hubs) Hub Switch