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ETHERNET LAN NETWORKING
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Ethernet1 Can it be used for Automotive Networks?
1. Introduction The purpose of this document is to provide an overview of the fast Ethernet technology as known today and review its suitability for use in automotive Infotainment application for streaming of audio and video data.
2. General Ethernet is a family of frame-based computer networking standards for local area networks (LANs). It defines wiring and signalling standards for the Physical Layer (PHY) Media Access Control (MAC)/Data Link Layer (DLL), and a common addressing format. Ethernet was originally developed at Xerox PARC in 1973–1975 based on the idea of computers communicating over a shared coaxial cable loop acting as a broadcast transmission medium. The coaxial cable was replaced with point-to-point links connected by hubs and/or switches to reduce installation costs, increase reliability, and enable point-to-point management and troubleshooting. Nowadays, Ethernet is standardized as IEEE 802.3. The most popular Ethernet versions are twisted-pair Ethernet systems. They have been developed since the mid-80s becoming widely known with 10BASE-T (10 refers to data capacity in Mb/s, BASE to the Baseband signalling and T to the Twisted Pair transmission medium)). 10BASE-T replaced early Ethernets (coaxial cable based loops) with a system of hubs linked with unshielded twisted pair (UTP), Fast Ethernet products based on 100BASE-T and 1000Base-T specifications are now entering the market for Local Area Networks.
3. The Ethernet Protocol Suite Adjacent diagram shows the structure of the Ethernet Protocol suite used in computer LANs starting with the Transmission Media and cable at the bottom of the diagram
APPLICATION
TRANSPORT
3.1
Ethernet Physical Layer
The Physical Layer translates communications requests from the Data Link Layer into hardware-specific operations to affect transmission or reception of electronic signals.The Physical Layer is a fundamental layer upon which all higher level functions in a network are based. The Physical Layer defines the means of transmitting raw bits rather than logical data packets over a physical link connecting network nodes. The Ethernet physical layer encompasses the physical media interfaces – cables and connectors. The speed ranges from 100 Mb/s to 1000 Mb/s while the physical is twisted pair
3.1.1
NETWORK
LINK
PHYSICAL
Cables & Connectors
Cables
Category 5 cable is specified for both 100 and 1000BASET, it includes four twisted pairs in a single cable jacket. Balanced lines are designed to preserve a higher signal-to-noise ratio despite interference from both external sources and crosstalk from other pairs. Cat 5 cable has three twists per 2.5cm of each twisted pair of 24 gauge copper wires within the cable. Cat5 performance characteristics and test requirements are specified only for frequencies of up to 100 MHz.
1
Ethernet is derived from the word "ether" a term used in the 19th century to describe a medium for the propagation of light. Copyright 2008 Caenso BV All Rights Reserved
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ETHERNET LAN NETWORKING
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The Cat 5e cable is an enhanced version of Cat 5. The Telecommunications Industry Association (TIA) formally defined in 2001 Cat5e for 1000BASE-T Ethernet Stricter performance specifications have been introduced to reduce cross talk. However, Cat 5e cable cables are still limited to a maximum of 100 m in length (normal practice is to limit fixed ("horizontal") cables to 90 m to allow for up to 5 m of patch cable at each end). TIA does not recognise Cat5 as a valid standard for 1000BASE-T.
3.1.2
Connectors
Nearly always, 8P8C modular connectors, are used for connecting category 5 cable. Ethernet is using 8P8C connections (often incorrectly referred to as "RJ-45") consisting of a male plug and the female socket wired according to a TIA/EIA standard. Each has eight equally-spaced conductors. On the plug, these conductors are flat contacts positioned parallel with the connector body. Inside the socket, the conductors are suspended diagonally toward the insertion interface. When an 8P8C plug is mated with an 8P8C socket, the conductors meet and create an electrical connection. Spring tension in the socket's conductors ensure a good interface with the plug and allow for slight travel during insertion and removal.
3.1.3
Key PHY specifications
100BASE-TX 100BASE-T4
802.3 (24) 802.3 (23)
4B5B MLT-3 coded signalling, CAT5 copper cabling with two twisted pairs. 8B6T PAM-3 coded signalling, Cat-5 cabling. Limited to half-duplex.
1000BASE-T
802.3 (40)
PAM-5 coded signalling, CAT5/CAT5e/CAT6 copper cabling with four twisted pairs (used in both directions)
All copper Ethernet segments that run the Collision Detect (CD) portion of CSMA/CD have a minimum cable length to function properly because of reflections. This applies only to 10BASE-T and 100BASE-T. 1000BASE-T supports half-duplex mode, making collisions possible. Consequently, the 1000BASE-T standard requires a minimum cable length for collision detection to function properly; to avoid this in Gigabit Ethernet, small frames are padded into the transmission in half-duplex mode. Full-duplex Ethernet connections, used in Fast Ethernet networks, work by making simultaneous use of all four physical pairs of twisted cable (which are inside the insulation), where two pairs are used for receiving packets and two pairs are used for sending packets, to a directly connected device. This effectively makes the cable itself a collision-free environment and theoretically doubles the maximum bandwidth that can be supported by the connection.
3.1.4
Autonegotiation and duplex mismatch
Many different modes of operations (10BASE-T half duplex, 10BASE-T full duplex, 100BASE-TX half duplex,) exist and will continue to exist in the fast Ethernet daomin for Ethernet over twisted pair, and most network interface hardware operating at physical and data link layer must support different modes of operations. Network interfaces connected to each other will negotiate the best possible shared mode of operation – Autonegotiation. Autonegotiation is a mechanism for detecting the speed but not the duplex setting of Ethernet peers that did not use autonegotiation. When two linked interfaces are set to different duplex modes, the effect of this duplex mismatch is a network that functions much slower than its nominal speed. Duplex mismatch may be inadvertently caused when an administrator configures an interface to a fixed mode (e.g 100 Mbit/s full duplex) and fails to configure the remote interface, leaving it set to autonegotiate. Then, when the autonegotiation process fails, half duplex is assumed by the autonegotiating side of the link. The resulting duplex mismatch results in a dramatically slow network, in which many collisions, and especially late collisions occur on the interface set to half-duplex, and frame check sequence errors are seen on the full-duplex side.
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4. Ethernet MAC and DLL Above the physical layer, Ethernet stations communicate by sending each other data packets, blocks of data that are individually sent and delivered. As with other IEEE 802 LANs, each Ethernet station is given a single 48-bit MAC address, which is used both to specify the destination and the source of each data packet. Network interface cards (NICs) or chips normally do not accept packets addressed to other Ethernet stations. Adapters generally come programmed with a globally unique address, but this can be overridden, either to avoid an address change when an adapter is replaced, or to use locally administered addresses 100Base-TX, and 1000Base-T supports Full-Duplex mode allowing two stations to simultaneously exchange data over a point-to-point between 2 stations only via an independent transmit and receive paths provided that both stations are configured for FD mode. FD Mode Ethernet can perform Link Aggregation to increase link availability and bandwidth between two Ethernet stations by allowing multiple "physical" links to be combined to operate as a single "logical" link. Bandwidth can be increased in linear increments e.g. from 100 Mb/s to 200 Mb/s, 300 Mb/s, 400 Mb/s etc. Link Aggregation operates by adding a new layer of function between the MAC layer and the higher layer protocols above. Each of the underlying Ethernet ports in an aggregated group transmits and receives frames with their own unique MAC address. As frames pass through the Link Aggregation layer, addresses are manipulated so the aggregated ports appear as a single link with one MAC address.
5. Ethernet Hardware 5.1
Hubs
A network hub or repeater hub operates at Physical Layer level. It is a device for connecting multiple twisted pair Ethernet devices together, making them act as a single network segment. Hubs work at the physical layer. The Hub is a form of multiport repeater. Repeater hubs participate in collision detection, forwarding a jam signal to all ports if it detects a collision. A hubbed Ethernet network behaves like a shared-medium, that is, only one device can successfully transmit at a time and each host remains responsible for collision detection and retransmission. With 100BASE-T links there are separate pairs for transmit and receive but they are used in half duplex mode in which they still effectively behave like shared medium links A network hub or repeater is does not manage any of the traffic that comes through it, and any packet entering any port is broadcast out on every other port (other than the port of entry). Since every packet is being sent out through every other port, packet collisions result--which greatly impedes the smooth flow of traffic.The availability of low-priced network switches has largely rendered hubs obsolete but they are still seen in older installations and more specialized applications
5.2
Switches
Ethernet switches are processing and routing data at the Data Link Layer (layer 2) of the OSI model. It has to be noted that currently available switches are not incorporating the AVB Transport protocol. Although, the integration of the AVB transport protocol in Ethernet switches will lead to a substantial loss of bandwidth, the availability of relatively low-cost high bandwidth Ethernet chips with e.g. 100Mb/s (100BaseT), 1Gb/s (1000Base-T), would suggest that it would be possible to use a standard high-bandwidth Ethernet for video, and audio data. However, given the need in new generation vehicle networks for up to four video channels, a capacity of several hundred Megabits per second is needed. The 100Mb/s Ethernet would not be sufficient, leaving the 1Gb/s and above as the only possible choice.
6. Real Time Data Traffic Ethernet has not been designed for real-time, low-latency data traffic needed for video/audio streaming. However, the increased availability of services requiring guaranteed delivery of time-critical data traffic targeting Small Office and Home (SOHO) applications has lead to the development of a transport protocol standard (IEEE 1722 AVB) defining the quality of service for this type of traffic. The purpose of the 1722 AVB protocol is to facilitate interoperability Ethernet between stations that stream time sensitive audio and/or video across an Ethernet networks. This new protocol will provide time synchronization and latency/bandwidth services by defining the packet format and stream setup, control, and teardown The IEEE1722 AVB working group is currently defining the AVB protocol. Once completed the AVB protocol will become a new sub-layer of the Data Link Layer.
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However, the IEEE 1722 AVB protocol does not deliver a true real time transfer for time critical audio/video data streaming, needed in cars. However, the Pseudo Real Time service offered can be used in vehicles in situation where the vehicle network architecture can be adapted to overcome its limitation.
7. Electromagnetic Compatibility (EMC) Ethernet systems are specified for compliance with Class B machines as defined by the Federal Communications Commission (FCC) Class B devices/systems are suitable for use in offices and homes. Class B indicates that the machine's radio frequency (RF) emissions are so low that they do not interfere with other devices such as radios and TVs. (FCC Class B compliance achieved 40dBuV/m) A new wiring standard Cat-6 with four twisted copper wire pairs, for Gigabit Ethernet, is currently being defined by TIA/EIA-568-B. Although, the Cat-6 is designed for more stringent specifications for crosstalk and system noise to provide performance of up to 250 MHz for 1000BASE-T, it is not designed to mitigate the PAM5 modulation used by Ethernet to send data over the PHY. PAM is a Pulse Amplitude Modulation technique that encodes message information in the amplitude of a series of signal pulses. The modulation scheme generates significant EMI. This is a very important factor in the choice of the automotive bearing in mind that modern car feature: : • Various antennas • Several kilometres of copper cables • Numerous processors with very high clock speeds • Engine starters with starting currents on >1,000A • Various signals spread over the entire RF spectrum up to 4GHz
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8. Definitions Protocol Suite Protocol Stack
TCP/IP 4B5B MLT-3 CAT5 PAM CSMA/CD
Host
Terminal
Client Application Server Subdevice
a computer networking protocol suite is the definition/description of the protocols governing the data exchange between computers is a particular software implementation of a computer networking protocol suite. The terms are often used interchangeably -- the protocol suite is the specification and the stack is the software implementation Is the name of the Internet Protocol Suite i.e. a set of communications protocols used for the Internet and other local area networkss Is a line coding format that works by mapping groups of four bits onto groups of 5 bits. The transitions provide clocking information for the signal Stands for Multi-Level Transmit and is a signalling method that uses three voltage levels. Is a multiple twisted pair cable type designed for Local Area Networks such as Ethernet for compliance EMC requirements for home and office networks Pulse-amplitude modulation is a form of signal modulation where the message information is encoded in the amplitude of a series of signal pulses. Carrier Sense Multiple Access With Collision Detection and is a network control protocol in which a carrier sensing scheme is used. A transmitting node detecting another signal while transmitting a frame, stops transmitting that frame, transmits a jam signal, and then waits for a random time before trying to send that frame again. Historically Host denoted a computer that provided services to smaller or less capable devices. In today’s computer networking terminology, Host is a computer connected to the Internet. A network host can host information as well as client and/or server software. A Terminal is an electronic device that is used for entering data into, and displaying data from, a computer or a computing system. A computer terminal is an instance of a humanmachine interface (HMI). A client is an application or system that accesses a remote service on another computer system, known as a server, by way of a network. Is a software module that employs the capabilities of a computer device directly and thoroughly to a task that the user wishes to perform. A Server is a computer dedicated to providing one or more services over a computer network A Subdevice is a unique type of hardware function related to a single role, running specific application software. The Subdevice can be itself part of a computer network or part of a device that combines several subdevices; all subdevices contained in the device are connected to the network. An HMI subdevice is in term of computer networking a client or terminal while a source subdevice is a server or host
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