INFO 330 Computer Networking Technology I

INFO 330Computer Networking Technology I Chapter 5 The Link Layer & LANs Glenn Booker INFO 330 Chapter 5

The Link Layer • So, let’s see where we’ve been • The transport layer provides process to process communication • The network layer provides host to host communication • Now the Link Layer provides the ability to send packets across a single … link • So this layer tells how to send a packet/segment/ datagram from one router/host to another INFO 330 Chapter 5

The Link Layer • There are two types of link layer channels • Broadcast channels, used in LANs, wireless LANs, satellite networks, and HFC cable networks • Point-to-point communication link, such as between two routers or between an ISP and a modem • We’ll focus on Ethernet and PPP (Point-to-Point Protocol) • Wi-Fi (IEEE 802.11 protocols) is in chapter 6 INFO 330 Chapter 5

Datagram Link Layer Terms • A node is a router or host – here we don’t care which one we’re dealing with! • Any connection between nodes is a link • The transmitting node puts the datagram in a frame, and transmits it into the link • The receiving node receives the frame, and extracts the datagram INFO 330 Chapter 5

Link Layer Services • A link layer protocol moves a datagram over a (one, individual, eins, uno) link • It defines the format of packets (frames) exchanged between nodes at each end of the link, and the actions the nodes do to send and receive these packets • Over a host-to-host route, links may use several different link-layer protocols – but only one per link • Typically, one link layer frame contains one network layer datagram INFO 330 Chapter 5

Link Layer Services • The link layer’s actions can also include • Reliable delivery • Error detection • Retransmission • Flow control • Random access • Link layer protocols include PPP, Ethernet, Token Ring, Wi-Fi, and some parts of ATM INFO 330 Chapter 5

Link Layer Services • Now elaborate a little on these services • Framing a datagram into a frame means we have data (the datagram) and one or more headers • Technically, can have header and trailer fields, but we’ll generically call both headers • Header format is defined by the protocol INFO 330 Chapter 5

Link Layer Services • Link Access uses the Medium Access Control (MAC) protocol to define how a frame is transmitted over a link • MAC negotiates transmission when many nodes share the same link • Reliable delivery is provided by high error- rate links (e.g. wireless) to keep the transport layer from retransmitting over the entire route INFO 330 Chapter 5

Link Layer Services • Flow control helps keep the sending node from overwhelming the receiving node • Error detection looks for bit errors, usually more elaborately than in the transport and network layers • Error correction – some protocols (ATM) can also fix errors detected INFO 330 Chapter 5

Link Layer Services • Half vs full duplex – with half duplex, a node can only send or receive at one time; with full duplex, it can send and receive at the same time • Yes, lots of the link layer services are similar to transport layer services • But the link layer only provides them between two nodes, whereas the transport layer does between hosts INFO 330 Chapter 5

Adapters • Most link layer protocols are implemented in an adapter (since we’re getting really close to the physical layer!) • Adapter = network interface card (NIC) • The adapter is the last connection between a host and the physical link to the network • Error checking occurs in the adapter, oblivious to the host • Only datagrams which come in cleanly are passed up the protocol stack to the application INFO 330 Chapter 5

Adapters • The main parts of an adapter are the link interface and the bus interface • The link interface connects to the physical network • The bus interface connects to the “parent” node’s I/O bus (e.g. PCI, PCI-X, Serial ATA, IDE, etc.) • Not much to it! INFO 330 Chapter 5

Error Detection and Correction • We can detect, and sometimes correct, bit errors at the link layer INFO 330 Chapter 5

Error Detection and Correction • We add error-detection and correction (EDC) to the data (D) to be sent across the link, in addition to other header info (address, sequence number, etc.) • At the other end of the link, the data could be changed (D’) and the EDC info could be corrupted (EDC’) • Telling from D’ and EDC’ if the original D was corrupted isn’t a perfect science! INFO 330 Chapter 5

Error Detection and Correction • Hence there could be undetected bit errors • The lower the undetected error rate, the larger the overhead to add to each frame • Three main methods for detection • Parity Checks • Checksum • Cyclic Redundancy Check (CRC) INFO 330 Chapter 5

Parity Checks • A simple error detection scheme, parity check adds one bit to the data • That one bit depends on the type of parity scheme • For even parity, the parity bit is chosen so that the total number of 1’s in the frame is … even • For odd parity, the parity bit is chosen so that the total number of 1’s in the frame is … odd INFO 330 Chapter 5

Parity Checks • If the receiver of an even parity link finds an odd number of parity, then there must have been some odd number of bit errors (1, 3, 5, …) • Notice that an even number of errors isn’t detected! • And yes, it helps if both sides of the link are using the same parity rules • Modems used to set even or odd parity INFO 330 Chapter 5

Parity Checks • A better approach is to break the data into a table with i rows and j columns, and define parity for each row and column • In this two-dimensional parity check, there are i+j+1 parity values (bits) • But by cross-referencing the parity errors, exactly which bit(s) were in error can be known, and hence fixed! INFO 330 Chapter 5

Parity Checks • If the receiver can detect and fix errors, it’s forward error correction (FEC) • Commonly used in audio devices to compensate for, e.g., scratched CD’s • In a network, this helps avoid retransmission, and the associated delays INFO 330 Chapter 5

Checksum Methods • Yup, this is just like the approach we saw before…here we call it an Internet checksum • Add the digits of the data • Take the 1s complement of the result – that’s the checksum • Data + checksum = 111111111… if not, there’s an error somewhere • See RFC 1071 INFO 330 Chapter 5

Cyclic Redundancy Check • A Cyclic Redundancy Check (CRC) code is widely used in the link layer • Checksums are easy to calculate in software, so they’re ok for the transport and network layers, but here we can use hardware to calculate CRC codes for us • A.k.a. polynomial codes • The use of CRC codes provides more sophisticated error checking INFO 330 Chapter 5

Cyclic Redundancy Check • CRC uses modulo-2 arithmetic, a.k.a. Boolean arithmetic • It’s equivalent to XOR (exclusive OR): • A B (A xor B) • 0 0 0 • 0 1 1 • 1 0 1 • 1 1 0 INFO 330 Chapter 5

Cyclic Redundancy Check • Multiplication by 2^k moves the bits left byk places • 1011 * 2^3 = 1011000 (11*8 = 64+16+8=88) • So much for the math lesson, so what? • The CRC code defines the ‘r’ CRC bits with a value of R • There’s a generator, G, which has some value starting with 1, and has r+1 bits INFO 330 Chapter 5

Cyclic Redundancy Check • Assume our data has ‘d’ bits, and is a string called D • The value of R is defined so that D * 2^r XOR R is equal to some exact integer multiple of G • (D * 2^r) XOR R = n*G • So R = remainder [D*2^r / G] INFO 330 Chapter 5

Cyclic Redundancy Check • The value of G is typically predefined by IEEE standards • Standard G lengths are 8, 12, 16, and 32 bits • Hence the corresponding lengths of R are r = 7, 11, 15, and 31 bits INFO 330 Chapter 5

Cyclic Redundancy Check • So how does this mess work? • Pick a length of G • Calculate R from the previous slide for each data frame, D • Send the frame • The receiver divides the d+r bits by G • If the remainder is zero, there are no errors • If the remainder is not zero, there were errors INFO 330 Chapter 5

Cyclic Redundancy Check • So what? Why all this work? • Errors tend to occur in bursts – not one error all by itself • Using CRC codes allows you to catch up to ‘r’ errors in a single frame • And errors of more than ‘r’ in a frame might be caught, (1 - 0.5r)*100 percent of the time • And this will catch any number of odd errors • So that’s why we use it a lot at the link layer INFO 330 Chapter 5

Multiple Access Protocols • Network links can be point-to-point (one sender and one receiver) or broadcast links • For a broadcast link • A node sends a frame to all of the other nodes • Used by wired, wireless, and satellite networks, plus the occasional cocktail party INFO 330 Chapter 5

Multiple Access Protocols • This motivates the multiple access problem – how do we control transmission onto a shared broadcast channel • Frames can arrive at a node (yes, technically the adapter on that node) at the same time, producing a collision (both frames on top of each other, a mess) INFO 330 Chapter 5

Multiple Access Protocols • Dozens of multiple access protocols have been defined, but they fall into three types • Channel partitioning protocols • Random access protocols • Taking-turns protocols INFO 330 Chapter 5

Multiple Access Protocols • We want multiple access protocols to provide • One node can send data at a rate of R bps • If M nodes want to transmit, each can transmit an average of R/M bps • The protocol should be decentralized, so that a single point failure doesn’t take down the system • It’s cheap to implement and simple INFO 330 Chapter 5

Channel Partitioning Protocols • Could use FDM or TDM (frequency or time division multiplexing) to share a channel’s bandwidth across some number of slots • Avoids collisions, which is good • But each slot only gets a fraction of the bandwidth, even if no one else is transmitting INFO 330 Chapter 5

Channel Partitioning Protocols • Instead use Code Division Multiple Access (CDMA), which assigns codes to each node which sends data • CDMA is also good for avoiding signal jamming, hence is used by the military • Is used widely for wireless protocols INFO 330 Chapter 5

Random Access Protocols • Here each node transmits as though it has the full channel bandwidth available • When a collision occurs, it waits a random amount of delay time before retransmitting • Keep retransmitting until the frame gets through • There are many protocols of this type, e.g. • Slotted ALOHA • ALOHA • CSMA (of which Ethernet is an example) INFO 330 Chapter 5

Slotted ALOHA • Suppose • All frames have size L bits • Time is divided into slots of duration L/R seconds (= time to transmit one frame) • Nodes only transmit at the start of a slot • Nodes all know when the slots start • If a collision occurs, the nodes know that before the end of the slot occurs • There is a probability, p, between 0 and 1 INFO 330 Chapter 5

Slotted ALOHA • Slotted ALOHA works like this: • When a node needs to transmit a frame, it waits until the next slot starts and transmits it • If there’s no collision, the node can transmit the next frame if needed • If there was a collision, the next time a random number is greater than p, transmit in that slot • So if the random value is less than 1-p, wait for retransmission INFO 330 Chapter 5

Slotted ALOHA • This takes advantage of the link when only one node is active – it gets the full rate • If there are multiple active nodes, some slots will be wasted because nobody is transmitting • The efficiency is the percent of slots where a successful transmission occurs • The efficiency for N active nodes is N*p*(1-p)^(N-1) • Bad part is: max efficiency is only 37% INFO 330 Chapter 5

ALOHA • What is we ignore the part about transmitting only at the start of a slot? • Transmit when you want to • If there’s a collision, retransmit immediately if value is >p, otherwise wait one slot duration and reevaluate retransmitting then • The icky part is that the efficiency of this is only half of Slotted ALOHA – the price for decentralized control INFO 330 Chapter 5

CSMA • CSMA (Carrier Sense Multiple Access) pays attention to whether anyone else is transmitting, before a node does so • Like listening for a break in conversation before jumping in, carrier sensing listens for a break in link traffic (basic CSMA protocol) • Collision detection is done by sensing if another node starts transmitting while you are (CSMA/CD) INFO 330 Chapter 5

CSMA • There are many variations on CSMA & CSMA/CD • Collisions can occur because of the time needed for transmitting frames – the channel propagation delay INFO 330 Chapter 5

Taking-turns Protocols • The ALOHA and CSMA protocols both take advantage of full bandwidth when available, but neither is good at assuring fair share of throughput when multiple nodes are active • To fix the latter, taking-turns protocols have been made – hundreds of them! • We’ll focus on two major kinds • Polling protocols • Token-passing protocols INFO 330 Chapter 5

Polling Protocols • Polling protocols make one node a master node • The master node polls each node in turn, and tells each node it can send some number of frames • This eliminates collisions and empty slots • But it adds a polling delay to notify each node it’s turn is up, and delays to check nodes which are inactive • And it’s really bad if the master node dies! INFO 330 Chapter 5

Token-passing Protocols • Token-passing protocols have no master node, but instead pass a small token frame among the nodes in a fixed order • Each node holds the token only if they have frames to transmit, up to some max number • Then keep passing the token • Failure of ANY node crashes the network! • Or if the token isn’t released, there’s trouble • FDDI and yes, Token Ring, are examples INFO 330 Chapter 5

Local Area Networks (LANs) • Local Area Networks use multiple access protocols extensively • Ethernet is the most common random access protocol • Token Ring had a slight speed advantage, so it was popular in the late 1980’s • A node sends a frame around the network, and it’s read by the recipient node • The sender removes it from the network INFO 330 Chapter 5

Local Area Networks (LANs) • FDDI (Fiber Distributed Data Interface) was designed for larger LANs, specifically Metropolitan Area Networks (MANs) • Under FDDI, the destination node removes the frame from the network • Hence it isn’t a pure broadcast channel, since nodes downstream will never get the frame INFO 330 Chapter 5

Link Layer Addressing • C’mon, we haven’t had an address format in at least two or three days • Here we’ll go over MAC and ARP • As stated earlier, the adapter is the real location of a link layer address • The MAC address (a.k.a. LAN address or physical address) is the link layer address of an adapter INFO 330 Chapter 5

MAC Address • A MAC address usually has 6 bytes, so there are 2^48 MAC addresses • 2^48 = 281,474,976,710,656 in case you wondered • Each byte is expressed as two hexadecimal numbers (0-9; A-F for 10-15) • 01:90:4B:5F:31:13 • Letters are case-insensitive INFO 330 Chapter 5

MAC Address • The IEEE makes sure each MAC address is unique • The first 24 bits are assigned to the hardware vendor; the rest are the item identifier • MAC addresses have no other structure, and shouldn’t change for a given adapter • MAC addresses were supposed to be permanent, but they can now be changed via software INFO 330 Chapter 5

MAC Address • Like the IP address, the MAC address is used to verify that the destination host (adapter) has been reached • The MAC broadcast address is all F’s, analogous to the 255.255.255.255 IP address • FF:FF:FF:FF:FF:FF INFO 330 Chapter 5

Address Resolution Protocol • The Address Resolution Protocol (ARP) (no, not AARP) translates between IP addresses and MAC addresses • RFC 826, and a nice tutorial in RFC 1180 • ARP only works within the local subnet • Unlike DNS, which resolves addresses anywhere • Each node (host / router) maintains an ARP table to map IP addresses & MAC addresses INFO 330 Chapter 5

INFO 330 Computer Networking Technology I