Bridged Ethernet Chapter 11 • Ethernet is a constantly evolving technology. The Ethernet today is far from what was Ethernet EltiEvolution: designed originally. • The first step in the evolution was the division Fast and of a LAN by bridges. • Bridges have two effects on an Ethernet LAN: Gigabit Ethernet raising the bandwidth and separating collision domains. 2 Figure 11-1 Figure 11-2 Sharing Bandwidth is s In a h n unb ared ared A Network with and without a Bridge b r etwee idged n E alltr ther n a et, the nsmit t total c ing st a apacit tions. A bridge divides the network into two or more segments. y In the above example, the 10-Mbps capacity is shared between 6 stations not 12 station. 3 4 Figure 11-3 Collision Domains in a Nonbridged andBd Brid ged dN Ne twor k Switched Ethernet • The idea of a bidbridge d LAN can be extddtended to a switched LAN –instead of having two to four segments, why not hihaving N segments where N is the number of stations on the LAN? • A layer‐two switch is an N‐port bridge with additional sophistication that allows faster handling of the packets. • In a switched Ethernet, the bandwidth is shared only between the station and the switch (5 Mbps each). • It was a big step toward an even faster Ethernet. With bridging the probability of collision is greatly reduced. 5 6 Figure 11-4 Switched Ethernet Full‐Duplex Ethernet • A traditional Ethernet is half‐duplex – a station can not send and receive at the same time. • The next step in the evolution was to move from switched Ethernet to full‐duplex switched Ethernet. • The full‐duplex mode increases the capacity of each dom ain from 10 Mbps to 20 Mbps. • In full‐duplex switched Ethernet, there is no need for the CSMA/CD access method, because there is no collision. The CSMA/CD function of the MAC sublayer can be turned off. 7 8 Figure 11-5 Figure 11-10 Full-Duplex Switched Ethernet Layers in Fast Ethernet Protocol (100 Mbps) 9 10 Fast Ethernet MAC Sublayer Fast Ethernet MAC Sublayer • The whole idea in the evolution of Ethernet is to • The sltlot time in bits is still 512. BtBut the sltlot keep the MAC sublayer untouched. time is only 1/10 that of 10‐Mbps Ethernet. • For full‐duplex Fast Ethernet, there is no need for Slot time = 5.12 µs CSMA/CD, however to keep backward • The maximum network length is related to the compatibility with traditional Ethernet, CSMA/CD slot time: was kept. MaxLength = PropagationSpeed × (SlotTime/2) • The only changes in Fast Ethernet (MAC sublayer) = (2 × 108) × (5.12 × 10‐6)/2 = 512 m are the slot time (in second)ds) and the maximum Practically, length of the network. MaxLength = 250 m 11 12 Figure 11-13 Fast Ethernet Phyyysical Layer Fast Ethernet Physical Layer • The reconciliati on sublayer replaces the PLS sublayer in 10‐Mbps Ethernet. But the encoding and decoding ftifunctions of the PLS sublayer are moved to the PHY sublayer because encoding in Fast Ethernet is medium ddtdependent. • In Fast Ethernet the MII replaces the AUI in 10‐Mbps Ethernet. – It operates at both 10 Mbps and 100 Mbps – It features a parallel data path (4 bits at a time) between the PHY and the reconciliation sublayers 13 14 Figure 11-14 MII Figure 11-15 Signals in MII The MII defines five types of signals. 15 16 Figure 11-16 Figure 11-17 MII Connector (40 Pins) MII Cable 17 18 Figure 11-18 Fast Ethernet Physical Layer Fast Ethernet Implementations • The titransceiver in FtFast Etherne t is calle d the PHY sublayer. Four-wire Two-wire Implementation • Besides the regular functions of 10‐Mbps Ethernet, Implementation the PHY is also responsible for encoding and decoding. • An transceiver can be external or internal. • The transceiver is medium dependent. • The MDI is just a piece of hardware that connects the transceiver to the medium. • The MDI is ilimplemen ttitation specific. Twisted-Pair Cable Fiber-Optic Cable Twisted-Pair Cable 19 20 Figure 11-19 100Base‐TX 100Base-TX • 100Base‐TX uses two pairs of titdtwisted‐pair cable (ith(either IlImplemen ttitation Cat‐5 UTP or STP) in a physical star topology. • The logical topology is bus for half‐duplex mode using a hub (CSMA/CD needed), or star for full‐duplex mode using a switch (CSMA/CD not needed). • The implementation allows either an external transceiver (with an MII cable) or an internal transceiver. • The transceiver is responsible for transmitting, receiving, detecting collision and encoding/decoding of data. 21 22 Encoding and Decoding Encoding and Decoding • Tra ditiona l Etherne t uses MhtManchester encoding • MLT‐3 (three levels, multiline transmission) is very – Advantage: it provides self‐synchronization similar to NRZ‐I – Disadvantage: may have two transitions per bit – There is a signal level transition only for bit 1. – Not a problem for 10‐Mbps Ethernet, because even – A maximum of one transition per bit is needed. voice grade UTP (Cat.3) can provide 25 Mbaud – Signaling rate is the same as the data rate signaling rate – Not self‐synchronized – For 100‐Mbps Enternet, 200 Mbaud signaling rate is • To maintain synchronization, 4B/5B encoding, which needed to achieve 100 Mbps data rate provides enough signal transition, is used (see Fig. 11‐ – Cat. 5 UTP can only support 125 Mbaud signaling rate 20). The output data at 125 Mbps rate is then • Fast Ethernet uses MTL3 + 4B/5B encoding encoded using MLT‐3 and transmitted at 125 Mbaud. 23 24 Figure 11-20 Figure 11-21 MLT-3 Signal Encodinggg and Decoding in 100Base-TX Two Levels of Encoding 25 26 Figure 11-22 100Base-FX 100Base‐FX Implementation • 100Base‐FX uses two pairs of fiber‐optic cables. • The output data from 4B/5B block encoder is encoded into a signal using NRZ‐I encoding scheme. • 4B/5B – NRZ‐I is effective over optical fiber, but not suitable for use over twisted pair. • All other features are the same as 100Base‐TX. 27 28 Figure 11-23 Figure 11-24 NRZ-I Encoding Encoding and Decoding in 100Base -FX 29 30 Figure 11-25 100Base-T4 Implementation 100Base‐T4 • A 100Base‐TX requires the use of category‐5 UTP or STP cable. But most buildings have already been wired for voice grade twisted pair cables (Cat. 3). Rewiring is expensive. • 100Base‐T4 was designed to use Cat. 3 or higher UTP. The implementation uses four pairs of UTP for transmitting data at 100 Mbps. 31 32 Figure 11-26 Example of 8B/6T Encoding Encoding and Decoding • To maintain synchronization and at the same time reduce the bandwidth, a three‐level line encoding method called eight binary/6 ternary (8B/6T) is used. • Each block of 8‐bit data is encoded as 6‐symbol ternary signal. • Three signal levels are used (+1, 0, and ‐1 V). • The mapping is chosen to ensure synchronization and DC balance. See Appendix K for a tabulation of 8B/6T code pairs. 33 34 Figure 11-27 Using Four Wires in 100Base-T4 Transmission Using Four Wires • 8B/6T reduces the bandwidth from 100 Mbaud to 75 Mbaud (ratio of 8/6) – assuming one transition / bit. • However it is not enough, because Cat. 3 voice grade UTP is designed to operate on 25 Mbaud bandwidth. • For unidirectional transmission, three pairs of cable aaere needed. To suppor t bbdidir ectoection al ttarans miss ssoion of certain control signals, four pairs are used. • Full duplex communication is not possible. Data is only flowing in one direction in the absence of collision. 35 36 Figure 11-28 Layers in Gigabit Ethernet (1000 Mbps) Gigabit Ethernet MAC Sublayer • The whlhole idea in the evoltilution of Etherne t is to keep the MAC sublayer untouched. • Two distinctive medium access methods are used: half‐duplex using CSMA/CD or full‐ duplex without CSMA/CD. • There are three implementations for half‐ duplex MAC – traditional, carrier extension and frame bursting. 37 38 Figure 11-29 Two Approaches in Gig abit Ethernet Gigabit Ethernet MAC Sublayer Medium Access • In traditional half‐duplex approach, the minimum frame length is kept the same as in traditional Ethernet (512 bits) for compatibility purpose. • The slot time is 1/100 that of traditional Ethernet. – Slot time = 0.512 µs – MaxLength = 25 m (practical lengg)th) • In carrier extension half‐duplex approach, the minimum frame length is increased to 4096 bits, eight times that of traditional Ethernet. – MaxLength = 200 m 39 40 Figure 11-30 A Frame Using Carrier Extension Method Gigabit Ethernet MAC Sublayer • Carrier extension is very inefficient if we have a series of short frames to send. • In frame bursting half‐duplex approach, multiple frames are sent. However, padding is added between the frames to make these multiple frames look like one frame, so that the channel is not busy. • In the full‐duplex approach, there is no need to follow the minimum length criteria, because CSMA/CD is not used. Most implementations follow full‐duplex approach. 41 42 Figure 11-31 Figure 11-32 Gigabit Ethernet Physical Layer Frame Bursting Approach 43 44 Figure 11-33 Gigabit Ethernet Implementations Gigabit Ethernet Physical Layer • The reconciliation sublayer is common to all Four-Wire implementations and is responsible for Two-Wire Implementation sending 8‐bit parallel data to the PHY via GMII. Implementation • GMII is part of NIC (cannot be external). • PHY (transceiver) is medium dependent and is also responsible for encoding and decoding. It can only be internal. • MDI connects transceiver to the medium.