1. 1CSS 432 1 CSS432 Shared Access Networks Textbook Ch2.6 - 2.7 Professor: Munehiro Fukuda

2. 2CSS 432 2 Ethernet Overview History  Developed by Xerox PARC in mid-1970s  Similar to IEEE 802.3 standard CSMA/CD  Carrier sense multiple access with collision detection Frame Format MAC (Media Access Control) Addresses  unique, 48-bit unicast address assigned to each adapter  example: 8:0:e4:b1:2  broadcast: all 1 s, multicast first bit is 1 Bandwidth: 10Mbps (10Base2=Thin Coax 200m, 10B5=Yellow Thick Coax 500m, 10BT=Twisted pair 100m), 100Mbps(10BaseT), 1Gbps Length: 2500m (500m segments with 4 repeaters) Dest addr 864 CRCPreamble Src addr TypeBody 26 Next frame Inter-frame gap bytes 46 ~ 1500 Min: 64bytes (512bits) ~ Max: 1518bytes Used by layer 3 IP: 0x0800 ARP: 0x0806 IPv6: 0x86DD 12

3. 3CSS 432 3 Ethernet Transmit Algorithm If line is idle…  Send immediately  Upper bound message size of 1500 bytes: MTU (Maximum Transmission Unit)  Must wait 9.6usec between back-to-back frames Why? (See the next slide.) If line is busy…  Wait until idle and transmit immediately  Called 1-persistent Transmit a packet with probability 1. (special case of p-persistent: transmitting a packet with P percent, where 0 < p ≤ 1)

4. 4CSS 432 4 Transmit Algorithm (cont) If collision…  Send a 32-bit jamming sequent, and then stop transmitting frame (64bit preamble + 32bit jam = 96bits)  Minimum frame is 64 bytes (header + 46 bytes of data) = 512bits Why? (See the next slide.)  Delay and try again 1st time: 0 or 51.2us 2nd time: 0, 51.2, or 102.4us 3rd time51.2, 102.4, or 153.6us nth time: k x 51.2us, for randomly selected k=0..2 n - 1 give up after several tries (usually 16) exponential backoff 10Mbps means 10bits/usec. 96bis needs 9.6usec 1010101… 64bits 1010.. 32bits

5. 5CSS 432 5 Collisions AB AB AB AB 500m x 5 = 2500m (with 4 repeaters) A collision occurred Time t Time t + d (d = 25.6us: approx. 0.01us/m) Time t + 2d (2d = 51.2us) Jam seq 10Mbps x 51.2us = 10 x 10 6 x 51.2 x 10 -6 = 512bits = (64bytes) Bandwidth10Mbps100Mbps1Gbps Bits/usec10bits/usec100bits/usec1000bits/usec Jamming sequence96bits => 9.6usec96bits => 0.96usec96bits => 0.096usec (96nsec) Max delay (RTT)512bits => 51.2usec512bits => 5.12usec512bits => 0.512usec Speed51.2usec/5000m = 10.2nsec/m51.2usec/200~400m = 12.8nsec/m0.512usec/25m (cupper) = 20.5nsec/m latency Network load 30% Ethernet Token Ring

6. 6CSS 432 6 Token Ring Overview Examples  16Mbps IEEE 802.5 (based on earlier IBM ring)  100Mbps Fiber Distributed Data Interface (FDDI) Idea  Frames flow in one direction: upstream to downstream  Special bit pattern (token) rotates around ring  Must capture token before transmitting  Release token after done transmitting Immediate release: FDDI Delayed release (after a sent frame came back) IEE802.5  Remove your frame when it comes back around  Stations get round-robin service Frame Format Control 888 24 CRC Start of frame End of frame Dest addr Body 48 Src addr Status 32 Priority bits

7. 7CSS 432 7 Token Maintenance Lost Token  No node emits a token when initializing ring.  A bit error corrupts token pattern.  A node holding token crashes. Lost Token Detection  IBM Token Ring: No more presence message from a monitor station  FDDI: No more message for more than 2.5ms Election Procedure  A station transmits a claim token with its MAC address if doubting the existence of a monitor station.  The highest address wins.  FDDI: the largest TTRT(Target Token Rotation Time) wins. Token Generation  Wait for NumStations x THT(Token Hold Time) + RingLatency

8. 8 Wireless Network Ad-hoc Network  WiFi Base Stations  WiFi, Bluetooth, Cellular CSS 432

9. 9 Wireless Network Bluetooth (802.15.1)  License-exempt band at 2.45GHz Wi-Fi (802.11) CSS 432 9 Cell Phone master active slave active slave parked slave parked slave 7255 : : Request Reponses oeoeoeoe TDM ABC DABC Hidden nodes: A and C are unaware of each other but their frames collide at B. Exposed node problem: B sends A a frame which C hears. This concludes C can’t send D a frame, which is actually possible. MACA: Multiple Access with Collision Avoidance: RTS: a sender sends a request to send. CTS: a receiver responds a clear to send. ACK: a receiver sends an ack upon a successful completion. Distribution system AP-1 AP-3 AP-2AP: Access point D A B C probe Probe responseAssoc request E 123 Base satations P Call P Handoff

10. 10 Wireless Network CSS 432 10 Direct sequence: n (e.g. 4)-bit chipping sequence CDMA: Code Division Multiple Access (Cellular-3G)  Each cell phone sends data with a different (but pre- assigned chipping code.)  A base station distinguish many cell phones using their unique chipping code.

11. 11CSS 432 11 Network Adapters CPU Memory I/O bus Controller Bus Interface Link Interface FIFO DMA Network Processor memory Interrupt System Bus PCI Bus Network Link Host Computer Network Interface Card (NIC) Programmed I/O 32bit, 33MHz = 1056Mbps 633Mbps STS-12 1000Mbps Ehternet Example  Myrinet Lanai series Myrinet Lanai series

12. 12CSS 432 12 Performance Considerations in Layer 2: Data Link Layer DMA  Initializing DMA channels versus programming CPU I/O Frame Size  Stuffing a full frame versus distinguishing one-time small-frame transfer and burst frame transfer channels Frame Transfer Strategy  Individual transfers versus pipelined transfers Fragmentation/Aggregation  Frame fragmentation versus frame aggregation Multicast through a switch  Software emulation or hardware implementation

13. 13CSS 432 13 Memory Bottleneck PCI(33bit, 33MHz) = 1056Mbps System Bus = 235MBps = 1880Mbps (Text example) ≈ real throughput  Memory Bus Controller Arbitration among CPU and DMA DRAM setup time  Data copy: Application memory space to OS OS memory space to NIC DMA-manageable space Zero copy/pin-downed communication  Bypassing OS Reducing memory-copying overhead Reducing interrupts Application DMA-manageable memory OS

14. 14CSS 432 14 Reviews  Ethernet: k-persistent, exponential back off, and the relationship between the minimum frame size and collisions.  Token ring network: immediate/delayed release, THT, TRT, and TTRT  Network adapters: writev/readv and memory bottleneck Exercises in Chapter 2  Ex. 42 (Ethernet)  Ex. 46 (Ethernet)  Ex. 53 (Wi-Fi)  Ex. 54 (Wi-Fi)