Local Area Networks A Local Area Network is a communications network that provides interconnection of a variety of data communicating devices within a small area. Typical Characteristics! High Data Rates (0.1 Mbps to 10 Gbps)! Short Distances (0.1 to 40 km) -8-11! Low Error Rate (10 to 10 ) LANS-1
Local Area Network Topologies LANS-2
Shared Media vs. Switched LAN Architectures LANS-3
LAN Transmission Media Twisted Pair: Consists of two insulated wires (unshielded or shielded) arranged in a regular spiral pattern.! Inexpensive! Bandwidth is limited! Cost effective for single building, low traffic network Coaxial Cable: Consists of a hollow outer cylindrical conductor which surrounds a single inner wire conductor.! Greater capacity than twisted pair! Can handle a considerable amount of data! Two types: - 50 ohm (used in baseband networks) - 75 ohm (used in broadband networks) Fiber Optic Cable: The fiber is a thin, flexible (glass or plastic) medium having a high index of refraction. The fiber is surrounded by a "Cladding" material with a slightly lower index of refraction. The Cladding layer isolates the fiber (no crosstalk). An absorptive jacket surrounds the cladding layer.! Low Noise Susceptibility, low loss, small size.! Very high data rates possible. LANS-4
LAN Transmission Media cont. LANS-5
LAN Protocol Architecture: IEEE 802 Architecture IEEE 802 Architecture vs. OSI Model 1. Logical Link Control (LLC): 2. Medium Access Control (MAC): Functions of the LLC and MAC layers:! Provide one or more Service Access Points (SAPs)! Provide frame assembly/disassembly! Perform address recognition! Perform error detection! Manage communications over the link 3. Physical Layer: Functions of the Physical layer:! Encoding/decoding of signals! Preamble generation/removal (for synchronization)! Bit transmission/reception LANS-6
LAN Communications Architecture IEEE 802 LAN Communications Model LANS-7
IEEE 802 PDU Structure LANS-8
IEEE 802 Standards IEEE 802 Subcommittees! 802.1: Higher Layer LAN Protocols! 802.2: Logical Link Control! 802.3: CSMA/CD Networks! 802.4: Token Bus Networks! 802.5: Token Ring Networks! 802.6: Metropolitan Area Networks! 802.7: Broadband Technical Advisory Group! 802.8: Fiber Optic Technical Advisory Group! 802.9: Isochronous LAN Networks! 802.10: LAN Security! 802.11: Wireless LAN networks! 802.12: Demand Priority Networks! 802.14: Cable Modem Networks! 802.15: Wireless Personal Area Networks! 802.16: Broadband Wireless Access Study Group! QoS/Flow Control Study Group LANS-9
LAN Standards LANS-10
IEEE 802.2 Logical Link Control (LLC) LLC Services 1. Unacknowledged Connectionless Service:! Datagram service! Supports point-to-point, multipoint, and broadcast 2. Connection-Mode Service:! Provides virtual circuit connections between a pair of SAPs! Provides flow control, sequencing, and error recovery 3. Acknowledged Connectionless Service:! A connectionless service that provides acknowledgements! Supports point-to-point transfers LANS-11
LOGICAL LINK CONTROL PRIMITIVES UNACKNOWLEDGED CONNECTIONLESS SERVICE DL-UNITDATA.request (source-address, destination-address, data, priority) DL-UNITDATA.indication (source-address, destination-address, data, priority) CONNECTION-MODE SERVICE DL-CONNECT.request (source-address, destination-address, priority) DL-CONNECT.indication (source-address, destination-address, priority) DL-CONNECT.response (source-address, destination-address, priority) DL-CONNECT.confirm (source-address, destination-address, priority) DL-DATA.request (source-address, destination-address, data) DL-DATA.indication (source-address, destination-address, data) DL-DISCONNECT.request (source-address, destination-address) DL-DISCONNECT.indication (source-address, destination-address, reason) DL-RESET.request (source-address, destination-address) DL-RESET.indication (source-address, destination-address, reason) DL-RESET.response (source-address, destination-address) DL-RESET.confirm (source-address, destination-address) DL-CONNECTION-FLOWCONTROL.request (source-address, destination-address, amount) DL-CONNECTION-FLOWCONTROL.indication (source-address, destination-address, amount) ACKNOWLEDGED CONNECTIONLESS SERVICE DL-DATA-ACK.request (source-address, destination-address, data, priority, service-class) DL-DATA-ACK.indication (source-address, destination-address, data, priority, service-class) DL-DATA-ACK-STATUS.indication (source-address, destination-address, priority, service-class, status) DL-REPLY.request (source-address, destination-address, data, priority, service-class) DL-REPLY.indication (source-address, destination-address, data, priority, service-class) DL-REPLY-STATUS.indication (source-address, destination-address, data, priority, service-class, status) DL-REPLY-UPDATE.request (source-address, data) DL-REPLY-UPDATE-STATUS.indication (source-address, status) LANS-12
IEEE 802.2 LLC Services Notes on Primitive Parameters: Priority: Amount: Priority desired for the data unit transfer. (Token Bus, Token Ring and FDDI are capable of giving prioritized service. CSMA/CD is not.) Specifies the amount of data that may be passed on the connection. The amount is dynamically updated by each request or indication. Reason (in Reset): Used to indicate who initiated the reset (LOCAL or REMOTE). Reason (in Disconnect): Used to indicate who initiated the disconnect (Other User or Service Provider). Service Class: Status: Specifies whether or not an acknowledge capability in the MAC layer is to be used for the data unit transmission. (So far only 802.4 supports this capability.) a) Success b) Failure (nature of failure) LANS-13
Unacknowledged Connectionless Service Unacknowledged Connectionless Service LANS-14
Connection-Mode Service: Connection Establishment Connection-Mode Service: Connection Establishment LANS-15
Connection-Mode Service: Data Transfer & Flow Control Connection-Mode Service: Data Transfer & Flow Control LANS-16
Connection-Mode Service: Reset Connection-Mode Service: Reset LANS-17
Connection-Mode Service: Disconnect Connection-Mode Service: Disconnect LANS-18
Acknowledged Connectionless Service: Data Transfer Acknowledged Connectionless Service: Data Transfer LANS-19
Acknowledged Connectionless Service: Poll Acknowledged Connectionless Service: Poll LANS-20
LLC - MAC Interface The LLC is intended to support multiple MAC protocols:! CSMA/CD! Token Bus! Token Ring! FDDI! DQDB IEEE 802.2 MAC Service Primitives and Parameters MA-UNITDATA.request (source-address, destination-address, data, priority, service class) MA-UNITDATA.indication (destination-address, source-address, data, reception-status, priority, service class) MA-UNITDATA-STATUS.indication (destination-address, source-address, transmission-status, provided-priority, provided-service class) LANS-21
MAC Service Expected by LLC LANS-22
LLC Flow Control & Error Control Mechanisms Flow Control:! Stop-and-Wait! Sliding Window Error Control:! Stop-and-Wait ARQ! Go-Back-N ARQ LANS-23
Basic Operation: Stop-and-Wait Flow Control Protocol! Sender transmits a data frame and then, before transmitting any other data frames, waits for a signal (an ACK) from the receiver, which indicates its willingness to receive more data. Stop-and-Wait Flow Control Protocol LANS-24
Stop-and-Wait Flow Control - Performance! Assume a half-duplex, point-to-point link! It can be shown that the efficiency of the link is U = 1/(1+2a) where a = Bit Propagation Time/Frame Transmission Time LANS-25
LANS-26
Sliding Window Protocol Basic Operation:! The sender and receiver each maintain a window. - Transmit Window: A list of the sequence numbers of frames that a transmitting station may send. - Receive Window: A list of the sequence numbers of frames that a receiving station is willing to receive.! The windows open and close (slide) as frames are transmitted and received.! The Maximum Transmit Window Size (N) determines how many frames can be in transit simultaneously.! The Window Size can be used as a flow control mechanism. LANS-27
Sliding Window Protocol LANS-28
Sliding Window Protocol Characteristics & Performance! The Maximum Transmit Window Size (N) determines how many frames can be in transit simultaneously.! The Window Size can be used as a flow control mechanism.! Efficiency is now a function of both 'N' and 'a'. U = 1 for N > 2a+1 U = N/(2a+1) for N < 2a+1 LANS-29
LANS-30
Error Control Types of Errors:! Lost Frame: The receiver is not aware that a frame has been transmitted.! Damaged Frame: A recognizable frame arrives at the receiver, however, some of the bits are in error. Error Control Ingredients:! Error Detection: Ascertain that there was an error.! Positive Acknowledgement: The receiver ACKs successfully received, error-free frames.! Retransmission After Timeout: The source retransmits a frame that has not been acknowledged after a predetermined amount of time.! Negative Acknowledgement and Retransmission: The destination NAKs frames in which an error is detected. The source retransmits such frames. Automatic Repeat Request (ARQ) Techniques:! Stop-and-Wait ARQ! Go-Back-N Continuous ARQ LANS-31
Stop-and-Wait ARQ Uses a Simple Stop-and-Wait Acknowledgement Scheme: Receiver sends either an ACK or a NAK (see code). Questions & Problems: 1. What if the transmitted frame is lost? Answer: Must equip the transmitter with a TIMER. 2. What if the ACK gets lost (receiver may get a duplicate)? Answer: Must put SEQUENCE NUMBERS (names) within the transmitted frames to allow the receiver to detect duplicates. 3. a) The transmitter sends out frame 0 which is properly received. b) The ACK is slow in getting back to the transmitter. The timer goes off causing the transmitter to send a duplicate of frame 0 which is also received correctly. c) The receiver sends back a second ACK. d) The first ACK arrives at the transmitter which then sends out frame 1 which gets lost. e) Here comes the second ACK. Answer: Must sequence the ACKs (e.g. ACK0 - ACK1). LANS-32
Stop-and-Wait ARQ Stop-and-Wait ARQ LANS-33
Go-back-N ARQ! A station may send a series of frames determined by the window size, using the sliding window flow control technique.! While no errors occur, the destination station will ACK incoming frames as usual. Error Control: Assume that A is sending frames to B. 1. Damaged or Lost Frames: a. A transmits frame i. B detects an error in frame i and has previously successfully received frame i-1. B sends a NAK i, indicating that frame i is rejected. When A receives this NAK, it must go back and retransmit frame i and all subsequent frames that it has transmitted. b. Frame i is lost in transit. A subsequently sends frame i+1. B receives frame i+1 out of order, and sends a NAK i. Upon receipt of the NAK, A proceeds as above. LANS-34
c. Frame i is lost in transit and A does not soon send additional frames. B receives nothing and returns neither an ACK nor a NAK. A will time out and retransmit frame i. 2. Damaged or Lost ACK: a. B receives frame i and sends ACK (i+1), which is lost in transit. Since ACKs are cumulative (e.g., ACK 6 means that all frames through 5 are acknowledged,) it may be that A will receive a subsequent ACK to a subsequent frame that will do the job of the lost ACK before the associated timer expires. b. If A's timer expires, A retransmits frame i and all subsequent frames (or alternately, A can create a checkpoint by requesting an ACK). 3. Damaged or Lost NAK: If a NAK is lost, A will eventually time out on the associated frame and retransmit that frame and all subsequent frames. LANS-35
Go-Back-N ARQ Go-Back-N ARQ LANS-36
Go-Back-N ARQ Questions & Problems: 1. What is the maximum size of the receive window? Ans. 1 2. What is the maximum size of the transmit window? n Ans. 2-1 Proof: n a) Assume window size is 2. n b) Sender transmits all 2 frames. n c) An ACK comes in for frame 2. d) There is no way to tell if all the frames have arrived correctly or none of the frames have arrived correctly. LANS-37
LLC Protocol Types and Classes LLC Type: Type 1 Operation: Supports unacknowledged connectionless service. Type 2 Operation: Supports connection-mode service. Type 3 Operation: Supports acknowledged connectionless service. LLC Class: The combination of services supported is given by the station class. LLC Class Types of Operations I II III IV Supported 1 X X X X 2 X X 3 X X LANS-38
LLC Protocol Data Unit Format I/G = Individual/Group DSAP 0 - Individual DSAP 1 - Group DSAP C/R = Command/Response 0 - Command 1 - Response LANS-39
LLC Command/Response Repertoire Format Commands Responses C-Field Format 8 7 6 5 4 3 2 1 Information Transfer I (Information) N(S) 0 N(R) P RR (Receive Ready) RR (Receive Ready) 0 0 0 0 0 0 0 1 N(R) P F Supervisory RNR (Receive Not Ready) RNR (Receive Not Ready) 0 0 0 0 0 1 0 1 N(R) P F REJ (Reject) REJ (Reject) 0 0 0 0 1 0 0 1 N(R) P F SABME (Set Asynch. Balanced Mode) 0 1 1 P 1 1 1 1 DM (Disconnect Mode) 0 0 0 F 1 1 1 1 DISC (Disconnect) 0 1 0 P 0 0 1 1 UA (Unnumbered Ack) 0 1 1 F 0 0 1 1 Unnumbered FRMR (Frame Reject) 1 0 0 F 0 1 1 1 UI (Unnumbered Info) 0 0 0 P 0 0 1 1 XID (Exchange ID) XID (Exchange ID) 1 0 1 P F 1 1 1 1 AC0 (Acknowledged Connectionless, Sequence 0) AC0 (Acknowledged Connectionless, Sequence 0) 0 1 1 P F 0 1 1 1 AC1 (Acknowledged Connectionless, Sequence 1) AC1 (Acknowledged Connectionless, Sequence 1) 1 1 1 P F 0 1 1 1 LANS-40
Examples of LLC Operation Type 1 Operation: Data Transfer LANS-41
Type 2 Operation: Link Establishment and Termination LANS-42
Type 2 Operation: Normal Data Transfer LANS-43
Type 2 Operation: Busy Condition LANS-44
Type 2 Operation: Reject Recovery LANS-45
Type 2 Operation: Timeout Recovery LANS-46
Type 3 Operation: DATA-ACK and REPLY Services LANS-47
Carrier Sense Multiple Access (CSMA): Listen Before Talking (LBT) Station listens to the channel to determine if it is idle. Non-persistent CSMA:! If the channel is sensed idle, the station transmits.! If the channel is sensed busy, reschedule.! If rescheduled, repeat algorithm at new point. 1-persistent CSMA:! If the channel is sensed idle, the station transmits.! If the channel is sensed busy, wait until the channel is sensed idle and then transmit.! If a collision occurs, reschedule. p-persistent CSMA:! If the channel is sensed idle, the station transmits with probability p and delays transmission for one time slot with probability (1-p) and repeats algorithm at new slot.! If the channel is sensed busy, wait until the channel is sensed idle and then proceed as above.! If a collision occurs, reschedule. LANS-48
CSMA Persistence and Backoff CSMA Persistence and Backoff LANS-49
Local Area Network Topologies Using CSMA Methods Bus Topology Tree Topology Star Topology LANS-50
Carrier Sense Multiple Access with Collision Detection (CSMA/CD) Listen While Talking (LWT) 1-persistent CSMA/CD:! If the channel is sensed idle, the station transmits (with probability 1).! If the channel is sensed busy, wait (persist) until the channel is sensed idle and then transmit.! If a collision is detected during transmission: - Cease transmitting the frame. - Send out a jamming signal to inform other stations. - Reschedule the frame for later transmission. LANS-51
IEEE 802.3 MAC Protocol! Uses 1-persistent CSMA/CD! Retransmission scheme: Truncated Binary Exponential Back-off - After each collision, the mean retransmission delay is doubled. - The doubling occurs only for the first ten attempts after which the mean retransmission delay is held fixed. - After 16 unsuccessful attempts, the station gives up. LANS-52
IEEE 802.3 Frame Format IEEE 802.3 Address Field Formats LANS-53
IEEE 802.3 Frame Format! Preamble: - A 7-byte pattern used to establish bit synchronization. - The pattern is an alternating sequence of 1s and 0s, with the last bit being a zero. - The nature of the pattern is that for Manchester encoding, it appears as a periodic square wave.! Start Frame Delimiter (SFD): - The sequence 10101011.! Destination Address: - Specifies the destination station(s). - Address may be unique, multicast, or broadcast. - The choice of 16-bit or 48-bit addresses is an implementation decision and must be the same for all stations on the LAN.! Source Address: - Specifies the source station address.! Length: - Specifies the number of LLC bytes that follow! LLC Data: - The data unit supplied by the LLC.! Pad: - Bytes added to insure minimum frame size for CD operation.! Frame Check Sequence: - 32-bit CRC (does not cover Preamble and SFD). LANS-54
IEEE 802.3 Physical Layer Medium Alternatives LANS-55
10baseT Configuration Multiport Repeater Operation:! A valid signal appearing on any input is repeated on all other links.! If two inputs occur, causing a collision, a collision enforcement signal is transmitted on all links.! If a collision enforcement signal is detected on any input, it is repeated on all other links. LANS-56
10baseT: Two-Level Star Topology LANS-57
Shared Media vs. Switched LAN Architectures LANS-58
Shared Media vs. Switched LAN Architectures LANS-59
Layer 2 Switch Benefits! No change is required to the software or hardware of the attached devices to convert a bus LAN or a hub LAN to a switched LAN. In the case of an Ethernet LAN, each attached device continues to use the Ethernet medium access control protocol to access the LAN. From the point of view of the attached devices, nothing has changed in the access logic.! Each attached device has a dedicated capacity equal to that of the entire original LAN: This assumes that the layer 2 switch has sufficient capacity to keep up with all attached devices.! The layer 2 switch scales easily: Additional devices can be attached to the layer 2 switch by increasing the capacity of the layer 2 switch correspondingly. LANS-60
Layer 2 Switch Alternatives Two types of layer 2 switches are available as commercial products: Store-and-forward switch: - The layer 2 switch accepts a frame on an input line, buffers it briefly, and then routes it to the appropriate output line. - Involves a delay between sender and receiver. - Boosts the overall integrity of the network. Cut-through switch: - The layer 2 switch takes advantage of the fact that the destination address appears at the beginning of the MAC frame. - The layer 2 switch begins repeating the incoming frame onto the appropriate output line as soon as the layer 2 switch recognizes the destination address. - The cut-through switch yields the highest possible throughput but at some risk of propagating bad frames, because the switch is not able to check the CRC prior to retransmission. LANS-61
Layer 2 Switch vs. Bridge A layer 2 switch can be viewed as a full-duplex version of the hub. Incorporates logic that allows it to function as a multiport bridge. Differences between layer 2 switches and bridges: - Bridge frame handling is done in software. - A layer 2 switch performs the address recognition and frame forwarding functions in hardware. - A bridge can typically only analyze and forward one frame at a time. - A layer 2 switch has multiple parallel data paths and can handle multiple frames at a time. - A bridge uses store-and-forward operation. - With a layer 2 switch, it is possible to have cutthrough instead of store-and-forward operation. Because a layer 2 switch has higher performance and can incorporate the functions of a bridge, the bridge has suffered commercially. New installations typically include layer 2 switches with bridge functionality rather than bridges. LANS-62
Layer 2 Switch Problems Broadcast overload: - A set of devices and LANs connected by layer 2 switches is considered to have a flat address space. The term flat means that all users share a common MAC broadcast address. - Thus, if any device issues a MAC frame with a broadcast address, that frame is to be delivered to all devices attached to the overall network connected by layer 2 switches and/or bridges. - In a large network, frequent transmission of broadcast frames can create tremendous overhead. - Worse, a malfunctioning device can create a broadcast storm, in which numerous broadcast frames clog the network and crowd out legitimate traffic. Lack of multiple links: - Current standards for bridge protocols dictate that there be no closed loops in the network. - Hence, there can only be one path between any two devices. - Thus, it is impossible, in a standards-based implementation, to provide multiple paths through multiple switches between devices. - This restriction limits both performance and reliability. LANS-63
Router Problems To overcome these problems, it seems logical to break up a large local network into a number of subnetworks connected by routers: - A MAC broadcast frame is then limited to only the devices and switches contained in a single subnetwork. - IP-based routers employ sophisticated routing algorithms that allow the use of multiple paths between subnetworks going through different routers. However, routers typically do all of the IP-level processing involved in the forwarding of IP traffic in software: - High-speed LANs and high-performance layer 2 switches may pump millions of packets per second. - A software-based router may only be able to handle well under a million packets per second. LANS-64
Layer 3 Switches Layer 3 switches implement the packet-forwarding logic of the router in hardware. Layer 3 switch categories: - packet-by-packet: Operates in the identical fashion as a traditional router. Because the forwarding logic is in hardware, the packet-by-packet switch can achieve an order of magnitude increase in performance compared to the software-based router. - flow-based: A flow-based switch tries to enhance performance by identifying flows of IP packets that have the same source and destination. This can be done by observing ongoing traffic or by using a special flow label in the packet header (allowed in IPv6 but not IPv4). Once a flow is identified, a predefined route can be established through the network to speed up the forwarding process. Huge performance increases over a pure software-based router are achieved. LANS-65
Typical Premises Network Configuration LANS-66
Fast Ethernet! 100Base-T (part of IEEE 802.3): - CSMA/CD at 100Mbps. - Supports multiple physical layer implementations:! 100Base-T4: 4-pair UTP! 100Base-TX: 2-pair STP or category 5 UTP! 100Base-FX: 2 optical fibers LANS-67
IEEE 802.3 100Base-T Physical Layer Medium Alternatives LANS-68
100Base-T Use of Wire Pairs LANS-69
100Base-X Signal Encoding: 4B/5B - NRZI LANS-70
100Base-X Signal Encoding: 4B/5B - NRZI cont.! 100Base-X refers to a set of options that use the physical medium specifications originally defined for FDDI. LANS-71
100Base-TX Signal Encoding! In order to concentrate most of the energy of the transmitted signal below 30MHz, 100Base-TX follows the 4B/5B conversion with a scrambler and an MLT-3 encoder which produces a pseudoternary signal. LANS-72
Gigabit Ethernet! Retains the CSMA/CD access protocol and Ethernet frame format of its predecessors.! For Shared Medium Hubs; - Carrier Extension: Appends a set of special symbols to the end of short MAC frames so that the resultant block is at least 4096 bit-times. - Frame Bursting: Allows for multiple short frames to be transmitted consecutively, up to a limit, without relinquishing control of the medium between frames. (Avoids the overhead of carrier extension.)! Physical Layer Alternatives: - 1000Base-SX: Short wavelength option. Supports links up to 275m- 550m depending on the type of fiber. - 1000Base-LX: Long wavelength option. Supports links up to 550m-5km depending on the type of fiber. - 1000Base-CX: Uses special shielded-pair copper cable that spans no more than 25m. - 1000Base-T: Uses 4 pairs of category 5 unshielded twisted pair wire over a range up to 100m.! The signal encoding scheme is 8B/10B. LANS-73
Example Gigabit Ethernet Configuration LANS-74
Gigabit Ethernet Distance Medium Options LANS-75
10-Gigabit Ethernet! Physical Layer Alternatives: - 10GBase-S (short): Designed for 850-nm transmission on multimode fiber. Can achieve distances of up to 300m. - 10GBase-L (long): Designed for 1310-nm transmission on single-mode fiber. Can achieve distances of up to 10km. - 10GBase E (extended): Designed for 1550-nm transmission on single-mode fiber. Can achieve distances of up to 40km. - 10GBase-LX4: Designed for 1310-nm transmission on single-mode or multimode fiber. Can achieve distances of up to 10km. Uses wavelength division multiplexing (WDM) to mulyiplex the bit stream across four light waves.! The signal encoding scheme is 64B/66B. LANS-76
Example 10-Gigabit Ethernet Configuration LANS-77