CS 536 ParkDirect Link Communication II:Wired MediaMulti-Access CommunicationTwo classes:• contention-based→ e.g., CSMA/CD, CSMA/CA→ used in Ethernet, WLAN• contention-free→ e.g., TDM, FDM, TDMA, CDMA, token ring→ used in telephony and broadband data networksOne more...CS 536 Park−→ also called MAC (medium access control)• broadband: FDM, TDMA, CDMA• baseband: TDM, multiple accessContention-based MAC for baseband:• Time slots are available for grab→ “on-demand” TDM• Can listen to channel activity...• To grab channel slot is to send→ shoot-first-ask-later (e.g., TV talk shows)• If ≥ 2 users grab at the same time, slot becomes junk→ collisionWhy not just used TDM?CS 536 ParkBenefits of contention-based MAC:• when not too many users, faster response time→ don’t need to go through registration & reservationphase (TDM)→ avoids admission control overhead• decentralized→ no central coordinator→ simple; “self-organization”Drawbacks of contention-based MAC:• when many users, degraded response & throughput→ collision wastes slots, i.e., bandwidth• lack of QoS (quality of service) assurances→ “you get is what you get”; best effort→ problematic for real-time traffic, e.g., telephonyThus when to use what?CS 536 ParkEthernet and CSMA/CD−→ copper, fiberTypes:• 10Base2 (ThinNet): coax, segment length 200 m, 30nodes/segment• 10Base5 (ThickNet): coax, segment length 500 m, 100nodes/segment• 10Base-T: twisted pair, segment length 100 m, 1024nodes/segment• 100Base-T (Fast Ethernet): category 5 UTP, fiber(also 100VG-AnyLAN)• Gigabit & 10 Gbps Ethernet: fiber, category 5 UTPCS 536 ParkConnectivity example:NIC NIC12transceiver (MAU)coax cableHUB10Base2, 10Base510Base-TBNC, DB-15 RJ-45category 3, 5 UTP• single-homed vs. multi-homed• unique Ethernet address per NIC• physical network: bus vs. hub vs. switch→ very old vs. old vs. not-so-oldCS 536 Park−→ hub: multi-tap junction−→ bus and hub: logically equivalentWire segments can be hooked up by repeaters, bridges,hubs or switches.• maximum of 2 (4 for IEEE 802.3) repeaters betweentwo hosts; 1500 m• for Fast Ethernet, 2 repeater hopsHigh-speed Ethernets have shorter network diameter• about 2500 m for 10 Mbps Ethernet• about 200 m for 100 Mbps Ethernet• even shorter for 1 Gbps Ethernet→ additional complications for medium-haulCS 536 ParkDIX Ethernet frame:sourceaddressdest.preambleaddressbodyCRCtypepostamble8662 4146 - 1500IEEE 802.3 Ethernet frame:sourceaddressdest.preambleaddressCRCpostamble8662 4146 - 1500length8LLC headerbody−→ IEEE 802.2 LLC (Logical Link Control)−→ common interface to different link protocolsCS 536 ParkEncoding: Manchester−→ recall: Ethernet is basebandAddressing:• 48 bit unique address• point-to-point• broadcast (all 1’s)Receiver: Ethernet adaptor accepts frames with “rele-vant” address.• accepts only own frame address• accepts all frames: promiscuous mode→ NIC feature→ sniffingCS 536 ParkCSMA/CD MAC:• CS (Carrier Sense): can detect if some other node isusing the link→ rule: if busy, abstein• MA (Multiple Access): multiple nodes are allowedsimultaneous access→ rule: if channel seems silent, send• CD (Collision Detection): can detect if collision dueto simultaneous access has occured→ rule: if collision, retry laterWired vs. wireless media:−→ CD is key difference−→ diffcult to detect collision while transmittingCS 536 ParkSignal propagation and collision:Bi-directional propagation−→ terminator absorbs signal: prevent bounce backterminatorBest-case collision: 2 stations−→ meet in the middle−→ worst-case?collisionCS 536 ParkWorst-case collision scenario:−→ τ: one-way propagation delay• sender needs to wait 2τ sec before detecting collision• for 2500 m length, 51.2 µs round-trip time (2τ)→ fact• enforce 51.2 µs slot time• at 10 Mbps, 512 bits; i.e., minimum frame size→ assures collision detectionCS 536 ParkTransmit at least 512 bits−→ 6+6+2+46+4=64B=512bits−→ note: delay-bandwidth productRetry upon collision: exponential backoff1. Wait for random 0 ≤ X ≤ 51.2 µs before first retry2. On i’th collision, wait for 0 ≤ X ≤ 2i−151.2 µs beforenext attempt3. Give up if i>16−→ a form of stop-and-wait−→ what’s the ACK?−→ guaranteed reliability?−→ pretty drastic measure: