Synchronous Digital Hierarchy (SDH) | NFON Knowledgebase UK [Contact](# "Contact") ![](/web-assets/f/290788/40x40/d98615a3ea/nfon-sales-contact.svg) Sales & General * [Talk to Sales](tel:+443303838000) * [Write to us](/gb/request-information) Search * [![Products](https://a.storyblok.com/f/290788/40x40/539cd331b9/nfon-products.svg) Products](#) Call & Work ![AI icon](/assets/ai_icon.webp) + [Cloud Telephony](/gb/products/cloud-telephony/) + [SIP Trunk](/gb/products/enablement/) + [Devices](/gb/products/devices) Build & Automate ![AI icon](/assets/ai_icon.webp) + [Assistants](/gb/products/ai/assistants) + [Bots](/gb/products/ai/bots) + [AI Agent Studio](/gb/products/ai/agent-studio) Engage & Support ![AI icon](/assets/ai_icon.webp) + [Contact Center](/gb/products/customer-contact/) Integrations & Add ons + [Integrations & Add ons](/gb/integrations/) * [![Solutions](https://a.storyblok.com/f/290788/40x40/da156cbbb8/nfon-solutions-industries.svg) Solutions](#) Industries + [Health & Wellness](/gb/solutions/health-wellness) + [Retail & E-Commerce](/gb/solutions/retail) + [Finance & Legal](/gb/solutions/finance-insurance) + [Travel & Hospitality](/gb/solutions/hospitality) + [Public Sector](/gb/solutions/public-sector) + [Energy](/gb/solutions/energy) Success stories + [Customer Stories](/gb/customer-stories/) * [![Partnerships](https://a.storyblok.com/f/290788/36x36/7bd105c67d/nfon-partner.svg) Partnerships](#) Partner with NFON + [Partner](/gb/partner/become-partner) + [NGAGE](/gb/partner/partner-programme) Partner Portal + [Partner Portal](https://partners.nfon.com/) * [![Help Center](https://a.storyblok.com/f/290788/40x40/d01fbb4f30/help-nfon.svg) Help Center](/gb/help-centre) * [Login](https://start.cloudya.com/ "Login") * United Kingdom + [International](/en) + [Deutschland](/de) + [España](/es) + [France](/fr) + [Italia](/it) + [Österreich](/at) + [Polska](/pl) + [Portugal](/pt) + [United Kingdom](/gb/get-started/cloud-telephony/lexicon/knowledge-base-detail/sdh) [![NFON Logo](/assets/logo-nfon.svg)](/gb/ "NFON Home") * [Products](#) ![](/web-assets/f/290788/36x36/a9f3ceed6f/nfon-call-work.svg) Call & Work ![AI icon](/assets/ai_icon.webp) Seamless communication ![](/web-assets/f/290788/36x36/d03c61cf40/nfon-build-automate.svg) Build & Automate ![AI icon](/assets/ai_icon.webp) AI Automation ![](/web-assets/f/290788/36x36/106823b73b/nfon-engage-support.svg) Engage & Support ![AI icon](/assets/ai_icon.webp) Omnichannel support ![](/web-assets/f/290788/36x36/50b60f5411/nfon-integrations-add-ons.svg) Integrations & Add ons Connect Teams & CRMs + [![](/images/products-call-work-cloudteleophny-navbar-int_Fjf8b.webp) ### Cloud Telephony ![AI icon](/assets/ai_icon.webp) Seamless cloud telephony for every device. High-fidelity audio with European-grade security.](/gb/products/cloud-telephony/) + [![](/images/products-call-work-siptrunk-navbar-int_ZH5NzW.webp) ### SIP Trunk Secure cloud connectivity for your existing hardware. Scales instantly with your business.](/gb/products/enablement/) + [![](/images/products-call-work-devices-navbar-int_Z2dP6.webp) ### Devices Professional telephones and hardware designed for clear calls and all-day comfort.](/gb/products/devices) + [![](/images/products-build-automate-assistants-navbar-int_2ghUIJ.webp) ### Assistants ![AI icon](/assets/ai_icon.webp) Pre-configured, plug-and-play digital roles like FrontDesk and Workdesk. Ready to deploy instantly with zero coding required.](/gb/products/ai/assistants) + [![](/images/products-build-automate-bots-navbar-int_Z1p57Vm.webp) ### Bots ![AI icon](/assets/ai_icon.webp) Prompt-driven, multi-channel conversational experiences. Automate custom customer journeys across Web Chat, Voice, and Livechat.](/gb/products/ai/bots) + [![A chatbot creation interface is shown with a readiness message, build report, agent prompt details, and created tools list, over a gradient background.](/images/products-build-agent-studio-navbar-int_Z1Y88dE.webp) ### AI Agent Studio ![AI icon](/assets/ai_icon.webp) Simply turn your ideas into AI agents for your business. Describe the task, create a working agent, then optimize it in one workspace.](/gb/products/ai/agent-studio) + [![](/images/products-engage-support-navbar-int_1xyFBE.webp) ### Contact Center ![AI icon](/assets/ai_icon.webp) Omnichannel customer service for modern teams. Manage conversations, automation, AI assistance, and customer context.](/gb/products/customer-contact/) + [![](/images/products-integrations-addons-navbar-int_Z1Ka5ea.webp) ### Integrations & Add ons Expand your capabilities and natively connect your everyday business to keep all workflows unified.](/gb/integrations/) * [Solutions](#) ![](/web-assets/f/290788/40x40/da156cbbb8/nfon-solutions-industries.svg) Industries Tailored solutions ![](/web-assets/f/290788/40x40/155f2a989d/nfon-success.svg) Success stories 54,000+ trust us + [![](/images/solutions-health-wellness-navbar-int_Z15q3kJ.webp) ### Health & Wellness Secure communication for better patient experiences and care delivery.](/gb/solutions/health-wellness) + [![](/images/solutions-retail-ecommerce-navbar-int_zUgjw.webp) ### Retail & E-Commerce Connected communication for modern retail and customer engagement.](/gb/solutions/retail) + [![](/images/solutions-finance-legal-navbar-int_1Dy5Ax.webp) ### Finance & Legal Trusted communication for regulated and security-conscious organizations.](/gb/solutions/finance-insurance) + [![](/images/solutions-hospitality-travel-navbar-int_17WoBe.webp) ### Travel & Hospitality Seamless communication for exceptional guest experiences and service.](/gb/solutions/hospitality) + [![](/images/solutions-publicsector-navbar-int_YyABB.webp) ### Public Sector Reliable communication for responsive public services and citizen support.](/gb/solutions/public-sector) + [![](/images/solutions-energy-navbar_FDoix.webp) ### Energy Reliable communication for responsive customer service, even during outages and peak demand.](/gb/solutions/energy) + [![](/images/solutions-costumerstroy-navbar-int_Z1MdOsM.webp) ### Customer Stories Real-world impact across every industry. Explore how we solve complex communication challenges for Europe’s leading brands.](/gb/customer-stories/) * [Partnerships](#) ![](/web-assets/f/290788/36x36/7bd105c67d/nfon-partner.svg) Partner with NFON Join the NFON network ![](/web-assets/f/290788/36x36/8c5c9a9c78/nfon-partner-portal.svg) Partner Portal Existing partner login + [![](/images/partnerships-partner-navbar-int_ZHYGKx.webp) ### Partner From onboarding to co-branded marketing, we provide the tools you need to win.](/gb/partner/become-partner) + [![](/images/partnerships-ngage-navbar-int_Zs3yY9.webp) ### NGAGE Discover our tiered rewards system designed to help you scale your business and revenue.](/gb/partner/partner-programme) + [![](/images/partnerships-partnerportal-navbar-int_2ki451.webp) ### Partner Portal Access partner tools, deal registration, training resources, and program benefits in one place.](https://partners.nfon.com/) * [Help Center](/gb/help-centre) * Search * [Login](https://start.cloudya.com/ "Login") * gb + [International](/en) + [Deutschland](/de) + [España](/es) + [France](/fr) + [Italia](/it) + [Österreich](/at) + [Polska](/pl) + [Portugal](/pt) + [United Kingdom](/gb/get-started/cloud-telephony/lexicon/knowledge-base-detail/sdh) * [Contact](#) [Contact](# "Contact") + ![](/web-assets/f/290788/40x40/d98615a3ea/nfon-sales-contact.svg) Sales & General Get in touch with us. ![](/images/getstarted-talktosales-navbar-int_ZMspQ4.webp) ### Talk to Sales Get a free consultation to see how NFON products can meet your needs. [+44 330 383 8000](tel:+443303838000 "+44 330 383 8000") ![](/images/getstarted-writetous-navbar-int_ZY7MOf.webp) ### Write to us Fill out our request form. Our experts will respond as soon as possible. [Write to us](/gb/request-information "Write to us") ### Synchronous Digital Hierarchy (SDH) [Basic functioning of synchronous digital hierarchy](/gb/get-started/cloud-telephony/lexicon/knowledge-base-detail/sdh#section-3) [The SDH components of the various layers](/gb/get-started/cloud-telephony/lexicon/knowledge-base-detail/sdh#section-4) # Synchronous Digital Hierarchy (SDH) The acronym SDH stands for Synchronous Digital Hierarchy and refers to a multiplex technology used in telecommunications. SDH allows data streams with low bit rates to be combined into high-rate data streams. Since the entire network is synchronous, individual bit streams can be embedded into and extracted from high-rate data streams relatively easily. Initial specifications were developed back in 1985 in the USA under the term SONET. As a new digital optic transmission system, SONET was to provide key advantages over plesiochronous digital hierarchy (PDH), while still being able to transport PDH. ITU-T standardised SDH based on SONET. SONET and SDH are interoperable. SDH is suitable as a transmission system for broadband [ISDN](/gb/get-started/cloud-telephony/lexicon/knowledge-base-detail/isdn) and for transporting ATM cells, PDH signals, Ethernet aggregations, SAN signals and other communication signals. ### Basic functioning of synchronous digital hierarchy SDH combines n signals with a bit rate of b to form data streams with bit rates of n x b on a synchronously clocked network. Unlike PDH, the individual transmission paths have minimal clock discrepancies. The synchronous mode of operation allows low-order multiplex systems, such as communication links for [telephone systems](/gb/en/products/cloudya), to be inserted in higher hierarchy levels and then removed again via add and drop. In accordance with the levels defined by the standard, SDH recognises different hierarchies such as STM-1, STM-4, STM-16 or STM-64 (155 MBits, 622 MBits, 2,488 MBits, 9,953 MBits) and more. About five per cent of the gross data rate is reserved for OAM tasks (Operations, Administration and Maintenance). The data is transparently transported over the SDH network in containers. ### The SDH components of the various layers On the physical layer, digital synchronous hierarchy uses connections based on copper lines, fibre optics or satellite and directional radio links. Distorted or muted signals can be refreshed via regenerators. On the superior layer, multiplexers combine the signals into high-bit-rate data streams. Finally, virtual containers transport the individual containers of usable data and control mapping of the various signals of different bit rates. [Back](#) ![NFON Logo](/assets/logo-nfon.svg) News * [Events](https://www.nfon.com/gb/news/events/) * [Press](https://www.nfon.com/gb/news/press/) Customers * [Cloudya Login](https://start.cloudya.com/auth/login) * [Admin Portal](https://admin.nfon.com/) * [NFON Status](https://status.nfon.com/) About NFON * [Corporate](https://corporate.nfon.com/en/) * [Investor Relations](https://corporate.nfon.com/en/investor-relations/ir-at-a-glance/) * [Sustainability & ESG](https://corporate.nfon.com/en/sustainability/) * [Career](https://corporate.nfon.com/en/careers/) Legal * [GTC-SLA](/gb/legal/gtc-sla) * [Legal Notice](/gb/legal/legal-notice) * [Trust Centre](https://www.nfon.com/en/trust-center/) * [Data Protection](/gb/legal/data-protection) * [Cookie Settings](javascript:onclick=UC_UI.showSecondLayer()) © 2026 - NFON | All rights reserved.