IEEE 802 Standards
Ethernet
- Ethernet: most widely used LAN
- developed by IEEE 802.3
- carrier sense multiple access with collision detection (CSMA/CD)
: random / contention access to media
(Pure) ALOHA
보내고 싶으면 언제든지 보냄 => collision 발생 많음 => 성능 낮음
- developed for packet radio networks
- applicable to any shared transmission medium
- opertion
- when station has frame, it sends at any time
- station listens for max round trip time + small increment
- if ACK, fine. if not, retransmit the frame
- if no ACK after repeated transmissions, give up at mac layer
- receiver performs error detection by Frame Check Sequence
- if frame is valid and destination address in the frame header matches receiver' address, send ACK
- frame may be invalid
- noise on the channel
- another station transmits a frame at about the same time (collsion) 거의 동시에 전송 - maximum utilization (throughput): 18%
Slotted ALOHA
pure aloha의 개선 버전
- time on the channel is organized into uniform slots whose size equals frame transmission time
- Need central clock or other sync mechanism
- Transmission begins at slot boundary
- only complete collision: collision 발생 빈도 낮음
- maximum utilization: 37% (offered load 1일 때)
CSMA
- CSMA (Carrier Sense Multiple Access) or LBT (Listen Before Talk)
- propagation delay: much less than transmission time
- all stations know that a transmission has started almost immediately
- operation
- first listen for clear medium (carrier sense) 아무도 전송하지 않는 거 확인
- if medium is busy, wait for some period of time & try again
- if medium is idle(clear medium), transmit
- if two or more stations attempt to transmit at about the same time, collision occurs
- wait reasonable amount of time for ACK (round trip plus ACK contention)
- if no ACK, then retransmit
- max utilization: depends on frame length and propagation time (medium length)
=> longer frame or shorter propagation gives better utilization
(순수 data를 보내는 시간이 길어짐, propagation: overhead)
Nonpersistent CSMA
- if medium is idle, transmit; otheriwe, go to step 2
- if medium is busy, wait an amount of time drawn from a probability distribution (retransmission delay) and repeat step 1
- use of random delays reduces probability of collisions
- capacity is wasted because medium will remain idle following end of transmission. medium이 전송 끝난 이후에 바로 전송 못하고 기다려야 함 => 시간 낭비
- nonpersistent stations are deferential(양보한다) => 단점
1-persistent CSMA
- If medium is idle, transmit; otherwise, go to step 2
- If medium is busy, continue to listen until idle; then transmit immediately
- 1-persistent CSMA avoids idle channel time
- 1-persistent stations are selfish (독점적)
- if two or more stations are waiting, collsion is guaranteed
P-persistent CSMA
- If medium is idle, transmit with probability p, and delay one time unit with probability (1-p)
: time unit is typically max propagation delaty - If medium is busy, listehn until idle and repeat step 1
- if transmission is delayed one time unit, repeat step 1
- compromise that attempts to reduce collisions, like nonpersistent, and reduce idle time, like 1-persistent
- what is an effective value of p?
Value of p?
- how many stations will attemp to transmit at t? => np
- if np > 1, multiple stations attemp to transmit, then collision occurs
=> select p such that np < 1 for expected peaks of n
- if heavy load is expected, p should be small
- as p is made smaller, stations wait longer / collision ⬇️
=> p: low , wait ⬆️
CSMA/CD or LWT
- CSMA / Collision Detection or Listen While Talk
- IEEE 802.3 / Ethernet
- With CSMA, collision occupies medium for duration of transmission
- stations listen while transmitting- if medium is idle, transmit; otherwie, go to step 2
- if medium is busy, listen until idle, then transmit immediately
- if collision is detected during transmission, transmit a jamming signal and then cease transmission 충돌 이후 쓸데없는 트래픽 안 보냄
- after transmitting the jamming signal, wait a random amount of time (backoff), then repeat from step 1
- IEEE 802.3 uses 1-persistent algorithm
- Binary Exponential Backoff
- attempt to transmit repeatedly for repeated collisions
- for the first 10 attempts, mean value of random delay is doubled
- mean value then remains the same for 6 additional attempts
- after 16 unsuccessful attempts, station gives up at mac and reports error to LLC
- as congestion increases, stations back off by larger amounts to reduce the probability of collision
- 1-persistent algorithm with binary exponential backoff is efficient over wide range of loads
: low loads => 1-persistence guarantees station can seize channel as soon as it goes idle
: high loads => at least as stable as other techniques
- backoff algorithm gives last-in, first-out effect
: stations with no or few collisions have chance to transmit before stations waited longer- Collision Detection
(for baseband bus)
- collision produces much higher voltage than normal signal
ex) 10Base5: 500m까지 higher voltage
- collision is detected if cable signal is greater than single station signal
- signal attenuates over distance
- limit distance to 500m (10Base5) or 200m (10Base2)
(for twisted-pair star-topology)
- more than one input into any port => collision
- special collision presence signal is generated
Gigabit Ethernet
- Full-Duplex Operation
- traditional ethernet: half duplex
- station can transmit adn receive simultaneously
- for 100-Mbps Ethernet in full-duplex mode, theoretical transfer rate is 200 Mbps
- attached stations must have full-duplex adpater cards
- must use switching hub
: each station constitutes separate collision domain
: no collision and CSMA/CD algorithm is no longer needed (collision detect 필요 없음)
: 802.3 MAC frame format is used and attached stations can continue to execute CSMA/CD algorithm
- Mixed Configurations
: fast ethernet supports mixture of existing 10-Mbps LANs and newer 100-Mbps LANs
- stations attach to 10-Mbps hubs using 10BASE-T
- hubs: connected to switching hubs using 100BASE-T
- high-capacity workstations and servers attach directly to 10/100 switches
- switches connected to 100-Mbps hubs use 100-Mbps links
- 100-Mbps hubs provide building backbone
- connected to router provideing connection to WAN
100-Mbps Fast Ethernet
-
IEEE 802.3 MAC protocol and frame format
-
100BASE-X: physical meduim specifications from FDDI
- two physical links between nodes: transmission and reception
- 100BASE-TX: STP or Cat. 5 UTP
- 100BASE-FX: optical fiber
- 100BASE-T4: Cat.3, voice-grade UTP (성능 안 좋음. 4페어 다 사용)
: uses four twisted-pair lines between nodes
: data transmission uses three pairs in one direction at a time -
Star-wire topology: similar to 10BASE-T
-
Gigabit Ethernet - MAC
- carrier extension: 짧은 mac frame 끝에 special symbol 붙임
- frame bursting: extra overhead 필요 X
- switching hub. collision 발생 X -
Gigabit Ethernet - Physical
10-Gbps Ethernet
-
Uses
- high-speed, local backbone interconnection betwen large-capacity switches
- server farm, backbone, and campuswide connectivity
- enables ISPs and NSPs to create very high-speed links at low cost 낮은 비용으로 고속 링크 생성
- allows construction of MANs and WANs that connect geographically dispersed LANs
- ethernet competes with ATM and other WAN technologies
네트워크 내의 인터넷, 인트라넷에서의 트래픽 계속 증가 => 고속 네트워크 필요
네트워크 커넥션 숫자 늘어남, 말단 커넥션 스피드 빨라짐, 말단 application이 high bandwidth 요함, 웹호스팅 많음 -
Advantages
- No expensive, bandwidth-consuming conversion between Ethernet packets and ATM cells is required; Network is Ethernet, end to end (conversion 필요 X)
- IP and Ethernet together offers QoS and traffic policing capabilities; Advanced traffic engineering technologies are available to users and providers
- Variety of standard optical interfaces (wavelengths and link distances) have been specified for 10 Gb Ethernet
-
Medium Options
- Maximum link distances cover 300 m to 40 km
- Full-duplex mode only
- 10GBASE-S (short): 850 nm transmission on multimode fiber, Up to 300 m
- 10GBASE-L (long): 1310 nm transmission on single-mode fiber, Up to 10 km
- 10GBASE-E (extended): 1550 nm transmission on single-mode fiber, Up to 40 km
- 10GBASE-LX4: 1310 nm transmission on single-mode or multimode fiber, Up to 10 km, Medium uses wavelength-division multiplexing (WDM) to multiplex the bit stream across four light waves -
Distance options: core 두께에 따라 differ
100-Gbps Ethernet
- Ethernet
- preferred technology for wired LAN
- dominate enterprise LANs, broadband access data center networking, MAN, WAN
- preferred carrier for bridging wireless techs into local Ethernet networks
- popularity due to cost-effective, reliable & interoperable - Market drivers for 100-Gbps Ethernet
- data center/internet media providers
- metro-video/service providers
- enterprise LANs
- internet exchanges/ISP core routing - IEEE 802.3ba: 40Gb/s & 100Gb/s
- Multilane Distribution
- IEEE 802.3ba
- physical link between end station and switch OR between switches may be implemented as multiple parallel channels
: separate physical wires, such as four parallel twisted-pair links between nodes
: separate frequency channels, such as WDM over a single optical fiber link
- virtual lanes are distributed into the appropriate number of physical lanes
- the number of VLs must be integer multiple of the number of PLs
