Data Link Issues Relates to Lab 2 This

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, CSMA, and Ethernet, 1

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

Data Link Layer • Datagram transferred by different link protocols over different links: § e. g. , Ethernet (802. 3) on first link, frame relay on intermediate links, Wi. Fi (802. 11) on last link • Each link protocol provides different services § e. g. , may or may not provide error control over link 4

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 5

Data Link Layer Implementation • in each and every host • data link layer implemented in “adaptor” (aka network interface card NIC) or on a chip – Ethernet card, 802. 11 card; Ethernet chipset – Implements: • Data Link layer, AND • Physical layer • attaches into host’s system buses • combination of hardware, software, firmware controller physical transmission 6

Types of “Single Segment” Networks • There are two types of communication networks: – Broadcast Networks: All stations share a single communication channel – Point-to-Point Networks: Pairs of hosts (or routers) are directly connected • Examples of single segment networks: – broadcast: Ethernet, Wi. Fi, – point to point link: Frame Relay • Typically, local area networks (LANs) are broadcast and wide 7 area networks (WANs) are point-to-point

Local Area Network (LAN) • Local area networks (LANs) typically connect devices within a building or a campus • Almost all LANs are broadcast networks • Typical topologies of LANs are bus or ring or star • LANs use an algorithm to gain access to shared channel to transmit Star 8

LANs: Data Link Layer - MAC and LLC • In any broadcast network, the stations must ensure that only one station transmits at a time on the shared communication channel • The protocol that determines who can transmit on a broadcast channel is called the Medium Access Control (MAC) protocol • The MAC protocol is implemented in the MAC sublayer which is the lower sublayer of the data link layer • The MAC is physical layer/topology dependent • The higher portion of the data link layer is often called the Logical Link Control (LLC) 9

Logic Link Controls • Frame Structure – Fields: types, lengths • Flow Control – Pacing control: • none • stop and go, window control • Error Control – None – Error detection/correction – Error recovery 10

MAC Protocols • multiple access protocol – an algorithm that determines how nodes share channel, i. e. , determine when node can transmit • Centralized – a master that controls how nodes share the channel • Distributed – no one in charge, nodes cooperate for access • three broad classes: – channel partitioning • divide channel into smaller “pieces” (time slots, frequency, code) • allocate a piece to each node a priori for exclusive use – random access • channel use not allocated - all nodes can use any piece (or full channel) • can result in collisions if nodes transmit at the same time • often implements a mechanism to “recover” from collisions – round robin “taking turns” • nodes take turns using pieces or full channel, but nodes with more to send 11 can take longer/larger turns – causing variable delays

Random Access • when node has packet to send – transmit at full channel data rate R (generally no pieces) – NO a priori allocation of channel among nodes • two or more transmitting nodes on channel ➜ “collision” • random access MAC protocol specifies: – how to detect collisions – how to recover from collisions (e. g. , via delayed retransmissions) • examples of random access MAC protocols: – slotted ALOHA – CSMA, CSMA/CD, CSMA/CA 12

Carrier Sense multiple access (CSMA) • CSMA: listen before transmit, i. e. , don’t interrupt others, wait until there is a pause: – if channel sensed idle: transmit entire frame – if channel sensed busy, defer transmission • When channel sensed idle: collisions can still occur: – propagation delay means two nodes may not hear each other’s transmission and the collision occurs in mid transmission • When channel sensed busy: collisions can still occur: – deferred nodes all detect pause at the same time after a transmission is completed and will attempt to transmit • Collision: entire packet transmission time wasted – distance & propagation delay play role in in determining collision probability 13

CSMA with Collision Detection (CSMA/CD) • CSMA/CD: carrier sensing, deferral as in CSMA § collisions detected within short time (propagation delay) § colliding transmissions aborted and a jam signal transmitted, reducing channel wastage • collision detection: § easy in wired LANs: measure signal strengths, compare transmitted and received signals – difficult in wireless LANs: received signal strength overwhelmed by local transmission strength – detection not functional --> don’t use /CD. Use Collision Avoidance (CA). Small reservation packets used to request channel usage. Possible because of centralized architecture (Access Point (AP). 14

CSMA with Collision Detection (CSMA/CD) 1. NIC receives datagram from network layer, creates frame 2. If NIC senses channel idle, starts frame transmission. If NIC senses channel busy, waits for pause, then checks if channel idle. If YES transmits, if NO, repeats - waits for next pause. 3. If NIC transmits entire frame without detecting another transmission, NIC is done with frame! 4. If NIC detects another transmission while transmitting, aborts and sends jam signal 5. After aborting, NIC enters binary (exponential) back-off: – after mth collision, NIC chooses a value K at random from {0, 1, 2, …, 2 m-1}. 6. NIC waits K·512 bit times, returns to Step 2 – longer back-off interval with more collisions 15

CSMA with Collision Detection (CSMA/CD) • spatial layout of nodes B and D 16

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 5 -17

Ethernet II frame structure • sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame type dest. source preamble address data (payload) CRC • preamble: § 7 bytes with pattern 1010 followed by one byte with pattern 10101011 § used to synchronize receiver and sender clock rates 5 -18

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, Apple. Talk) or ARP • CRC: cyclic redundancy check at receiver § error detected: frame is dropped type dest. source preamble address data (payload) CRC 5 -19

Ethernet II standards: link & physical layers • many different Ethernet standards – common MAC protocol and LLC frame format – different speeds: 2 Mbps, 100 Mbps, 1 Gbps, 10 G bps – different physical layer media: fiber, cable application transport network link physical MAC protocol and LLC frame format 100 BASE-TX 100 BASE-T 2 100 BASE-FX 100 BASE-T 4 100 BASE-SX 100 BASE-BX copper (twisted pair) physical layer fiber physical layer 5 -20

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 < 38 bytes • Ethernet II: • FCS is CRC • Data field is padded if payload is less than < 46 bytes 21

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 (Wi. Fi access points are virtually hubs) Hub Switch 22