LOCAL AREA NETWORKS

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LOCAL AREA NETWORKS

  1. 1. EE6364 LOCAL AREA NETWORKS -- Network Configurations and Components -- Transmission Techniques and Media -- Access Control Protocols -- Performance Analysis -- Virtual LAN • Ethernet • Fast Ethernet • Ethernet Switch • Gigabit Ethernet • Token Ring DCW LAN-1
  2. 2. EE6364 LAN CONFIGURATIONS In general, a local area network can have the one of the following four configurations: (1) bus, (2) ring, (3) star, and (4) ring-star hybrid. Different configurations use different network access techniques and yield different performance. User Node Bus Bus Bus Interface Terminator U ser N ode A ccess Interface Ring Interface Ring C entral C on trol N ode User Node Bus and ring configurations use shared transmission media for message transfer between user nodes. The star configuration uses dedicated transmission medium. LANs of bus configuration (e.g., Ethernet) and ring configurations (e.g., IBM Token Ring) are very popular. However, recent LAN technology such as ATM LAN uses the star configuration. DCW LAN-2
  3. 3. EE6364 LAN FUNCTIONAL ELEMENTS There are four major functional elements for a LAN. They are: 1. A Transmission Medium In the bus and ring topologies, all messages sent by the users are transported on a common transmission medium (a bus or ring). The protocol used to detect and mediate contention among users is called the medium access protocol. 2. Network Access Stations An network access station is responsible for (1) implementing the medium access protocol, (2) placing user messages onto the transmission medium, (3) inspecting the header of the messages received to select those intended for local reception, (4) performing error and flow control, and (5) buffering messages to be sent and received. 3. Network Controller A network controller is used to perform admission control and call processing in the connection oriented network. In star configuration LANs, this performs the switching function. 4. Gateway Gateways are used to connect the LAN to external users via another LAN, and/or wide area networks. A gateway could be a bridge or a router. DCW LAN-3
  4. 4. EE6364 TRANSMISSION TECHNIQUES AND MEDIA Two transmission methods have been used for LANs. (1) Broadband: It uses analog technologies. Analog signals are multiplexed onto to the same transmission medium using frequency modulation technique. It requires amplifiers and modems. The advantages of broadband method are (1) it has large capacity, (2) it can broadcast over large areas using amplifier, and (3) it can be adapted to existing CATV technology. The disadvantages are its complexity and cost. (2) Baseband: Baseband transmission is totally digital. The entire frequency spectrum is used for transmission. Baseband LANs are limited in distance due to signal attenuation. Repeaters can be used to join different LAN segments to extend the distance. Transmission media include: (1) Twisted pair: These are the existing wires in the building. So they are economical. However, they are limited in bandwidth to few megabits per second and are susceptible to noise. (2) Coaxial cable: Coaxial cable has better performance, provides higher capacity, can support a larger number of devices, and can span greater distance. It can be used by both broadband and baseband techniques. (3) Fiber DCW LAN-4
  5. 5. EE6364 LAN PROTOCOLS IEEE has standardized protocols for local area networks. They are equivalent to the first (physical) and second (data link) layers in the OSI 7-layer reference model. OSI Reference Model Layer IEEE 802.2 Logical Link Control (LLC) Data 802.3 802.4 802.5 802.6 802.11 Link CSMA/CD Token-Bus Token-Ring MAN CSMA Wireless Layer MAC MAC MAC MAC MAC Physical Physical Physical Physical Physical Physical Medium Medium Medium Medium Medium Layer 802.2 specifies the logical link control protocol which is applicable to all the network configurations. Functions performed include error control, flow control and sequencing. 802.3 defines the media access control (MAC) protocol and the physical medium specification for the bus configuration LAN. 802.4, 802.5, 802.6 and 802.11define the MAC protocols and the physical medium specifications for the token-bus, token ring LANs, metropolitan area networks, and wireless LANs receptively. The MAC protocols define the procedures and message formats for the network access stations to access the transmission media. The combination of the MAC protocols of 802.x and 802.2 is equivalent to the second layer (i.e., data link layer) of the OSI reference model; the physical medium specifications of 802.x are the physical layer of the OSI reference. DCW LAN-5
  6. 6. EE6364 802.2 LLC PROTOCOL 802.2 specifies two alternatives of service to higher layer entities: (1) connectionless service and (2) connection-oriented service. These services are defined by specifying the service primitives and parameters exchanged between an LLC entity and its users. Two primitives (L_DATA.request, L_DATA.indication) for connectionless services and fourteen primitives (e.g., L_DATA_CONNECT.request, L_DATA_CONNECT.indication, L_DATA_CONNECT.confirm) for connection-oriented services are supported. The LLC protocol has similar formats and functions to those of the HDLC. The LLC frame consists of four fields: (1) destination service access point (SAP) address, (2) source SAP address, (3) control field, and (4) data field. Octets 1 1 1 or 2 => 1 DSAP SSAP Control Data Some examples of SAPs are: SAP Value (Hexadecimal) Assignment E0 Novell Netware F0 NetBios 06 TCP/IP AA subnet access protocol (SNAP) 00 null SAP 7F ISO 802.2 FE OSI protocol BC Banyan VINE 80 XNS Depending on the type of frame, the length of the control field can be either 1 or 2 octets. There are three types of frames: (1) information transfer frame, (2) supervisory frame, and (3) unnumbered frame. DCW LAN-6
  7. 7. EE6364 Formats of the control fields for these three types of frames are: Bit 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 0 N(S) P/F N(R) Information Transfer 1 0 SS XXXX P/F N(R) Supervisory 1 1 MM P/F MMM Unnumber where N(R)= transmitter send sequence number, N(S)= transmitter receive sequence number, S= supervisory function bit used to indicate RR (00) , RNR (10) or REJ (01), M= modifier function bit used to indicate unnumbered frames, such as, unnumbered information frame for connectionless service, set asynchronous balanced mode extended frame (SABME) (1111P110) and unnumbered acknowledgment frame (UA) (1100F110) for connection-oriented service, etc. P/F= poll/final bit, X= reserved. The type of frame is recognized by examining the first two bits. DCW LAN-7
  8. 8. EE6364 SLOTTED ALOHA SYSTEMS In the late 1960s, University of Hawaii developed a system, called Aloha, to connect computers. It is a packet-switched radio communication network. The central node listens to packets transmitted by other nodes at the radio frequency f0=407 Mhz and broadcasts the received packets at the radio frequency f1=413 MHz. Central Node F 1 = 413 M Hz F 0 = 407 M Hz The Aloha system was originally designed to be unslotted and then later improved to be slotted. In the slotted Aloha system, time is divided into contiguous fixed - length intervals called time slots, each of duration T seconds. Each user sends packets with fixed length corresponding exactly to the duration of one time slot. Each user knows its propagation delay to the central node. Whenever a user seeks to transmit a packet, it synchronizes its transmission such that, at the central node, the packet falls precisely into one time slot, and no user does a packet overlap two time slots. When two or more packets arrive at the central node at the same time, a collision occurs, and the central node can not broadcast any of the packets collided. To avoid repeated collisions among the same set of users, each user waits some random time slots before attempting to retransmit again. DCW LAN-8
  9. 9. EE6364 Let p be the probability that a given user attempts to access a given time slot, i.e., p is the probability that a user has a least one packet ready for transmission. There could be more than one user wants to access the same time slot. Their probabilities are independent and identical. The number of users, Q, is a binomial distributed random variable. The probability that k out of the total N users are competing for the same slot is: ⎛N⎞ P(Q = k ) = ⎜ ⎟ p k (1 − p ) N −k ⎜ ⎟ ⎝k⎠ If k=0, there is no transmission, the time slot is wasted. If k >1, collision occurs and again the time slot is wasted. A successful transmission is when only one user attempt to transmit at a given time slot. Thus, the probability of a successful transmission is when k=1, and ⎛N⎞ ps = Pr ob( success) = P (Q = 1) = ⎜ ⎟ p (1 − p) N −1 ⎜1⎟ ⎝ ⎠ = Np(1 − p) N −1 To find the probability, p, that maximizes the probability of a successful transmission, ps , we have dps = − N ( N − 1) p(1 − p) N − 2 + N (1 − p) N −1 = 0 dp and p=1/N. Thus the maximum success probability is p( Max ) = N (1 / N )(1 − 1 / N ) N −1 = (1 − 1 / N ) N −1 s and lim p( Max ) = lim (1 − 1 / N ) N −1 = 1 / e = 36.8% s N →∞ N →∞ For a slot Aloha system, at most only 36.8% of the available time slots can be effectively used. This is the efficiency of the network. DCW LAN-9
  10. 10. EE6364 RESERVED SLOTTED ALOHA SYSTEM The reservation ALOHA protocol is for nodes that use the same channel for the reservation and for the transmission of packets. The R.ALOHA protocol begins with a reservation phase. During this phase, the nodes use the slotted ALOHA protocol to attempt to access the channel. At the end of the reservation phase, the node which made the reservation transmits the packet. After the transmission phase is finished, the reservation phase begins again. In the reservation phase, the utilization efficiency is 36.8%, and the efficiency at the transmission phase is 100%. Ttransmission efficiencyR.ALOHA = Treservation + Ttransmission TStransmission = TSreservation + TStransmission 0. 368 1 = TSreservation 2. 72 * +1 TStransmission where TSreservation = time slot for reservation (fixed length) TStransmission = time slot for transmission (fixed length) Usually, TSreservation << TStransmission . If TSreservation = 0.05* TStransmission efficiency = 88%, much higher than pure slotted ALOHA system. DCW LAN-10
  11. 11. EE6364 UNSLOTTED ALOHA SYSTEM In the unslotted Aloha system, a user can attempt to transmit a packet at any time. We assume each packet takes T seconds to be sent. Thus, packets may arrive at the central node while another user is already transmitting. Let p be the probability that a given user attempts to begin transmitting within an arbitrarily chosen window of width T. If two or more packets begin transmission within the window, collisions occur. An arbitrarily chosen T-second test window will see the start of a successful transmission if the three following conditions are met: 1. Only one of the N users begins access within that window; 2. Once the first user begins transmission, no other user transmits within the T-second window needed to complete the transmission of the packet; 3. No user attempted to begin transmission within the T-second window preceding the beginning of the test window. These three conditions are independent, with probabilities p1, p2, and p3, given by N p1 = ∑ p = Np i =1 p2 = (1 − p) N −1 p3 = (1 − p) N −1 Thus, the probability of a successful transmission is ps = p1p2 p3 = Np(1 − p) 2 N − 2 The optimizing value of p is dp s = − N ( 2 N − 2 ) p (1 − p ) 2 N − 3 + N (1 − p ) 2 N − 2 = 0 dp 1 we get p = 2N − 1 DCW LAN-11
  12. 12. EE6364 And, we have 1 1 p ( Max) = (1 − ) 2 N −2 s 2 2N − 1 lim ps ( Max ) = 1 / 2e = 18. 4% N →∞ Thus, the efficiency of unslotted ALOHA is only half of that of the slotted ALOHA. 0.6 0.5 0.4 eff( N ) 0.3 0.2 0.1 0 0 10 20 30 40 N DCW LAN-12
  13. 13. EE6364 ETHERNET Ethernet was first invented by Xerox company in the 70s. In the 80s, the IEEE 802.3 standard defines a group of LANs with various physical layer standards (i.e., physical media and rates) and the media access control (MAC) protocol based on Ethernet. The standard designates the 802.3 LANs by short-hand notations. 10Base5 means 10Mbps, baseband, and 500-meter segment. This type of LANs uses 50-ohm coaxial-cable. It is called thick Ethernet (3/8 inch). 10Base2 means 10Mbps, baseband, and 200-meter segment. It uses thin flexible coaxial cable and is called thin Ethernet (3/16 inch). 10BaseT means 10Mbps, baseband, and unshielded twisted pair (UTP). The length is 100 meters and the impedance is 100 ohms. 10BaseF means 10Mbps and uses fiber. 10Broad36 means 10Mbps, broadband, and up to 1800 meter segment. The MAC for IEEE 802.3 is called carrier sense multiple access/ collision detection (CSMA/CD). There are three variations of CSMA: non-persistent, 1-persistent, and p-persistent. In non-persistent CSMA, the user listens to the medium before transmission. If the medium is idle, it can transmit. If the medium is busy, wait an amount of time drawn from a probability distribution. These procedures are repeated. In the 1-persistent CSMA, the user listens to the medium, 1. If the medium is idle, transmit. 2. If the medium is busy, continue to listen until the medium is idle, then transmit immediately. DCW LAN-13
  14. 14. EE6364 In the p-persistent CSMA, the user listens to the medium, and 1. If the medium is idle, transmit with probability p, and delay one time unit with probability (1-p). 2. If the medium is busy, continue to listen until the medium is idle and repeat step 1. 3. If transmission is delayed for one time unit, repeat step 1. IEEE 802.3 adopts the 1-persistent CSMA. The CSMA/CD protocol adopted by IEEE 802.3 is as follows: 1. When a node has data to send, it first sets the backoff factor k=0. 2. The node monitors the medium and waits for an idle. 3. Once the medium becomes free, the node waits for an inter- frame gap (IFG) period of time and then sends its frame. The IFG is the time needed to send 96 bits. 4. While transmitting, the node also monitors for any collision. If a collision is detected during the transmission, the node immediately ceases transmitting data, and transmits a brief jamming signal to assure that all other nodes are aware that there has been a collision. 5. The node increases the backoff factor k by 1. If K is greater than 10, the node aborts the transmission. Otherwise, it goes back to step 2 to retransmit the data after waiting for a period of backoff time. It uses a truncated binary exponential backoff algorithm to calculate the backoff time. It randomly chooses a variable r, where 0 <= r <= (2k – 1). The backoff time is r x (slot time). The length of a time slot is at least twice the propagation delay in the bus, i.e., 2τ = 2 × (length of the bus)/(electrical wave speed). The standard specifies the time slot as 512-bit time. The original Ethernet frame format is as following: Octets 8 6 6 2 46 - 1500 4 Preamble DA SA Type Data FCS DCW LAN-14
  15. 15. EE6364 Where Preamble is used to establish bit synchronous and to locate the first bit of the frame. Destination Address (DA) specifies the unique physical address (MAC address) of the receiving device. Source address (SA). Type indicates the type of network layer protocol used in the data field. Frame check sequence (FCS) is a 32-bit CRC based on all field starting from DA. The MAC addresses are uniquely assigned by the Ethernet device (e.g., NIC card, router port) manufacturers, and are hard-coded into the devices. The Ethernet MAC address is 6 bytes long. The format is as follows: Organizational Unique Identifier Network Interface Controller (OUI) (NIC) Specific Identifier 0: Unicast 1: Multicast Most significant Bit (MSB) 0: Global unique 1: Locally administered The address space is grouped into two parts. The first three bytes contains the organizational unique identifier (OUI), which identifies the manufacture. The second group is the NIC specific identifier, which is assigned by the manufacture to each NIC card. The least significant bit (LSB) of the first byte indicates whether this address is for unicast (=0) or multicast (=1). The second least significant bit of this byte indicates whether this address is globally unique (=0) or locally administration (=1). If the MAC address is all 1’s, then it is a broadcast address locally. Some examples of the Ethernet type assignments are as follows: DCW LAN-15
  16. 16. EE6364 Type Value (Hex)1 Assignment 06-00 Xerox Network Service 08-00 Arparnet Internet Protocol (IP) 08-05 X.25 level 3 08-06 Address resolution protocol (ARP) 0B-AD Banyan VINE 70-00 Ungermann-Bass 70-34 Cabletron 80-35 Reverse address resolution protocol (RARP) 80-38 Digital Equipment Corp 80-F3 AppleTalk ARP 90-00 Loop Back The IEEE 802.3 Ethernet frame format is different from that of the original Xerox Ethernet. The 802.3 Ethernet frame format is as following: Octets 7 1 6 6 2 1 1 1 3 2 =< 1492 4 Preamble SFD DA SA Length DSAP SSAP Control OUI Type Data FCS Where Preamble is used to establish bit synchronous and to locate the first bit of the frame. This field contains 7 bytes of alternating 0s and 1s. Start Frame Delimiter (SFD) indicates the start of the frame. This byte has a bit pattern of 10101011. Destination Address (DA) specifies the receiving device MAC address. Source Address (SA) specifies the source device MAC address. Length indicates the number of octets in the data field starting from DSAP to before the FCS field. DSAP and SSAP are the destination and source service access points. Control is the control field of the LLC. OUI is the organization unique identifier Type indicates the type of network layer protocol used in the data field. Frame Check Sequence (FCS) is a MAC frame 32-bit CRC based on all field starting from DA. In the above frame format, the field from DSAP to Control is the IEEE 802.2 LLC field. The combination of the OUI and Type fields is called the sub-network access protocol (SNAP). Examples of OUI assignment are as follows: OUI Value (Hex) Assignment 1. For complete listing, see IETF RFC-1340 or RFC-1700. DCW LAN-16
  17. 17. EE6364 00-00-0E Fujitsu 00-00-1B Novell 00-00-F8 Digital Equipment Corp. 00-80-C2 IEEE 802 committee 00-A0-3E ATM Forum The reason for adding SNAP field to the 802.3 frame is that the Type field used by the Ethernet frame has been assigned by IEEE802.3 committee to be the length field. A field is needed to identify the network layer protocol (i.e., the Type field). The Type field requires two octets, SAP field is only one octet long. So, SNAP is used. If the 802.3 Ethernet frame contains an IP packet, then the LLC and SNAP fields are defined as: DSAP-SSAP-Control : AA-AA-03 (hex) OUI-Type: 00-00-00-08-00 (hex). This special LLC (DSAP=AA, SSAP=AA) indicates a SNAP to follow. In the SNAP field, OUI of 00-00-00 indicates that Ether type to follow, and Type=08-00 identifies the network layer protocol used is IP. DCW LAN-17
  18. 18. EE6364 CSMA/CD Performance Analysis: Let the probability that a user has data to send = p and there are N users. Then the probability, ps, that a successful transmission is initiated when the bus becomes idle is the probability that only one user attempts access. So, p s = Np (1 − p) N −1 Let M be number of time slots which elapse when a successful transmission is initiated. If M=k, then there are k-1 unsuccessful or wasted time slots before the successful attempt. Thus, we have p( M = k ) = p S (1 − p s ) k −1 , k≥1 Let the time needed to transmit a packet is T seconds. The total elapsed time F from completion of the last successfully transmitted packet to completion of the next successful transmitted packet is: F = 2τM + T-2τ and the average elapsed time is <F> = 2τ<M> + T-2τ ∞ ∞ where <M> = ∑ kp ( M = k ) = p s ∑ k (1 − p s ) k −1 k =1 k =1 −d ∞ k = ps ∑ (1 − p s ) dp s k = 0 ps 1 = = [1 − (1 − p s )]2 ps 2τ Thus, <F> = + T − 2τ ps Since τ and T are fixed, <F> is minimized if Ps is maximized. To obtain the p which yields the maximized ps, we have d ps = 0 . dp This yields p=1/N. Thus, 1 p s MAX ) = (1 − ) N −1 ( N DCW LAN-18
  19. 19. EE6364 When N→∞, ps ( MAX ) = 1/ e . Thus, <F>= 2τ(e-1) + T. The channel efficiency η of the Ethernet using CSMA/CD is 2 η = (Duration needed to transmit a packet) / (Total time required) T 1 = = T + 2τ (e − 1) 1 + (e − 1) / a where a= T/2τ. The throughput, Th, can be defined as size _ of _ packet _(in _ Bits ) Th = time _ required _ to _ send _ the _ packet Let the size of the packet be s bits, the transmission speed in the Ethernet be t bits/second, the length of the shared bus be l meters, and the propagation delay in the transmission media be c meters/sec. Then T = s/t, and τ = l/c. s Th = l s 2 * (e − 1) + c t Note that the shorter the length of the bus, l, the higher the throughput. This is the reason that the length of the bus is limited and the number of users on each bus has been limited too. This is called micro- segmented. 1 2 This is the theoretical value. In reality, the measured efficiency is η= 1 + 2.5 / a DCW LAN-19
  20. 20. EE6364 FAST ETHERNET3 In the analysis of shared media Ethernet, it has been shown that the higher the transmission speed, t, the higher the throughput. Ethernet, most popularly known as 10baseT, operates at 10 Mb/s. To increase the speed, fast Ethernet was proposed and standardized by IEEE as 802.3u. 802.3u Ethernet uses shard bus configuration and operates at 100 Mb/s data rate. The transmission media used are as follows: 100BaseT4 100BaseTX 100BaseFX Medium Four pairs; UTP Two pairs; UTP Optical fiber, Category 34 Category 5 multi-mode Max. Length 100m 100m 2Km To maintain compatibility with Ethernet (10baseT), fast Ethernet uses the same frame format as that for the Ethernet. It also uses CSMA/CD to access the bus. Many fast Ethernet can support both 10Mb/s and 100Mb/s interfaces with the host computers. Fast Ethernet performs auto-negotiation with the NIC cards to detect the speed. Exercise: Compare the throughput of 10baseT and 100baseT. 3. IEEE 802-3u-1995, “Media Access Control (MAC) Parameters, Physical Layer, Medium Attachment Units and Repeater for 100 Mbps Operations, Type 100Base-T,” IEEE Press, Piscataway, NJ, 1995. 4. Category 3 unshielded twisted pair (UTP) wire is used for regular phone line. DCW LAN-20
  21. 21. EE6364 GIGABIT ETHERNET5 With the increasing demand of bandwidth of many applications and the increasing processing power of the new computers, there appears to be a need for faster Ethernets, i.e., Ethernets with bandwidth higher than fast Ethernet (100Mb/s). IEEE 802.3z task force has defined the specification of the gigabit Ethernet, which operates at 1000Mb/s. Highlights of differences between 10Mb/s, 100Mb/s and Gigabit Ethernets are: Ethernet Fast Ethernet Gigabit Ethernet Data rate 10Mb/s 100Mb/s 1Gb/s Cat 5 UTP 100m 100m 100m STP/Coax 500m 100m 25m Multimode Fiber 2Km 412m (half duplex) 220-550m 2Km (full duplex) Single Mode Fiber 25Km 20Km 5Km In the gigabit Ethernet physical layer, four physical media have been specified. They are (1) long reach single mode fiber (denoted as 1000BaseLX), (2) short reach multimode fiber (1000BaseSX), (3) 150- ohm balanced copper cable (1000BaseCX), and (4) category-5 unshielded twisted pair (1000BaseT). The first three physical layers have been defined by 802.3z (denoted as 1000Base-X), while 802.3ab committee has defined specifications for the category-5 twisted pair physical layer. In 1000base-X, the coding of the Ethernet frames in the physical media is 8B/10B coding. In this coding, an eight bit data is coded into a 10-bit code group. This 8B/10B coding has excellent properties, such as transition density, run-length limiting, DC balance, and error robustness. The physical layer also performs AutoNegotiation to determine the data rate, i.e., 10, 100 or 1,000 Mb/s. 5 H. Frazier and H. Johnson, “Gigabit Ethernet: From 100 to 1,000 Mbps”, IEEE Internet Computing,pp. 24-31, January/February 1999. DCW LAN-21
  22. 22. EE6364 The frame format is similar to that for the 10 and 100 Mb/s Ethernets. It is as follows: /I/ /I/ /S/ Preamble sfd MAC Header Upper Layer Header + Data CRC /T/ /R/ /I/ /I/ Idle Gigabit Ethernet Frame Idle Where: /I/ is the idle code, /S/ signifies the start. Each frame ends with a pair of code groups, /T/ and /R/. In the link layer, the operation can be either half-duplex or full duplex. If half-duplex is used, the media access control (MAC) protocol is CSMA/CD with two extensions. These two extensions are: o The carrier extension appends a set of special symbols to the end of short frames so that the resulting frames are at least 4096 bits, instead of the 512-bit imposed by the 10 and 100 Mb/s Ethernets. This extension is used to overcome the inherent limitation of the CSAM/CD algorithm that mandates the round- trip signal propagation delay between any two host computers not to exceed the time required to transmit the smallest allowable frames. o An optional frame bursting was defined. Frame bursting allows multiple short frames to be transmitted consecutively up to a limit, without relinquishing control of the signaling control. If full duplex is used, CSMA/CD is disabled. A link level flow control is used to prevent receiver buffer overflow. A pause protocol is adopted that a congested receiver can request the transmitter to pause its transmission. The congested receiver can sent to the transmitter a pause frame which contains a timer value expressed as a multiple of 512 bit-time. Once the pause frame is received, the transmitter should then stop transmitting. If the receiver becomes uncongested before the timer expires, the receiver can send another pause frame with timer value set to zero. The transmitter then resumes transmitting. The physical and link layer diagram of gigabit Ethernet is summarized as follows: DCW LAN-22
  23. 23. EE6364 Media Access Control (MAC) 802.3z Gigabit Media-Independent Interface (GMII) 802.3ab 1000Base-X PHY Physical 8B/10B AutoNegotiation 1000Base-T coding Phy. Coding Sunlayer Sub-Layer 1000Base-LX 1000Base-SX 1000Base-CX Physical 1000Base-T Medium Fiber-Optic Fiber-Optic Copper Phy. Medium Attachment Attachment Xcvr Xcvr Xcvr Sub-layer Single mode Fiber Multimode Shielded Categorr-5 (10μ 5Km) Fiber Copper Unshield Multimode Fiber (50 μ 500m Cable (25m) Twisted pair (100m) (50 μ 550m 62.5μ 220m) 62.5μ 500m) A gigabit media independent interface is defined so that the MAC layer can interface with various physical media. GMII defines independent 8-bit- parallel transmit-and-receive synchronous data interface. DCW LAN-23
  24. 24. EE6364 SWITCHED ETHERNET6 Ethernet and fast Ethernet use shared medium to transmit packets from different computers. Because all computers share the same transmission medium, collisions do occur which reduce the throughput as we have already seen. In addition, all computers shared the total bandwidth (10Mb/s or 100 Mb/s). Thus, the more computers, the less bandwidth used by each computer. Switched Ethernet or Ethernet switch has been developed to eliminate the collision and to improve the throughput. It is a packet switch, and users do not share the transmission medium. Just like other switches, the Ethernet switch has buffers to perform store-and-forward functions to send packets from one user to another user based on the destination MAC address. The users can keep using their network interface cards (NIC) which have already been installed in their PCs. These NICs perform the CSMA/CD algorithm as before and send packets to the Ethernet switch using the Ethernet packet format. Because in the Ethernet switch, there is no shared medium, the transceiver in the NIC always detects no activity in the Ethernet. Thus, the NIC can always send packets to the switch. The switching can be done by either store-and-forward or cut-through method. In the store-and-forward method, the switch does not switch until the whole packet is received and examined. In the cut-though method, the switch starts to send the packet to the destination once it receives the source and destination MAC addresses. 6. M. Molle and G. Watson, “100BaseT/ IEEE 802.12/ Packet Switching,” IEEE Communications Mag., pp. 64-73, August, 1996. DCW LAN-24
  25. 25. EE6364 TRANSPARENT BRIGES7 Gateways are used to connect users of one LAN to external users via another LAN, and/or wide area networks. A gateway could be a bridge or a router. Bridges interconnect networks based on link layer addresses (e.g., MAC addresses), while routers interconnect networks based on the network layer addresses (e.g., IP addresses). In general, bridges are used for LAN-LAN interconnection, and routers are used for LAN-WAN interconnection. We will discuss routers later when we discuss Internet protocols. In a shared media LAN, such as Ethernet, the distance is limited. Bridged LAN can be used in a campus environment to extend LANs or to interconnect existing LANs that are deployed separately by different departments. An example of two LANs connected by a bridge is shown in the following figure. LAN1 LAN2 H1 1 2 H4 Bridge H2 H5 H3 H6 H7 In the above network configuration, two Ethernets are connected using a bridge. If host, H1 in LAN1 wants to send a message to host H3, the message is broadcasted to all hosts of LAN1 and the bridge. By examining the destination MAC address at the header of the message frame, H2 and the bridge discard the message received, and H3 accepts it. If H1 sends a message to H7, the message is broadcasted to LAN1, and both H2 and H3 will discard it. The bridge receives the 7. F. Backes, “Transparent Bridges for Interconnection of IEEE 802 LANs,” IEEE Network, vol. 2, no. 1, pp. 5-9, January, 1988. DCW LAN-25
  26. 26. EE6364 message. After examining the destination MAC address, the bridge determines that the destination is on LAN2. It then broadcasts the message to LAN2 through port 2. Thus, H4 to H7 of LAN2 all receive the message. However, only H7 accepts it, other hosts will discard it. In the above example, the bridge not only extends the LAN, it also knows which user is on which LAN. This type of bridges is called transparent bridges. Transparent bridges were defined by IEEE 802.1d committee. They are called transparent, because the users are unaware of the existence of the bridges. Thus, the introduction of the bridge does not require the hosts to be configured. A transparent bridge performs the following three basic functions: - Forwards frames from one LAN to another. - Learns where hosts are attached to the LAN. - Prevents looping in the topology. Bridge Learning Each transparent bridge has a forwarding table. When a frame arrives on one of its interface ports, the bridge has to perform table look up to decide whether or not to forward the received frame to another port based on the destination MAC address. Each entry of the forwarding table has at least two fields: the MAC address of a host and the associated port of the bridge. Initially, the forwarding table is empty. The bridge builds the table by learning. Before the forwarding table is completely filled, when a frame is received with the destination MAC address not on the table, the bridge will broadcast the frame to all its ports except the port that it receives the frame. In the above network configuration, the bridge has two ports (ports 1 and 2) attached to LAN1 and LAN2. When the bridge receives a frame broadcasted by H1, it learns from the source MAC address that H1 MAC address is associated with port 1. In the same manner, the bridge learns that H7 MAC address is associated with port 2 after it DCW LAN-26
  27. 27. EE6364 receives a frame broadcasted by H7 to LAN2. After a while, a forwarding table as shown below is established. MAC address Port H1 1 H2 1 H3 1 H4 2 H5 2 H6 2 H7 2 If H5 sends a frame to H2, the bridge receives the frame from port 2. By examining the destination MAC address and the table look up, the bridge routes the frame to port 1 and broadcasts the frame to LAN1. The LAN environment is dynamic, hosts may be added, removed, or moved. The bridge needs to adapt to the dynamics of the network. First the bridge adds a timer associated with each entry. The timer is decremented periodically. When the timer reaches zero, the entry is removed. When the bridge receives a frame with the source MAC address matches with the one in the table, the entry is refreshed. Secondly, when the bridge receives a frame and finds a match in the source address but the port number in the entry is different from the port number on which the frame arrives, the bridge updates the entry with the new port number. Exercise: Show the forwarding tables of bridges A and B of the following network configuration. How do these bridges build these two tables? LAN1 LAN2 LAN3 H1 1 Bridge 2 1 2 B Bridge A H7 H2 H5 H3 H8 H6 DCW LAN-27
  28. 28. EE6364 Spanning Tree Algorithm8 One potential problem of the learning process is that it does not detect looping. Looping can cause flood of frames and bring down the network completely. To remove loops in a network, a spanning tree algorithm (STA) has been specified by IEEE 802.1D committee. The spanning tree algorithm requires that each bridge have a unique bridge ID number, each port within a bridge have a unique port ID, and all bridges on a LAN recognize a unique MAC group address. The algorithm is as follows: 1. Select the bridge with lowest bridge ID as the root bridge for all the bridges of all the LANs. 2. Calculate the cost of each path to the root bridge. The cost is assigned according to some pre-defined criteria. The path cost is the sum of the costs along the path. An example is each LAN costs 1. If a path traverses through four LANs, the path cost is 4. 3. Determine the root port for each bridge except the root bridge. The root port is the port with the least-cost path to the root bridge. In case of ties, the root port is the one with the lowest port ID. 4. Select a designated bridge for each LAN. The designated bridge is the one that offers the least cost path from the LAN to the root bridge. The port that connects the LAN and the designated bridge is called designated port. 5. Place all root ports and all designated ports in the forwarding states in the spanning tree. They are the ports that are allowed to forward frames. The other ports are placed into a blocking state, and are not allowed to forward frames. The algorithm is implemented using a distributed algorithm. Each bridge exchange special messages called configuration bridge protocol data units (configuration BPDUs). Each configuration BPDU contains the bridge ID of the transmitting bridge, the root bridge ID, and the cost of the least cost path from the transmitting bridge to the root bridge. The following shows a Configuration BPDU. 8. IEEE 802.1D, IEEE Standard for Local Area Network MAC (Media Access Control) Bridges DCW LAN-28
  29. 29. EE6364 Octets 2 1 1 1 8 4 8 2 2 2 2 2 Protocol Protoccol BPDU Flags Root Root Bridge Port Message Maximum Hello Forward ID Version Type Bridge Path ID ID Age Age Time Delay ID ID Cost Protocol ID: All zeros for STP Protocol version ID: All zeros for current STP version BPDU type: All zeros for configuration BPDU Flags: Only the least and the most significant bits are used. When the least significant bit is set to 1, it indicates this BPDU as a topology change message. When the most significant bit is set to 1, the BPDU is a topology change acknowledgement. Root Bridge ID: Bridge ID for the root bridge. The first two bytes identify the priority of the bridge, and the last six bytes are the MAC address of a port of the root bridge. Root path cost: The cost of the shortest path from this bridge to the root bridge. Bridge ID: The ID of this bridge. Port ID: The ID of the port that sends this configuration BPDU. It consists of two parts. Six bits are used to indicate the priority and the remaining ten bits are used to indicate the port number. Message age: The time elapsed from the generation of the configuration BPDU by the root bridge and its receipt by the bridge processing the BPDU. It has a 1/256 second increment. Maximum age: The maximum amount of time the configuration BPDU can be used. Hello time: The time interval that the root bridge should send the configuration BPDU. Forward delay: The time that a bridge must remain in each intermediate processing state before transition from blocking to forwarding. Each bridge records the best configuration BPDU it has so far. A configuration BPDU is the best if it has the lowest root bridge ID. If there is a tie, the configuration BPDU is best if it has the lowest cost to the root bridge. DCW LAN-29
  30. 30. EE6364 The configuration BPDU uses the ordinary Ethernet frame format. The destination MAC address is a special multicast address assigned to all bridges. The source MAC address is the address of the port. The SAP value is assigned to be 0x42 (i.e., 01000010 in binary). Each bridge initially assumes that it is the root bridge. Each bridge transmits configuration BPDUs periodically on each of its ports. When a bridge receives a configuration BPDU from a port, the bridge adds the path cost to the cost of the LAN that this BPDU was received from. The bridge then compares the configuration BPDU with the one recorded. If the bridge received a better configuration BPDU, it stops transmitting on that port and save the new configuration BPDU. Eventually, only one bridge, the designated bridge, on each LAN will be transmitting configuration BPDU on that LAN. Each bridge maintains a timer for the saved configuration BPDU. The timer is reset when the bridge receives a configuration BPDU. If the timer expires due to some bridge failure, the bridge starts the spanning tree algorithm again. The following is an example of six LANs connected by five bridges. LAN1 LAN3 1 1 3 LAN4 B1 B2 2 2 1 LAN2 2 B4 1 3 2 B3 3 LAN5 1 B5 2 LAN6 DCW LAN-30
  31. 31. EE6364 Note that some LANs are connected by two bridges. This could be constructed to provide redundancy. The corresponding spanning tree configuration is as follows: LAN1 LAN3 D 1 R 1 3 LAN4 B1 B2 D D 2 2 R 1 LAN2 2 B4 D R 1 3 2 B3 D 3 LAN5 R 1 B5 D 2 LAN6 The ports with dashed lines to the bridges are in the blocking state. STP can take a long time to converge. The Rapid Spanning Tree Protocol (RSTP) was introduced by IEEE 802.1w. RSTP basically is the same as STP, but it provides faster convergence time from topology changes. RTSP provides faster recovery by monitoring the link status of each port and generating a topology change after the link status change. RTSP improve recovery time by adding alternate port for a port that acts as a backup to the root port. In addition, RTSP reduces the number of port states. Exercise: Explain how the above spanning tree configuration is constructed. DCW LAN-31
  32. 32. EE6364 VIRTUAL LAN9 Typically, an organization that has many nodes (or stations) deploys separated LANs, which are connected by routers. Nodes on different geographic areas or in different groups are physically connected to different LANs. However, this physical association between a node and a specific LAN is not flexible. An employee may be moved to another floor and is still in the same group, thus wants to be in the same LAN. One way to solve this problem is to re-wire, which is costly and time consuming. Another problem arises that nodes in the different group but in the same area may want to share the same LAN to share cost. In this case, nodes from a group do not want to receive any broadcast from the other group. Virtual LAN (VLAN) has been developed to allow logical partition of nodes in the LAN into communities of interest called VLAN groups. Members of a VLAN group are not constrained by the physical location. The following figure shows an example of VLAN groups supported by two Ethernet switches. Ethernet Switch A Ethernet Switch B VLAN 1 VLAN 2 VLAN 3 9. IEEE 802.1Q-1998, IEEE Standard for Local and Metropolitan Area Networks: Virtual Bridged Local Area Networks. DCW LAN-32
  33. 33. EE6364 Note that nodes that are attached to the same Ethernet switch are assigned to different VLANs; while nodes from different Ethernet switches can be in the same VLAN. IEEE 802.1Q defines the architecture and protocol for VLAN. It specifies the MAC frame format as follows: Octets 7 1 6 6 4 2 4 Type/ Preamble SFD DA SA TAG LLC + Data FCS Length Bits 16 3 1 12 C TPID (= 0x8100) Priority F VLAN ID I A new field, called Tag, of four bytes lenth is introduced. The Tag Protocol ID (TPID) field has a value of 0X8100, which indicates the following two byte field is Tag Control Information (TCI). Note that 0X8100 (hexadecimal) is greater than 1500 (decimal). Thus, the switches can easily identify that it is not a length field. The TCI contains a three-bit priority field, which is defined in IEEE 802.1p, a one-bit canonical format identifier (CFI), and a twelve-bit VLAN ID. The priority bits support up to eight different priority levels. The CFI field is used for IEEE 802.5 token ring LANs. The VLAN ID field can support up to 4094 VLANs. (Two possible values are reserved.) The membership of a VLAN can be defined based on - Ethernet switch port; - MAC address; - IP address; - Policy based. DCW LAN-33
  34. 34. EE6364 IEEE 802.1ad10: VLAN is useful within an enterprise. Network service providers who are providing network service to connect LANs from different customer sites find that VLAN is not enough. This is because the VLAN tags defined in IEEE 802.1q is controlled by the customer. Thus, Ethernet frames from different customers that are transmitted in the service provider’s network may have the same VLAN tag. In order for the customers to share the same public network, another tag is needed in order to identify the customers. IEEE 802.1ad (Provider Bridges) is an amendment to IEEE standard IEEE 802.1Q. It is also called Q-in-Q or Stacked VLANs. IEEE 802.1ad defines the insertion of a S-VID (service VLAN ID, aka S-tag) by the service provider’s edge Ethernet switch (or bridge) at the UNI to provide separate instances of the MAC services to multiple independent customers of a public Bridged Local Area Network. With IEEE 802.1ad, the Ethernet frames from different customers in the service provider’s network can be easily identified, and it does not require cooperation among the customers, and requires a minimum of cooperation between the customers and the service provider. The following figure illustrates the operation of 802.1ad: UNI UNI Provider Network (Q-in-Q) Customer Customer Network Network S-Tag S-Tag added removed 10 IEEE 802.1ad-2005, Virtual Bridged Local Area Network – Amendment 4: Provider Bridges DCW LAN-34
  35. 35. EE6364 The frame format of an Ethernet frame with IEEE 802.1ad is as following: Octets 7 1 6 6 4 4 2 4 DA Type/ Preamble SFD SA S-TAG C-TAG LLC + Data FCS Length Bits 16 3 1 12 Bits 16 3 1 12 D C TPID (=0x88a8) PCP E S - VLAN ID TPID (=0x8100) PCP F C - VLAN ID I I Note that the Service –TAG and the customer-TAG have the same format. Inside the customer Ethernet network, the Ethernet frames have only the customer tag. When the frames enter the service provider’s network, at the UNI, the provider Ethernet switch inserts the service tag. Inside the service provider’s network, the switches forward the Ethernet frames based on the S-TAG and the destination MAC address (DA). At the egress switch, the S-tag is removed before the Ethernet frame is transmitted to the customer’s network. S-Tag, in general, is administered by the service provider. DCW LAN-35
  36. 36. EE6364 IEEE 802.1ah11: IEEE 802.1ad is useful for a metropolitan Ethernet network that connects Ethernet local area networks from different sites for a customer. However it has limitations: (1) The S-VID field can only accommodate 4096 customers. For a large network, especially a backbone network, this number may not be enough. (2) Most Ethernet control protocols, such as bridge protocol data units (B-PDUs) used by the customer networks can not interact with the service provider’s Ethernet switches/ bridges. These control protocol data units must be tunneled through the service provider’s network. IEEE 802.1ah specifies the tunneling of the customer’s Ethernet frames through the service provider’s backbone bridged network (PBBN). This protocol is also called MAC-in-MAC. The following figure illustrates the Ethernet frame format defined in IEEE 802.1ah: Octets 7 1 6 6 4 18 4 4 2 4 Type/ Preamble SFD B-DA B-SA B-TAG I-TAG S-TAG C-TAG Length LLC + Data FCS Bits 16 3 1 12 Bits 16 3 1 1 1 2 24 48 48 D D U TPID (=0x88a8) B-PCP E B - VLAN ID TPID (=0x88e7) I-PCP E C Res1 Res2 I-SID C-DA C-SA I I A Note: B-DA: Backbone Destination Address, B-SA: Backbone Source Address, B-TAG: Backbone Tag, TPID: Tag Protocol Identification B-PCP: Backbone Priority Code Point, DEI: Discard Eligible Indicator, I-PCP: Service Instance Priority Code Point, UCA: Use Customer Address, Res: Reservation, I-SID: Backbone Service Instance Identifier, 11 IEEE 802.1ah, Virtual Bridged Local Area Networks – Amendment 6: Provider Backbone Bridge. DCW LAN-36
  37. 37. EE6364 C-DA: Customer Destination Address, C-SA: Customer Source Address The value of TPID for B-TAG is 0x88a8, and that for I-TAG is 0x88e8. Since I-SID field has 24 bits, it can support up to 16million service instances. The number of backbone MAC addresses is much smaller than that for the customers. So it is much easier for the backbone bridges to learn these addresses. The transport of the Ethernet frames in PBBN uses the B-DA and B-VLAN ID. The following figure shows an example of a network which constists of the customer networks, provider bridge networks (PBN), and a provider backbone bridge (PBB) network. CN (802.1q) CN (802.1q) PBN CN (802.1ad) PBN (802.1q) (802.1ad) PBBN CN (802.1ah) CN (802.1q) (802.1q) PBN (802.1ad) PBN CN (802.1ad) (802.1q) CN (802.1q) CN CN (802.1q) (802.1q) CN (802.1q) DCW LAN-37
  38. 38. EE6364 TOKEN RING The token ring LAN is based on the use of a single token that circulates around the ring when all users are idle. A user wishing to transmit must wait until it detects token passing by. It changes the token from “free token” to “busy token”. The user then transmit a frame immediately following the busy token. The frame makes a round trip around the ring. Every user on the ring will examine the destination address in the header of the frame. Only the intended user will receive the frame. When the transmitting user receives the frame it sent, it purges that frame, and transmits a free token to the ring. The format of the token is O c te ts 1 1 1 SD AC ED and the frame format is Octets 1 1 1 2,6 2,6 1 1 SD AC FC DA SA Data ED FS where SD is an 8-bit pattern as starting delimiter. Access control (AC) identifies the frame priority and indicates whether this is a token or a data frame. Frame control (FC) indicates whether this is an LLC data frame. If not, bits in this field control operation of the token ring MAC protocol. Destination address (DA). Source address (SA). Frame check sequence (FCS). Ending delimiter (ED). Frame status (FS) contains the address recognized (A) and frame copies (C) bits. In the above Token Ring architecture, only one token is available, only one packet is outstanding in the ring, and only the transmitting user can generate a new token. Thus, this architecture wastes valuable time on the ring. DCW LAN-38
  39. 39. EE6364 An alternative is to let the user who has just transmitted a packet immediately reinsert a token to the ring, rather than waiting for the packet to travel completely around the ring. The packet is relayed around the ring and is copied by the receiver. The trailing token is also relayed until it arrives at a user having a packet waiting for transmission. There, the token is removed, the new packet inserted, and a new token is immediately reinserted following the new packet. Again, only the transmitting user can remove the packet from the ring. This architecture allows more than one packet in the ring and is more efficient. IEEE 802.5 specifies this alternative as the medium access method. In a token ring LAN, there are two factors that affect the delay performance. One is the propagation delay in the ring, which is determined by the length of the ring. The other factor is the processing delay by each access station. Each access station has to examine the circulating packets. While the processing delay for each station is fixed, as the number of access stations increases, the processing delay starts to dominate. Performance Analysis: Assume every node send a packet with duration T (i.e., the insertion time of the packet), and let’s denote Dx,y the propagation between nodes x and y; δ the duration (or the insertion time) of the token; DH the processing time at each node; N total number of nodes. DCW LAN-39
  40. 40. EE6364 Data Token Stripping 1 2 3 .... N 1' Station 1 0 T T+δ D1,2+D2,3+...+Dn,1+(N-1)DH DP+NDH+NT Dp+NDH Station 2 1 2 0 D1,2+DH D1,2+DH+2T D1,2 1 2 3 Station 3 0 D1,2+DH+D2,3+DH D1,2+D2,3+2DH+3T D1,2+DH+D2,3 1 2 3 N-1 N Station N 0 D1,2+D2,3+...+DN-1,N+(N-1)DH+NT D1,2+D2,3+...+DN-1,N+(N-2)DH Let D p = D1, 2 + D2 , 3 + ... + DN −1, N + DN ,1 Then the token ring efficiency is NT η= NT + NDH + D p And 1 limη = N →∞ DH 1+ T Thus, the larger the T, the higher the efficiency, η. DCW LAN-40

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